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INTRODUCTION: What follows began as a response to specific landscape containers proposed on a site plan under review by the Cranford Environmental Commission. More information, in greater detail, has been added to broaden the base of the original material covered. Design elements and aesthetic considerations – worthy topics for another venue – are only alluded to in general terms. The focus here is on building and maintaining a healthy community environment. Relevant updates and revisions will be made when appropriate. While gardeners have greater freedom and flexibility in many choices they can make, they will find useful information here. Discussions on plant selection, compatibility, hardiness, container culture and water conservation apply to residential container gardening as well.  The goal is to produce healthy, attractive, durable and easily maintained containers that create a welcoming atmosphere and draw people to the area.  Thanks to a resurgence in container gardening in recent years, there is a wealth of plant material – beyond familiar trailing vinca vines and centerpiece dracaenas – available for use in municipal containers. Many newer introductions have environmentally sound traits, such as disease and pollution resistance, drought tolerance and weather-hardy foliage. The scope of any “recommended plants” palette will expand, if there are storage facilities available for dormant plants that are not winter-hardy.  Mixed groupings of plants in the same container is like a small garden, except the shared growing space is both restricted and constricted. Root systems are left vulnerable to temperature extremes and fluctuations in oxygen, nutrient and moisture levels, due to their close proximity to container walls and the absence of garden soil’s reserves and protection around them. Understanding this is critical to plant selection and successful container environment management.


-2Using too many plants, underestimating container size and combining plants with incompatible features – root systems, growing habits, growth rates, life expectancies and/or cultural requirements – will negatively impact plant health, appearance, water and nutrient consumption and maintenance needs. This can be avoided in the design phase with thorough research and careful planning.


Most trees, shrubs and perennials do not like to be in containers for long periods of time. Perennials usually need division after two to three years. Shrubs last three to five years, after which time they are better off planted in the ground. Suitable trees in optimal conditions may last ten years. Plants will likely require root pruning or repotting into larger containers during these time frames.


Mixing permanent and temporary plantings in the same container is not recommended. Repeated disturbance of the soil will cause root injury detrimental to any permanent plant’s health.


Hardiness zones for containerized plants are different from their in-ground hardiness. Plants designated to overwinter on site need to be two zones hardier than the zone in which they are located. For Cranford’s climate (Zone 6), plants need to be hardy in Zone 4 to reliably survive a winter outdoors in a frost-proof container. Exceptions do exist, but they are contingent upon container placement (microclimates; a protected site). Failure to address winter survival needs of plants will seriously jeopardize their life expectancies and prove unnecessarily costly.


Select only container-raised trees and shrubs for container use, not B & B (balled and burlapped).

-3 Assigning trees to planters rather than planting them in the ground can prove costly in maintenance effort and dollar expenditures. Moreover, the environment will be denied benefits that only mature trees can offer.  The recommended minimum size for tree planters is 5 ft. wide by 2 ft. deep. Trees suited for such restricted spaces need to be small or dwarf, container-hardy, low maintenance specimens with disease and pest resistance, site-specific environmental stress tolerance, good structure and multi-season interest.  Plants with garden-worthy attributes do not always make good candidates for municipal settings because public safety and cost effective maintenance of streets and sidewalks have to be daily concerns. These are important considerations to keep in mind when evaluating plants with features such as: toxic or irritating parts; sharp thorns; low hanging or drooping branches that can interfere with pedestrian or vehicular traffic; messy berries that attract congregations of birds; seed pods that create a lot of litter; or flowers that are magnets for bees, wasps and yellow jackets.  Groupings of annuals and tender perennials (plants, native to warmer climates, that need indoor winter protections) – by themselves or in combinations – can also face compatibility issues that require resolution in the design phase. For annuals, roots are usually not a problem unless too many plants are being squeezed together to fill the contained space. Roots are then forced to form intertwined mats which rapidly use up available water and nutrients. Repeated stress like this will lead to impaired plant performance, susceptibility to disease and insect problems, and increased maintenance time and costs. Since their roots are much shallower, annuals are not as forgiving as woody plants and perennials. They deteriorate more rapidly and often do not recover.

-4 Lack of shade, shelter and wind protection will increase water consumption and subject plants to a higher incidence of physical injury. Porous containers will dry out faster than non-porous ones. Containers with southern, western and open exposures are most vulnerable to rapid moisture loss. Using containers in full sun and wind-exposed sites that are larger than the size used in shaded or protected areas is one way to handle these environmental stresses; the increased soil volume will provide additional moisture and support. Drought tolerant plants whose structures and foliage will hold up under adverse conditions are the best choices for these locations.  Potassium-based hydrogels are highly recommended as an environmentally safe watermanagement tool. They do not contaminate soil or ground water and help in the control of water run-off. They are capable of holding several hundred times their dry weight in water. Since they release their stores to plant roots as needed, they would be invaluable during drought or hot weather. They encourage root penetration, reduce soil compaction and minimize transplant shock. They help prevent leaching of nutrients – a big problem in container gardening – and provide plants with soluble potassium for their development. Following their application, hydrogel granules are effective for at least two years, but they do need to be incorporated sparingly into the growing medium. More is not better. Hydrogels can save 25%-50% in water use. Salt-based hydrogels are not recommended.  Watering and good drainage are the most critical factors in container culture. A plant’s growing medium must have sufficiently large pore space to retain moisture, oxygen and nutrients, yet simultaneously permit the free movement of water. Additional drainage holes may have to be drilled in container bases to help water exit more quickly. The larger the container size, the more drainage holes are needed. Whenever drainage gravel

-5is used as a bottom layer inside deep containers, landscape fabric will need to be placed in between the gravel layer and the growing medium to prevent clogging and soil loss.  Container plantings require a supportive, quick-draining, water-retentive, nutrient-rich growing medium that has good aeration for root development. Building site soil is not suitable for container use; there is a strong risk of contamination by weed seeds, soilborne pests, disease pathogens and pollutants that will negatively impact the environment, plant health and ground water. Even sterilized, this soil is simply too heavy and lacks the needed pore space to drain quickly. It will become easily compacted and discourage good root development. These are conditions that make plants prone to fungal disease and rotting.  The length of time a plant spends inside a given container is a very important variable in determining the composition of its growing medium. Research indicates that sphagnum peat moss starts to decompose after nine months of use; while this happens, its pore spaces begin to close up. Any plant remaining in a container for longer than nine months without repotting will require a growing medium that has a lot of coarse particle content, such as fine bark, and smaller amounts of sphagnum peat moss content. Professional bark-based blends are available. (Check current wholesale catalogs.)  The specific nutrients needed by containerized woody plants, perennials and tender specimen plants will have to be addressed by custom-tailoring the components of their growing media. The yearly addition of weed-free organic material offers many benefits and should not be overlooked. Also recommended for use is an environmentally safe product which combines mycorrhizal fungi with hydrogels, soil conditioners and biostimulants. This product is available in premixed packets. It is applied to the growing

-6medium prior to planting and will improve both soil fertility and fine absorbing root development.  In contrast to woody plants and perennials, annuals and tender plants depend on rapid growth within a very short period of time. Their needs, which are immediate under any circumstance, are intensified within the confines of a container. For optimal performance, seasonal plants need an uninterrupted supply of nutrients. Despite all their pluses, soilless blends are nutrient-poor. Incorporating weed-free organic matter (compost; dried manure) and soil conditioners, as well as sterilized top soil or potting soil ( 1part soil: 2 parts soilless mix) into these blends will improve their fertility and permit a slow release of nutrients. The addition of hydrogels will reduce leaching of available nutrients.  A two to three inch layer of organic mulch is especially important for trees, plant groupings drought-intolerant plants and specimen containers. It conserves water, cools soil temperatures, suppresses weeds, prevents soil crusting and compaction and supplies nutrients as it breaks down. For municipal settings, compost, shredded bark and shredded oak leaves are probably the best options; they will require renewal to maintain their depth. Pea gravel, which provides everything but nutrients, is another option, but using coarse wood chips or gravel is not recommended. Compost or a trailing plant that completely hides the soil can be used as mulch in densely planted annual containers. Just like in the garden, good air circulation is essential around every plant base, tree trunk and shrub stem to prevent rotting and problems with insects. Mulch is best kept 3-6 inches away from these vulnerable areas.  Flowers are ephemeral. More often than not, they require constant grooming and extra nutrients to extend their productivity, keep their appearance attractive and in proportion,

-7and, in some cases , prevent their reseeding. Their display can be marred or interrupted by adverse weather conditions such as drought, heat and excessive rain. Low maintenance containers, especially in a municipal setting, demand lasting characteristics or an everchanging series of seasonal attributes to keep them interesting. Plants in close proximity to one another must work well together visually as well as culturally. This is an arena in which outstanding foliage, stem and bark traits, plant structure and textural contrasts need to step to the forefront; flowers become integral to a grouping of plants as accents. A suggested ratio for effective containers is 2/3 foliage plants to 1/3 flowering plants.  Minimal use of fertilizer is recommended. Too much fertilizer – regardless of derivation or method of application – generates a lot of weak, succulent growth that cannot tolerate drought, attracts insects such as aphids and scale, and increases a plant’s need for water. Too much fertilizer prevents water uptake, causing a plant to wilt even if sufficient water is present in the growing medium. Excessive or misapplied synthetic fertilizer kills beneficial microorganisms in organic matter which the plant uses to defend against disease, burns plant roots and foliage from salt build-up, and contaminates ground water. Plant selection, container size, organic components in growing media and hydrogels are all part of a strategy to help ensure a constant supply of nutrients for plants in containers. Further recommendations are currently being researched.

C.E.C. March 2003

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