Guidelines for Growing Calla Lily Aethiopica

Calla Aethiopica has a long history in the United States as the classical tall, white chalice-shaped cut flower. Its most traditional use is for both weddings and funerals. It has recently gained additional popularity as a potted plant, due in part to the registration and use of the growth retardant Bonzi, and fuller, more compact cultivars.

Aethiopica is more temperamental in regards to exact potted flower scheduling compared to colored callas. However, as a potted plant, Aethiopica is a robust grower that withstands variations in light and temperature and frequently can be grown outdoors, or under partial (or full) fover in temperate climates. Cooler temperatures below 70°F (21°C) maximum, preferably in the 60's or 50's on average, will help reduce disease incidence and severity. At these optimal regimes, you will see increased flowers due to superior growth and lack of stress.

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Calla Aethiopica rhizomes are available in the northern hemisphere from the month of August to November. Upon arrival, unpack rhizomes and put in well ventilated trays until potting up. Care must be taken in holding rhizomes too long or desiccation and weakening may occur.

For a potted crop or covered fall planting, allow approximately 11 to 14 weeks for initial bloom flushing. Some first flowers may come as early as 8 to 10 weeks, especially for later plantings of more sprouted rhizomes. Although sprouted and larger bulbs are sometimes earlier, bulb size alone does not consistently affect time to flower.

Unlike spring colored callas, Aethiopica do not respond with an initial gibberellic acid (GA) induced flower flush and application would have little use on a potted crop. However, there is some evidence that a delayed GA response occurs six to ten months post application and may benefit late spring cut-flower production. Rates from 80-250 ppm may be trialed, with higher rates possibly increasing flower deformity and height. GA affects cultivars Green Goddess, Pink Mist and Childsiana more strongly compared with the standard White Aethiopica. in these varieties and for cut flower plantings, use GA as a foliar spray at above rates.

Aethiopica come as elongated and naked (without multiple side bulblets attached) rhizomes called 'horns,' or as broader, more compressed rhizomes with some attached bulblets called 'clusters,' or somewhere in-between.

Horns tend to grow with a more dominant apical shoot, have fewer side shoots (which produce lower canopy leaves), and have generally larger, taller and heavier stemmed flowers.

Clusters tend to have more or multiple breaks or budding around the main apical shoot, fuller, more compact plant habit, and lower bloom heights in relation to the leaf canopy.

Although interchangeable to a certain degree, clusters are favored somewhat for potted production and horns for cut flowers depending on your flower and/or plant habit objectives. Flower counts on average are similar for equal sizes.

Rhizomes can be planted in either raised beds or containers and perfer a deep, well-draining soil. Plant bulbs 2" deep in containers and 3-4" deep in open bed. Pot or plant in a well draining media with a pH of 5.5-6.5 and high in organic matter. Successful media generally have 50-65% peat with other components being coarse for drainage. Lime, gypsum and dolomite as calcium sources in the media are recommended for plant health. Incorporation of Terraclor (PCNB) and/or Trichoderma into media helps reduce disease.

Water freshly planted rhizomes, then fungicide drench immediately. Maintain slightly moist until substantial sprouts emerge. Keep constantly moist, but not soggy, until peak bloom, then decrease moderately.

Becase of the robust nature of the Aethopica potted plant, larger pots or tubs of 10-12" or medium pots of 6-8" are generally used, although some experimentation has been done with smaller, more growth-regulated pots. Greater flowering and growth comes with greater root space. Cluster type bulbs or multiple (2-3), smaller diameter horns 1 1/4" to 1 3/4" will help fill out potted plantings and maximize bloom per pot. Larger horns simply will not fit in the medium or smaller sized pots.

Fertility maintenance is critical to continued flower production and full, well-colored plant growth in Aethiopica. We recommend that a pre-plant charge of nitrogen and potassium, with little or no phosporous, be incorporated into the media. This will produce good, early establishment of roots and improved, early vigor. Use only a 30 day release formula then begin a balanced liquid feed at 200 ppm weekly or 100 ppm constant or as needed. Modifications can then be based on individual growth objectives and environments.

Night temperatures of 45-55°F and day temperatures of 55-65°F are optimum for plant growth and flowering and must be followed or greater disease problems can occur, especially in warmer conditions. Therefore, best outdoor growth generally occurs in the spring and fall. Prolonged warmer greenhouse temperatures, especially at night, can inhibit flower production and can soften and stretch existing blooms. Moderate early warming to 65-70°F, only until sprouting however, can help hasten rooting and establishment. Avoiding high temperature spikes will help minimize root problems.

Calla Aethiopica withstands a wide range of light conditions from 500 to 5,00 foot-candles. It is not day-length sensitive nor does it require temperature treatments to induce bloom. 50% shade is optimal, although the plants can handle full sun to 70% shade without becoming too badly stressed. Stem length is increased, but weakened, under shaded conditions.

Callas are somewhat frost sensitive and foliage will not withstand prolonged freezing temperatures. Rhizomes will take some frost but cannot withstand a soil freeze to bulb depth. High daytime temperatures, such as in hoop houses, are conducive to the proliferation of fungal and bacterial diseases. Cool nights and days will promote stronger and shorter stems.

Aethiopica, particularly if potted or in beds, benefit greatly from an aggressive preventative fungicide program. Incorporation of PCNB (Terrachlor) into media at 3-5.5 oz/yd3, and Trichoderma (Root Shield) at 1 lb/yd3 will help maintaing healthy roots. The two products are compatible. Either product used alone will also yield benefits. Immeidately after initial watering of media, follow with a thorough fungicide drench of Subdue (1-2 oz/100gal) for water molds, combined with either the newly available Medallion (1-2oz/gal), Compass or Heritage (0.4 oz/gal).

If these new (and improved) fungicides are not available in your area, use Cleary's 3336 or FungoFlo at recommended rates for Rhizoctonia, along with the Subdue. PCNB is most effective as an incorporate, but can be drenched with above products if not added to the media. Including Agrimycin in the drench at 100-200 ppm (0.5-1 lb/100 gal) will also help suppress Erwinia soft rot, a bacterial disease which often becomes established in the damaged root tissues caused by the more aggressive fungal pathogens. Alteration or addition of Aliette at 13 oz/00 gal) can help manage watermolds.

Always examine roots weekly, or more often for symptoms of clearing or browning or other disorders. A second drench is recommended at 14-30 days including PCNB and/or Trichoderma to maintain long-term pot health. Avoid prolonged and pverly soggy or overly dried soil conditions. Separate any diseased or rotted pots. Avoid temperature and humidity extremes in greenhouses below 40°F or above 70°F for best health.

A preventative foliar disease program is best. Use copper Kocide or Champion sprays at 1 lb and at 1 qt per 50 gal respectively. These can be rotated with Daconil (do not mix Daconil with copper). This foliar treatment is especially recommended for pathogen-conducive environments such as outdoor plantings and in cut flower blocks. We also encourage, on a trial basis, the use of new chemicals or mix combinations, and good record keeping for future reference. Aethiopica tend to be tolerant of most pesticides.

To control aphids, thrips, shoreflies and fungus gnats (shoreflies and fungus gnats can spread bacteria), Orthene, Diazinon, Malathion, Tempo, Dursban and Maverik have shown effective and non-phytotoxic. Gnatrol is also very effective on fungus gnat larvae. Check registrations in your area.

In Aethiopica, Bonzi (Paclobutrazol) growth regulator use can improve the potted plant growth habit dramatically. Trials on rates, timing, multiple applications, bulb size and configeration, soil media, and environmental differences are absolutely essential. Aethiopica is sensitive to Bonzi and too high a rate or too early an application can cause leaf crinkling, plant rosetting, lack of bloom or disproportionate (disparity in) flower-foliage height.

A rule of thumb for a single dose is to make and appliaction when plants are at 1/2 to 3/4 desired plant height. Again, the rate will depend primarily on the light and temperature effects on stretching and your bulb size and type. The best range to start trials is 1/4 - 1/3 oz Bonzi per gal (7-10 ppm) and going down to a 1/8 - 1/4 oz for lighter effects, earlier or multiple applications or use of smaller or clustered bulbs. Rates higher than 10 ppm may arrest plant height completely and may be evaluated under more extreme stretching conditions or if final desired has been attained.

Use of Bonzi is truly an art as well as a science and these recommendations have been tested on the central coast of California. Drenching is best done one to two days after watering to insure uniform soil moisture. It should be noted certain organic media components such as composted or uncomposted pine bark, widely used in the eastern USA, have shown up to 10 times reduced efficacy of Bonzi. Temperature, nitrogen fertility, light and water management, morning cold pulsing, or far red light (twilight) exclusion manipulations can separately or together affect growth habit and thus Bonzi applications.

Bonzi in Aethiopica, as in spring colored callas, reduces foliage height, darkens leaves and toughens or tones plants, which can improve shipping and post-harvest life. However, it can also reduce foliage height, and perhaps reduce later flower numbers at the higher rates. Use care and experiment, always using untreated controls.

Cut flowers at earliest when yellow spadix (the little finger-like thing in the middle of the bloom) is just visible at 10-15% open spike stage. Tighter spikes will not open naturally. Aethiopica can be cut at late stages to fully open, but may not ship as well due to packing and bruising. Flowers should always be cut before pollen shed.

Flowers can be picked either by pulling stems or by cutting stems. There are advantages and disadvantages to both techniques. Pulling stems increases stem length and is a much quicker technique. However, some research shows that younger flower primordia may be damaged this way and stem "stretching" or bruising can take place if picking is not done correctly. We recommend pulling stems, but the soil must have proper moisture or stem "stretching" will occur.

Cutting stems reduces chance of flower primordia damage as well as stem stretching, but the resulting stem length is slightly shorter. Some diseases caused by virus or bacteria may be transmitted, so knife disinfection should be practiced.

It is up to each individual grower to decide which method is best for their operation.

Flowers should last up to three weeks or longer when cared for properly.

Flowers should be placed in clean water immediatly after picking in field then re-cut and bunched according to stem length and quality. They should be kept in a commercial flower keeping solution such as Floralife or in a solution of suger and bactericide (Clorox). Too high a concentration of suger in the solution will encourage premature flower opening and increase microbial growth in the solution. Stem splitting, in our experience, is worse in pure or less hard water.

Flowers should be stored at 36-40°F and no lower than 34°F. Stems should be re-cut and solutions changed every two days, if not daily. Bucket and work surface sanitation is very important to vase life and flower quality. When bunching flowers, take care not to bunch stems too tightly thereby restricting solution uptake.

* Growing information provided courtesy of Golden State Bulb Growers.