Successful pot culture of callas begins with purchasing quality “grower” product. “Grower” callas should be young and vigorous with several growing points or “eyes” showing on the tuber. It is also important to purchase preconditioned tubers as these will have been treated with fungicides and bactericides as well as bloom-enhancing gibberelic acid.
Because today’s colored calla hybrids were originally produced from crosses with species native to high light and free-draining mineral soils of South Africa, choose a media that drains well and has good air porosity. Use only coarse peat, and avoid fine particles. Strive for a pH of 6-6.5. Use coir with caution only and be sure to drench as pathogen pressure increases.
Provide the highest possible light, with the exception of perhaps 30 percent shade required in “high summer” outdoor forcing conditions. High light yields more compact plants, more flowers and reduces the need for Bonzi (paclobutrazol).
Keep pots moist, but avoid excessively wet and/or dry conditions. After initial watering, the first thorough watering should be the four-part chemical drench. Then, water sparingly until leaves unfurl. Avoid pooling and splashing as well as ebb-and-flood systems as these can spread pathogens.
Stage 1. From planting until the sprouts are 1?2-3 inches, keep temperature at 75° F days and 65° F nights, or a constant 68° F. Uniform early heat improves determinance. The first Bonzi drench may also be applied at this stage.
Stage 2. Begins one week after sprouting and initial Bonzi application at about day 21-28 and continues to approximately day 50. Maintain 70-75° F days and 55-60° F nights. Nights can be run cooler by 2-5° F with shorter, tougher plants but longer bench times resulting.
Stage 3. This stage is marked by flower buds beginning to push and first color showing — generally between day 50 and 75. Maintain 65° F days and 50-55° F nights. Cooler temperatures will produce stouter and more brightly colored blooms as will a morning cold pulse between 5 and 8 a.m.
Feed early and regularly with a balanced 20-10-20 plus minors at 100- to 150-ppm nitrogen. Maintain EC levels of 1.5-2.0, and avoid levels above 2.5. Fertigate with every watering, but leech pot salts with clear water every third to fifth irrigation beginning with leaf unfurling in Stage 2 and especially during the last six weeks. Two or three total applications of iron chelate in Stage 2 forward will darken foliage.
Growth regulator requirements vary by variety and cultural conditions. Varieties are grouped into x-low, low, medium and high use. Paclobutrazol has been proven to be the most reliable PGR for callas. And, the x-low, low, medium and high-use varieties generally use the same concentration of material with only the number/frequency of applications varying.
For an 8- to 10-ppm concentration of 4-percent paclobutrazol, use 1?4 to 1?3 ounces per gallon. For a 10- to 15-ppm solution, use 1?3 to 1?2 ounces per gallon. Begin the first PGR application at the end of Stage 1 and when the sprouts are between 1?2 and 3 inches tall. Be sure pots are uniformly moist prior to application. Grade for sprout length to ensure application at this short sprout stage. Come back and apply to the segregated slower-to-sprout pots when they have 1?2- to 3-inch sprouts. Repeat applications to previously treated pots are to be made at six- to 10-day intervals. Do not treat after 40 days post emergence. This generally means no more than three applications even on the high-use varieties. Too much paclobutrazol will reduce flower counts and add bench time.
Precise concentrations and a precise amount of fluid per pot are essential. For this reason, do not use fertilizer injectors to apply PGRs as they are too prone to deviation. Our recommendation is to prepare a tank-mixed final solution at the desired concentration and to apply it using a dosing/metering system.
Control fungus gnats and shore flies as these can spread bacteria and other pathogens. Control aphids, thrips and white flies on an as needed basis. Aphids and thrips can vector viruses. Callas are relatively tolerant to phytotoxicity of most foliar sprays.
For additional cultural information, see Technical Information for Callafornia Callas at www.goldenstatebulb.com .