GROWING CANNAS IN CONTAINERS By Carl E. Whitcomb PhD A container is a unique man-made environment where no plant is native. Containers have three major complications compared to growing in field soil: a very limited volume, a bottom which restricts downward movement of water and, because of the bottom normal productive field soil does not work. The mix or container growth medium must be comprised of coarse, non-soil, components. The most productive container growth media are comprised of conifer bark, peat and coarse sand. Three parts by volume of ground pine bark (ground through a one inch screen) plus one part good peat (a compressed bale of peat will expand to about double the volume when used) and one part coarse concrete sand works well for containers of one quart volume and larger. When components are mixed together the smaller particles fit in among larger particles causing the volume to shrink. For example, if three cubic yards of pine bark, one cubic yard of peat and one cubic yard of sand are mixed together the end result is about four cubic yards or shrinkage of 20%. Any nutrients added to the mix should be added relative to the final volume as that is what the plant will experience. Slow release N-P-K fertilizers such as Osmocote 19-5-9 or 19-6-12 standard-start (12 to 14 month release) at 15 pounds per cubic yard works well across the south in hardiness zones 7, 8 and 9. In zones 6,5 and 4, use 6 pounds of Osmocote 18-6-12 (8 to 9 month release) plus 3 pounds of Osmocote 19-6-12 (3 to 4 month formulation). In addition add 1.5 pounds of Micromax micronutrient fertilizer per cubic yard, plus from zero to 6 pounds of dolomite per cubic yard. If irrigation water contains 60 ppm calcium or more, add no dolomite. If water contains from 30 to 50 ppm calcium add 4 pounds of dolomite per cubic yard. If water contains less than 30 ppm calcium, add 6 pounds of dolomite per cubic yard. You must know the chemistry of your irrigation water in order to grow good plants in containers. A common belief is that pH of the container growth medium must be 6.5 to 7.0. This is fiction. In field soils, pH is important because as pH goes up complexes form and reduce solubility of micronutrients and likewise as pH goes down, micronutrients become more soluble and available to plants. But in a container you are not growing in soil. Components of a soilless container growth medium contain few micronutrients so pH change has little influence. In short, if you add Micromax to the mix the micronutrients are there. If you do not they are absent and lowering pH will not increase availability. A typical pine bark, peat, sand mix with nutrients added as noted above will typically have a pH of 4.5 to 5.5. Do not use hardwood bark or any compost as part of the container growth medium. These materials typically contain large quantities of calcium and excess of other elements. When chlorosis develops due to reduced availability of iron and manganese, the problem is not pH, but excess calcium. Calcium is the dominant and controlling element in any container growth medium. Do not add phosphorus beyond that included in the Osmocote. Adding more phosphorus does not increase flowering. Excess phosphorus forms complexes with calcium and further reduces availability of iron and other micronutrients. Manage irrigation water carefully. During summer, two or three irrigation cycles of moderate quantity per day are typically superior to one heavy watering. High root zone temperature in containers, especially in zone 7 and southward restrict root functions. Root zone temperature can be moderated 7 to 10 degrees F by using support pots which also stop blow over*. RootSkirts® on individual containers or support pots reduce root zone temperature 20 to 25 degrees*. Remember, plants run on energy produced in the leaves. The more effective you are in providing all of the components used by the leaves to produce energy [full sun, all 12 essential nutrients, oxygen and moisture to the root system in a suitable root environment] the greater plant growth and flowering. Cannas grow well in containers when proper mix, nutrition and water are provided. If plants begin to slow in growth, check the root system as the plant has likely utilized all of the volume in the container and needs to be shifted into a larger one. Summary: Preferred materials and nutrition program. Container Growth medium --- 3-1-1 mix of pine bark, peat and sand. Do not use hardwood bark or compost. Nutrition ---- 3 components; Osmocote, Micromax and dolomite. South -- Osmocote 19-5-9 or 19-6-12 6 (12 – 14 month release) @ 15 lbs per cubic yard. North – Osmocote 18-6-12 (8-9 month) @ 6 lbs per cubic yard, PLUS, 3 lbs of Osmocote 19-6-12 (3-4 month release). Micromax micronutrients – 1.5 lbs per cubic yard. Dolomite – Rate depends on the amount of calcium in the irrigation water; None – When water contains 60 ppm calcium or more. 4 lbs per cubic yard – When water contains 30 to 50 ppm calcium. 6 lbs per cubic yard – When water contains less than 30 ppm calcium. * For details read Plant Production in Containers II by Carl Whitcomb. To order go to www.lacebarkinc.com.
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