April 2003

Calla Lilies–the personification of cool

I’ve just returned from the California Pack Trials, where I found the trials to be fairly conservative this year, with breeding companies mostly focusing on refining the major crops. There were a lot fewer “new” releases this year, and considering the economy, it makes sense to play to your strengths until things begin to settle out. However, while on the west coast, I took my first tour of Golden State Bulb Growers and was amazed at the advances in calla lily breeding and production that Tom Lukens and his staff have been working on. I have always loved this plant for its simple elegant flowers, but living in the Deep South, I obviously have not seen it produced under optimal conditions. The tour was really phenomenal, not only to see extremely high-quality calla lilies, but also to realize that I (and possibly some of the industry) may have been operating under some misconceptions about these long-lasting flowering potted crops, and also missing the boat on many landscape and cut flower uses.

Golden State had a comprehensive trial, showing more than 40 different cultivars of tuberous callas with various plant growth regulator regimes, planting dates and production methods. It was very impressive. While availability of newer colors is still limited, there were many colors there I hadn’t seen before. The focus of Golden State’s trials was production-related issues, solving problems for forcers of this crop, and the handouts were detailed and very helpful. I recommend you contact Golden State if you want to learn more about this crop, as it isn’t possible for me to relay all of their information in this column.

Basics and Misconceptions

The colored flower types (Zantedeschia hybrida) are complex hybrids of South African species and grow from deciduous tubers. These plants come from higher mountain regions where temperatures are cool but the ground never freezes. The native light levels are high, and the soil drains quickly. The second type of calla commonly found on the market is the larger white variety Zantedeschia aethiopica, which grows from rhizomes (horns) and is from the more tropical valley conditions of the western Cape in South Africa, so this species prefers a very moist to semi-aquatic location once established. Z. aethiopicais marketed as a bog plant or for use in water gardens as well as garden plantings.

The most common misconception is that the multi-colored calla hybrids used in flowering potted production and the much larger Zantedeschia aethiopica should be grown under the same conditions and can be treated in the same manner. These two crops have different seasons of flower and very different watering requirements. We’ll talk about that more in a minute, but with regards to watering: Z. aethiopica can tolerate semi-aquatic conditions once established, while the colored forms are susceptible to soil-borne bacteria if the media is kept too wet. Also, once growing actively Z. aethiopica will continue to throw flowers as long as conditions are optimal, while the colored hybrids must undergo a dormancy between flowerings.

Many growers also get confused about the optimum environment for callas. They assume that since these are tropical flowers they prefer high temperatures and high humidity. The truth is that, in general, callas prefer cooler temperatures than tropical foliage plants. Warm temperatures and low light cause stems to elongate beyond their ability to support themselves, and the flowers do not last as long under warmer temperature regimes. Callas need bright light and cool night temperatures to avoid stretching and to get the highest-quality flowers.

One of the most recent advancements in calla production is that a bigger tuber is not necessarily better than a smaller one. Calla hybrids form flowers in relation to the number of eyes present in the tuber (or clump of tubers). A large tuber with one eye, will produce fewer flowers than a smaller, multi-eyed tuber. Also, it is easier to control a smaller tuber in production than a large one.

On Tubers

In the colored flower forms, tubers are marketed from seed and clonally propagated material. There is some speculation on what is best for potted flowering plant production, speculation aside, in general, you want as many eyes as possible on each potted tuber to give you the highest number of flowers per pot.

Tubers need to be pretreated with gibberellic acid (GA) at roughly 125 ppm to overcome dormancy requirements; however, GA alone may cause deformed flowers in some cases, so growers should pre-treat with promalin (a combination of GA and benzyl adenine), which will break dormancy, as well as increase branching, without causing flower deformities. Pretreatment of tubers is essential to uniform sprouting and flowering and can either be done in-house or with pretreated tubers. Fungicides can also be applied prior to planting and will help avoid disease issues from mechanical damage during shipping and handling.

Golden State is currently promoting a concept for finish growers that they call High Input Potted Product (HIPP). This method of production avoids the field aspects of tuber production entirely and generates tubers from greenhouse-grown material. The result, according to Tom, is that tubers are much cleaner and the risk of disease is lower. But the big advantage is that the overall vigor of the tubers is much higher, allowing forcers to use smaller tuber sizes (and also smaller pot sizes) with the same effect as larger, field-produced tubers.

I was very impressed with the 4-inch material coming out this program. Growing a heavily flowered 4-inch hybrid calla can be problematic because to get a lot of flowers you usually need a tuber that is too large for a 4-inch pot. The HIPP program seems to get around this issue. There were up to seven flowers on the 4-inch HIPP material on trial at Golden State. HIPP tubers will have a higher price point, but since you can use a smaller tuber, the price may even out and work for your production.

My Favorites

I think if I were to select something unique or appealing from this collection of beautiful cultivars, it would be the gold- and rose-toned cultivars, as I’m not a big fan of the smaller calla hybrids. The yellow and Flame types were robust, with spotted foliage and a larger stature with really striking spathe colors. By the way, a calla bloom is separated into two parts: the spathe, which is the colorful modified leaf, and the spadix, which is the central column containing the real flowers. I was most impressed with ‘Millennium Gold’, ‘Flame’ and ‘Hot Flashes’, but this is splitting hairs because all the offerings were beautiful.

I think many growers can use this crop to expand their niche and should definitely look at avoiding the gallon perennial trade with this crop. It will take a bit more work and growing skill to really do a great job with callas, so don’t take all your effort and feed it into the lowest price point size. Instead, I encourage growers to experiment with a quality 4-inch crop, as well as larger pot sizes (10- to 12-inch or color bowls), which can command a specialty price structure.