The Pros and Cons of Growing Plants Cooler - PDF

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                          The Pros and Cons
    greenhouse            of Growing Plants Cooler
    temperatures will     Ryan M. Warner, John E. Erwin, Michael J. McDonough and Neil Mattson
    reduce heating        Department of Horticultural Science, University of Minnesota
    costs but will
                             It is estimated that                  many growers are deciding      and branching, insect and
    likely increase
                          approximately 80% of the                 whether to lower green-        diseases incidences and
    production times.     cost of heating a green-                 house temperatures.            control and whether the
    Here we weigh         house is incurred at night.              Lowering greenhouse tem-       increased production times
    the pros and          Given the long nights of                 peratures will reduce heat-    under cooler temperatures
    cons of lowering      winter, heating costs are                ing costs but will likely      will minimize the savings
                          clearly a major cost in                  increase production times.     growers are hoping to real-
    greenhouse            plant production in the                  This article will weigh the    ize by reducing green-
    temperatures and      north. The dramatic                      pros and cons of lowering      house temperatures. We
    provide an idea       increase in fuel prices last             greenhouse temperatures        will also discuss ways to
    of what impact        winter caught many people                and provide an idea of         physically save on heating
                          ‘off-guard’ and reduced                  what impact cooler tem-        costs.
    cooler                profits for many green-                  peratures will have on crop
    temperatures will     house growers. The fore-                 quality, timing, and or pest   Impact on Plants
    have on crop          cast for fuel costs this win-            control. Specifically, we      Seed Germination
    quality, timing,      ter and beyond is uncer-                 will discuss how tempera-        It is critical to provide
                          tain, but it is likely that the          ture impacts seed germina-     appropriate temperatures
    and or pest           higher fuel prices will be               tion, plant development        during seed germination!
    control               with us for some time.                   rate, plant quality charac-
                          The ‘bottom line’ is that                teristics such as flowering            Continued on next page

                               Figure 1. Leaf unfolding rate in response to temperature (Erwin, 1995).
                                    Leaf unfolding rate per day

                                                                  Average daily temperature (°F)

  Seed germination and                   ºF cooler than at bench         Figure 2. Impact of temperature on days to flower of
  early seedling establish-              level and 2) on clear nights    three bedding plant species (Mattson and Erwin,
  ment is one of the most                plant temperature is lower      unpublished data).
  temperature-sensitive                  than air temperature.
  stages of plant develop-               Never place seed trays on
  ment. Temperatures too                 the floor! For these rea-
  warm or cool during seed               sons, do not try to save
  germination will delay ger-            money by dropping
  mination, reduce the per-              temperatures during this
  cent germination and                   stage!
  decrease the uniformity in
  germination time. Also,                Plant development rate
  cool temperatures (<70 ºF)                Plant development rate
  can promote ‘damping off’              (i.e. leaf unfolding rate) is
  diseases, such as Pythium              dependent on the average
  spp.                                   daily temperature at which
     For most species, ger-              plants are grown. Leaf
  mination media tempera-                unfolding rate increases as
  ture should be maintained              average daily temperature
  between 72 and 76 ºF.                  increases (Fig. 1). Above
  When your crop is estab-               a ‘maximum’ temperature,        growing plants ‘too hot’        The impact of reducing
  lished and actively grow-              leaf unfolding rate             will increase production        temperature on days to
  ing, most species are much             declines. Therefore, plants     time!                           flower differed across
  more tolerant of cool tem-             will come into flower ear-         We conducted research        species (Fig. 2). That is,
  peratures than at the seed             lier when grown under a         at the University of            growing plants cooler will
  germination stage. It is               warmer temperature              Minnesota on the effects        increase production time
  important to remember                  regime compared to a            of temperature on time to       of some crops more than
  that 1) the temperature on             cooler temperature regime       flower of a few major bed-      others. For instance,
  the floor can be 10 to 20              to a point. However,            ding plant crops (Table 1).
                                                                                                                   Continued on next page

  Table 1. Effect of temperature on the number of days to flower for Impatiens wallerana Hook f., Petunia x hybrida
  Hort., and Gams. (pansy) cultivars. Plants were grown in the reported temperatures from when the cotyledons
  expanded to when the first flower opened (Mattson and Erwin, unpublished data).

  Cultivar                                          54° F       61° F     68° F      75° F      Delay in Flowering if 24-h Temperature is
                                                                                                          Reduced 1 ºF (days)*

  I. wallerana ‘Super Elfin Lipstick                  -          72        54         47                          1.8
  P x hybrida ‘Avalanche Pink’                        88         74        47         39                          2.5
  P x hybrida ‘Dreams Rose’                           84         67        46         37                          2.3
  P x hybrida ‘Purple Wave’                          112         88        57         45                          3.3
  V x wittrockiana ‘Colossus Yellow Blotch’           95         82        63         58                          1.9
  V x wittrockiana ‘Crystal Bowl Supreme Yellow’      72         63        51         46                          1.3
  V x wittrockiana ‘Delta Pure White’                 88         71        61         53                          1.6
  V x wittrockiana ‘Sorbet Blackberry Cream’          68         60        50         45                          1.1

  * for temperatures between 54 and 75 ºF
  - plants died in this treatment

         Figure 3. Easter lily ‘Nellie White’ reduction in time                  dependent on the operating     pathogen that causes
         from visible bud to flower opening under increasing                     cost of your facility.         ‘black root rot’ and, unfor-
         temperature (Erwin and Heins, 1995).                                       Temperature also            tunately, is becoming a
                                                                                 impacts rate of develop-       more common problem for
                                                                                 ment from visible bud to       pre-finished plants, partic-
                                                                                 flower opening. For exam-      ularly pansies, vinca and
  Days from visible bud to flower

                                                                                 ple, as temperature            petunias. Thielaviopsis
                                                                                 increases from 55 ºF to        can be a problem under
                                                                                 75ºF, days from visible        either cool or warm tem-
                                                                                 bud to flower opening of       peratures, but
                                                                                 Easter lily decreases from     Thielaviopsis activity is
                                                                                 42 to 27 days (Fig. 3). A      reported to be optimal at
                                                                                 general ‘rule of thumb’ is     62 ºF (Nameth, 2001).
                                                                                 that there is little benefit   Another widespread dis-
                                                                                 in increasing average daily    ease promoted by cool
                                                                                 temperature above 72 ºF        temperatures is gray mold,
                                                                                 with any crop.                 caused by Botrytis.
                                                                                                                Botrytis is a problem for
                                    Average Daily Temperature (°F)
                                                                                 Disease pressure               pre-finished plants and
                                                                                    One very important          favored by cool tempera-
                                                                                 consideration when pro-        tures. Previous research
                                                  decreasing temperature         ducing plants under cool       has identified that 71ºF is
                                                  from 68 ºF to 61 ºF            temperatures is the poten-     optimal for Botrytis
                                                  increased days to flower       tial for increased disease     growth and 59 ºF is opti-
                                                  for Impatiens ‘Super Elfin     pressure. This is the result   mal for Botrytis spore pro-
                                                  Lipstick’ from 54 days to      of three things: 1) some       duction (Nameth, 2001).
                                                  72 days, an increase of 18     diseases are more active at       When plants are grown
                                                  days. In contrast, decreas-    cool temperatures, 2)          cool, they typically require
                                                  ing temperature from 68 ºF     plants, benches and floors     less water than when they
                                                  to 61 ºF increased days to     stay wet longer after each     are grown warmer.
                                                  flower for Petunia ‘Purple     watering when tempera-         Therefore, each time you
                                                  Wave’ from 57 to 88 days,      tures are lower, providing a   water, the media will stay
                                                  an increase of 33 days!        larger window for              wet longer, and any stand-
                                                  Most of you are probably       pathogen spores to germi-      ing water will take longer
                                                  considering lowering your      nate, and 3) plant health of   to evaporate. Combined,
                                                  greenhouse temperature         some species is weakened.      these two factors can
                                                  less than the 7 ºF of this        Diseases caused by          increase the root rot dis-
                                                  experiment. Still, each 1 ºF   Pythium, Rhizoctonia and       ease pressure plants are
                                                  decrease in temperature        Thielaviopsis are promoted     under. Many diseases
                                                  will delay ‘Purple Wave’       under cool temperatures.       require standing water for
                                                  flowering by approximate-      For instance, ‘damping         spores to germinate.
                                                  ly 3 days (Table 1). Would     off’ caused by Pythium is      Keeping greenhouses cool
                                                  it be cheaper to keep your     most severe at 53-68 ºF        will increase the opportu-
                                                  temperature higher and         (Nameth, 2001).                nities for diseases to
                                                  decrease your production       ‘Damping off’ diseases are     become established. It is
                                                  time, or lower the tempera-    not the only diseases pro-     very important to actively
                                                  ture and increase produc-      moted by cool tempera-         scout for diseases and have
                                                  tion time? This, in part, is   tures. Thielaviopsis is the    a plan for applying fungi-

  cides ready before the              expensive to install.          slow the life cycle of
  need arises.                        However, you may want          insects (Table 2). For
     Here are some sugges-            to consider installing         example, decreasing tem-             When plants are
  tions for reducing disease          them as a long-term            perature from 80 ºF to 55            grown cool, they
  severity when growing               strategy to reduce heat-       ºF increases fungus gnat             typically require
  under cool temperatures             ing costs.                     life cycle time (from egg            less water than
  (Nameth, 2001):                      Keep in mind that             to adult) from 12 to 27              when they are
  1) Don’t place seed               chemical activity is             days (Lindquist, 1998).              grown warmer.
    flats/seedlings on the          reduced at lower tempera-        Similarly, decreasing tem-
                                                                                                          Therefore, each
    floor! – floor tempera-         ture. Therefore, it may          perature from 86 ºF to 68
    ture can be 10 to 20 ºF         take longer to see results                                            time you water,
                                                                     ºF increases western
    cooler than the air tem-        of an insecticide/fungicide      flower thrips life cycle             the media will
    perature a few feet off         application. It is impor-        time from 16 to 31 days              stay wet longer.
    the ground; also, having        tant that you don’t increase     (Robb, 1989). Therefore,             Combined, these
    flats on the floor increas-     the amount of chemical           you may need to increase             two factors can
    es the amount of time           applied.                         the amount of time in                increase the root
    that free standing water                                         between pesticide applica-           rot disease
    is present, creating ger-       Insect control                   tions to maintain the same           pressure plants
    mination opportunities             Temperature is the            number of applications               are under.
    for fungal spores.              greatest factor determining      during a single life cycle.
    Raising flats off of the        the development rate of
    floor even a few inches         insects. Cool temperatures                 Continued on next page
    will raise temperature
    and reduce direct contact                         Table 2. Impact of temperature on life cycle duration
    with standing water.                                      of common greenhouse insect pests.
  2) Keep floors and bench-
    es as dry as possible.
  3) Keep air circulating -                                      Impact of temperature on time
    keeping air moving at all       Insect                       from egg to adult                                 Reference
    levels, not just at bench
    level, of the greenhouse
    will reduce the length of       Aphid – Green Peach*         Increases from 6 to 20 days as temperature
                                                                 decreases from 68 to 50 ºF.                       El Din, 1976
    time you have standing
    water after each water-         Aphid – Cotton/Melon*        Increases from 7 to 10 days as temperature        van Steenis and
    ing.                                                         decreases from 77 to 68 ºF                        El Khawass, 1975
  4) Use active bottom heat-        Fungus Gnats                 Increases from 12 to 27 days as temperature
    ing if possible - heating                                    decreases from 80 to 55 ºF                        Lindquist, 1998
    of seed trays to the            Shore flies                  Increases from 10 to 16 days as temperature
    appropriate temperature                                      decreases from 93 to 73 ºF                        Lindquist, 1998
    will reduce disease             Spider Mite – Two Spotted    Variable, but generally increases from 8 to
    severity by promoting                                        28 days as temperature decreases from
    actively growing plants,                                     86 to 59 ºF                                       Lindquist, 1998
    and by raising the tem-         Thrips – Western Flower      Increases from 7 to 13 days as temperature
    perature above the opti-                                     decreases from 98 to 62 ºF                        Robb, 1989
    mum for ‘damping off’           Whitefly – Greenhouse        Approximately 21 to 26 days at 81 ºF and          Lindquist, 1998 &
    disease growth. If you                                       32 to 39 days at 65-75 ºF.                        Sanderson, 1998
    do not already have heat-       Whitefly – Silverleaf        Increases from 16 to 31 days as temperature
    ing pads, etc. they are                                      decreases from 86 to 68 ºF                        Lindquist, 1998
                                    *Aphids do not reproduce by eggs. Reported times are from birth to adult.
PAGE 10 - JANUARY 2002                           UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN                     50+ YEARS OF SERVICE

            Figure 4. Effect of day and night temperature on flower                  Flowering                       6 flowers, and increased
            diameter of Dendranthema x grandiflora ‘Bright                              One positive impact of       flower width from 46 to 77
            Golden Anne’ (12 ºC=54ºF, 16 ºC=61 ºF, 20 ºC=68 ºF, 24                   reducing greenhouse tem-        mm.
            ºC=75 ºF).
                                                                                     perature is that flower size
                                                                                     and flower number often         Branching
                                                                                     increase as greenhouse             Another potential posi-
                                                                                     temperatures decrease.          tive aspect of reducing
                                                                                     For example, decreasing         greenhouse temperature is
                                                                                     temperature from 76 ºF to       increased plant quality due
                                                                                     59 ºF increases flower          to increased branching.
                                                                                     diameter of chrysanthe-         For example, in Fuchsia
                                                                                     mum ‘Bright Golden              ‘Dollar Princess’ single
                                                                                     Anne (Fig. 4). Flower           stem pinched cuttings,
                                                                                     number per inflorescence        decreasing greenhouse
                                                                                     of zonal geranium               temperature from 72 ºF to
                                                                                     ‘Veronica’ increases from       55 ºF increased lateral
                                                                                     approximately 15 to 50          branch number at flower-
                                                                                     flowers as temperature          ing from 6 to 12 branches
                                                                                     increases from 85 to 54 ºF      (Fig. 6).
                                                                                     (Fig. 5). Similarly,
                                                                                     decreasing temperature          Dry weight
                                                                                     from 77 ºF to 59 ºF                Decreasing night tem-
       Figure 5. Impact of temperature on number of flowers                          increased Fuchsia ‘Dollar       perature while maintaining
       per inflorescence of zonal geranium ‘Veronica’ (Erwin                         Princess’ flower number         the same day temperature
       and Heins, 1992).                                                             per inflorescence from 3 to     increases plant dry weight,
  Flower number per inflorescence

                                                                                     Figure 6. Impact of temperature on number of later
                                                                                     branches at flowering for Fuchsia ‘Dollar Princess’
                                                                                     (Erwin et al, 1991).
                                                                            Lateral branch number

                                    Average Daily Temperature (°F)

                                                                                                    Average Daily Temperature (°F)

    Table 3. Effect of day and night temperature on stem, leaf, flower and total shoot
        dry weight of ‘Mimas’ New Guinea impatiens (Erwin, 1995). Data on the
             percent of total shoot dry weight are presented in parentheses.                     The difference
                     Plants were grown under a 9-hour photoperiod.
              Data were collected after 51 days in temperature treatments.                       between day
                                                                                                 and night
                                                DAY TEMPERATURE (ºF)
  Night Temp.                                                                                    is known to
  (ºF)                           59             68              77              86               impact stem
                                                Total shoot dry weight (g)                       elongation
  59                             2.8            6.3            7.1              6.7
  68                             4.6            6.3            6.7              6.8
  77                             4.7            5.9            6.3              6.8
  86                             2.3            4.8            4.7              4.2

                                                Stem dry weight (g)
  59                             0.06 (2%)      1.02 (16%)    1.28 (18%)        1.43 (21%)
  68                             0.07 (1%)      0.86 (14%)    1.14 (17%)        1.42 (21%)
  77                             0.07 (1%)      0.83 (14%)    1.08 (17%)        1.30 (19%)
  86                             0.04 (2%)      0.72 (15%)    0.82 (17%)        0.83 (20%)

                                                Leaf dry weight (g)
  59                             2.69 (95%)     3.87 (62%)    4.46 (63%)        4.41 (66%)
  68                             2.88 (63%)     3.28 (52%)    4.29 (64%)        4.49 (66%)
  77                             3.30 (71%)     3.71 (63%)    4.16 (66%)        4.95 (72%)
  86                             2.26 (97%)     3.67 (77%)    3.71 (79%)        3.37 (79%)

                                                Flower dry weight (g)
  59                             0.08 (3%)      1.38 (22%)    1.34 (19%)        0.87 (13%)
  68                             1.60 (35%)     2.12 (34%)    1.29 (19%)        0.87 (15%)
  77                             1.30 (28%)     1.33 (23%)    1.05 (17%)        0.59 (9%)
  86                             0.03 (1%)      0.38 (8%)       0.18 (4%)       0.04 (1%)

  an attribute of plant quali-    ing on New Guinea impa-       ly, it is estimated that 80%
  ty. For example, for New        tiens.                        of the cost of heating a
  Guinea impatiens ‘Mimas’           Plants often have an       greenhouse is incurred at
  grown at a day tempera-         optimal temperature for       night. Therefore, if you
  ture of 77 ºF, decreasing       flowering. Flower dry         are planning on reducing
  night temperature from 77       weight of New Guinea          temperature, it is likely
  ºF to 59 ºF increased total     impatiens ‘Mimas’ was         that you will focus on
  stem dry weight from 6.3        greatest when plants were     reducing night tempera-
  g to 7.1 g, a 13% increase      grown at constant 68 ºF. If   ture. The difference
  (Table 3). However,             either day or night temper-   between day and night
  changing temperatures           ature deviated from 68 ºF,    temperatures is known to
  may affect how a plant          floral dry weight decreased   impact stem elongation.
  uses that dry weight. For       (Table 3).                    The greater the difference
  instance, dropping temper-                                    (‘DIF’) between day and
  ature from 77 ºF to 59 ºF       Plant height
  almost eliminated flower-         As mentioned previous-              Continued on next page

     Figure 7. Internode length of Easter lily ‘Nellie White’              before dawn and maintain-      temperature will impact
     increases as the difference between day temperature                   ing a lower temperature for    overall fuel usage during
     (DT) and night temperature (NT) increases (Erwin and                  the first few hours of the     the entire production time
     Warner, 1999).
                                                                           day will reduce the amount     of a crop. From the equa-
                                                                           of plant growth regulators     tion in Fig. 2 we can pre-
                                                                           needed on a crop. This         dict that ‘Purple ‘Wave’
                                                                           practice is called a morn-     will flower in 64 days at
                                                                           ing temperature ‘DIP’ or       68 ºF. For this example,
                                                                           ‘DROP’. For more infor-        let’s assume that you want
                                                                           mation on ‘DIF’ and            to reduce your temperature
  Internode Length (cm)

                                                                           ‘DIP’, see the May 2001        from constant 68 ºF to a
                                                                           issue of the OFA Bulletin.     regime of 68 ºF day tem-
                                                                                                          perature and 58 ºF night
                                                                           Will you really                temperature (because, as
                                                                           save any money?                mentioned previously, 80%
                                                                                                          of heating costs are
                                                                              Keep in mind that,
                                                                                                          incurred at night).
                                                                           depending on how much
                                                                                                          Therefore, your average
                                                                           you reduce greenhouse
                                                                                                          daily temperature is 63 ºF.
                                                                           temperature, you may be
                                                                                                          Again from the equation in
                                                                           decreasing the total num-
                                                                                                          Fig. 2 we can predict that
                                                                           ber of crops you can turn
                                                                                                          ‘Purple Wave’ petunias
                          Difference between DT and NT ª°F)                through your greenhouse.
                                                                                                          will flower in 80 days at
                                                                           If you are strictly a spring
                                                                                                          63 ºF.
                                                                           bedding plant grower, or
                                                                                                             Using information from
                                                                           you are dependent on pro-
                                                                                                          the Natural Resource,
                                                                           ducing a certain number of
                                             night temperatures, the                                      Agriculture, and
                                                                           crops (or turns) each sea-
                                             more stem elongation or                                      Engineering Service
                                                                           son, decreasing green-
                                             ‘stretch’ you will see on                                    (NRAES) bulletin
                                                                           house temperature may not
                                             your crop (Fig. 7).                                          Greenhouse Engineering
                                                                           ultimately save you any
                                             Therefore, if you are                                        (Aldrich and Bartok,
                                                                           money. As discussed pre-
                                             reducing your night tem-                                     1994), we can compare the
                                                                           viously, reducing green-
                                             perature only, expect                                        approximate total fuel
                                                                           house temperature will
                                             increases in stem elonga-                                    usage for growing ‘Purple
                                                                           increase crop production
                                             tion. As a result, your                                      Wave’ under a day/night
                                                                           time by slowing the rate of
                                             crops may require more                                       temperature regime of
                                                                           development. Therefore, it
                                             growth regulator applica-                                    68/68 ºF or 68/58 ºF. In
                                                                           is important to compare
                                             tions, or application at                                     Minneapolis, MN we need
                                                                           the total fuel cost of pro-
                                             higher concentrations.                                       to provide 19.2 heating
                                                                           ducing a crop under the
                                             Alternatively, using a                                       degree days (HDD; in this
                                                                           temperature you would tra-
                                             morning temperature dip                                      case defined as a 24-hour
                                                                           ditionally use with the
                                             can help to reduce the                                       period in which the inside
                                                                           lower per day costs, but
                                             necessity for growth regu-                                   temperature is 1 ºF higher
                                                                           increased production time,
                                             lators. Plants are stretch-                                  than the outside tempera-
                                                                           of a cooler temperature
                                             ing the most at the end of                                   ture) during the night to
                                             the night and early in the                                   maintain a 68 ºF night
                                                                              Let’s use ‘Purple Wave’
                                             morning. Reducing tem-                                       temperature and 16.3 HDD
                                                                           petunia as an example to
                                             perature 30-60 minutes                                       to maintain a night tem-
                                                                           illustrate how reducing

  perature of 58 ºF.              Which crops should not         How to Physically
  Therefore, the approximate      be grown cool?                 Save on Heating
  number of HDD provided             Not all bedding plant                                        Reduce your
  at night to grow ‘Purple        crops will adapt well to a     Costs
                                                                 Impact of greenhouse
                                                                                                  energy bill, both
  Wave’ at constant 68 ºF is:     cool temperature regime
                                                                 glazing material on heat         short and long-
                                  after germination and early
    19.2 HDD/day *64 days         seedling growth. The most      loss                             term
       = 1229 total HDD           obvious example is vinca.         Different greenhouse
                                  Celosia, Cleome, Cosmos,       glazing materials lose heat      s Install energy
     In contrast, growing         Gomphrena, Portulaca and       at different rates. Single         curtains
  plants at 68 F/58 ºF would      sunflowers are other crops     layer glass and single layer
                                                                 plastic film lose heat at the    s Reduce air
  require approximately:          that are adversely affected
                                                                 highest rates (approximate-        leakage
                                  by cool greenhouse condi-
    16.3 HDD/day * 80 days        tions. To deal with this       ly 1.1 Btu/hr·ºF·ft2;
                                                                                                  s Provide heat
       = 1304 total HDD           problem, there are two         Aldrich and Bartok, 1994).
                                                                                                    only where it is
                                  main options: 1) don’t         In comparison, double
                                                                                                    needed, i.e.
     Therefore, the total         grow these crops or 2)         layer polycarbonate or
                                                                                                    under benches
  amount of fuel required to      group these crops together     acrylic loses heat at less
  produce a ‘Purple Wave’         and maintain a subsection      than half of the rate (0.5       s Circulate air to
  crop under the cooler tem-      of your greenhouse             Btu/hr·ºF·ft2) of single           reduce/eliminate
  perature regime is approxi-     warmer than the rest of the    layer glass or plastic.            cold spots
  mately 6% higher than           greenhouse or use heat            Installing a thermal
  growing that crop under         mats or skirted benches        energy blanket can greatly       s Install energy
  warmer temperatures and         with under-bench heaters.      reduce fuel usage for two          efficient heating
  crop time is increased by                                      reasons. First, adding the         system
  16 days. Keep in mind           Crops that grow well           energy blanket reduces
                                                                 that rate of heat loss from      s    Calibrate
  that this is an extreme         under cool conditions                                               temperature
  example. The delay in              Several bedding plant       the greenhouse. For exam-
                                                                 ple, adding a thermal ener-          sensors
  flowering for growing           crops will perform very
  plants under an average         well under cool tempera-       gy blanket to a greenhouse       s Insulate
  daily temperature of 68 ºF      tures, including snapdrag-     covered with single layer          greenhouse
  compared to 61 ºF for           ons, alyssum, Dianthus         glass reduces the heat loss        perimeter
  other crops presented in        and pansy. For these           rate from 1.1 Btu/hr·ºF·ft2
  Table 1 is as little as 9       crops, crop quality will not   to 0.5 Btu/hr·ºF·ft2             s Use a double
  days for some pansy culti-      be compromised under           (Aldrich and Bartok,               layer glazing
  vars, and 22 to 27 days for     cool conditions but crop       1994), the same rate as for        material
  other Petunia cultivars         production time will still     double layer acrylic.
                                                                 Second, energy blankets          s Install
  examined. However, it is        be longer than under
                                                                 reduce fuel costs by reduc-        windbreaks
  important to consider that      warmer conditions.
  increasing crop production                                     ing the volume of air that
                                                                                                  s Use the cheapest
  time under a cool tempera-                                     you are heating. Why
                                                                                                    fuel available
  ture regime may eliminate                                      spend money to heat the
  any fuel savings you gain                                      roof of your greenhouse          s Explore
  by reducing the green-                                         when your plants are 10 to         alternative fuel
  house temperature in the                                       20 feet below? The short-          sources
  first place.                                                   term cost of installing

                                                                         Continued on next page

                           energy blankets can quick-      altering your current struc-      ly be very true as growers
                           ly be recovered by reduced      tures to increase energy          search for creative ways to
    The current            fuel costs.                     efficiency, but some of the       reduce energy costs with-
    ‘energy crisis’ is                                     tips listed above can be          out sacrificing plant quali-
                              Here are some tips for       employed in any green-            ty and, ultimately, prof-
    not likely to go
                           reducing your energy bill       house situation. It is often      itability.
    away any time          in both the short- and          said that growing plants is
    soon.                  long-term (adapted from         as much an art as it is a
    Therefore,             Both, 2001):                    science. This will certain-
    optimizing             s Install energy curtains
                           s Reduce air leakage
    greenhouse             s Provide heat only             References                           Minn. Commercial Flow.
    heating                   where it is needed, i.e.     Aldrich, R.A. and J.W. Bartok,       Growers Bull. 44(5):1-11.
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