Cooling Easter lilies

Departments - Production Pointers

Growers need a clear cooling strategy to successfully grow Easter lilies.

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When they arrive from California or Oregon, Easter lily bulbs need to be cooled in order to develop.
Photo © AB Photography | Adobe Stock

Easter lily bulbs are dug from fields in California and Oregon and are prepared for distribution across the country to be received by growers in mid-October. But what then? Easter lilies have to be cooled in order to develop flowers. There are different strategies to cool Easter lily bulbs, each with advantages and disadvantages. Thankfully, there is also an insurance policy for bulbs that aren’t sufficiently cooled. This article is going to review how to cool Easter lily bulbs to produce flowering plants in the greenhouse.

Cooling requirements

In order to form flowers, Easter lilies require a period of cold temperatures, or vernalization. Once the requirements for vernalization have been satisfied, Easter lily flowers can form. However, if the cold period is insufficient and vernalization requirements are not met, then flowers will not develop. And there is no consumer demand for vegetative Easter lilies! Cooling Easter lilies is imperative to produce a flowering plant. So what does it take to satisfy the vernalization requirements of Easter lilies? Six weeks at 40 to 45 F, with 41° F being a common target temperature. Once they have received this treatment, Easter lilies can form flowers by the end of January, while shoots are emerging and elongating. This cold treatment can be provided to Easter lilies in two different methods, each with their own distinct advantages and disadvantages.

Controlled-temperature forcing

Controlled-temperature forcing (CTF), also referred to as “pot cooling”, is providing vernalization to Easter lily bulbs that are already potted and rooted into containers. Uncooled bulbs are received, potted up, then grown at 60 to 62° F for three weeks to establish roots. After this, the containers with rooted-in bulbs are cooled for 6 weeks at 40 to 45° F, with 41° F being a common target temperature.

For root development, plants are usually placed in a greenhouse after they are potted up. One challenge that can arise during November, when Easter lilies are rooting for CTF, and poinsettia crops are in full swing, is the question, “Is there sufficient bench space?” The requirement for space for rooting potted Easter lilies can conflict with poinsettias on the greenhouse bench if both crops are grown. However, some growers have space in hallways, headhouses, and other spaces where racks of potted lilies can be rooted as long as appropriate temperatures are provided.

Using CTF to cool Easter lily bulbs can produce high quality plants, and this approach should be taken for premium finished lilies. However, the primary drawback is the space and infrastructure required. Coolers are the most common approach for its predictable temperature management. While some growers attempt cooling potted Easter lilies in the greenhouse, it can be too difficult to maintain consistently cool target air temperatures with the solar radiation. If there are other ways that coolers used to chill CTF lilies can be used at other times of the year for other crops, it can helps justify the investment.

Growers have two options for Easter lily bulb cooling: controlled-temperature forcing and case cooling.
Photo © Kimberly Boyles | Adobe Stock

Case-cooling

Case-cooling, as the name implies, is providing the vernalization to Easter lily bulbs that remain in the cases they are stored and shipped in. Once the bulbs have received 6 weeks of cooling, they are potted up for rooting and are rooted as described above (3 weeks at 60 to 62° F)

Like CTF, case cooling uses coolers for the vernalization period; however, unlike CTF, the cooler space requirement is much less for case-cooled bulbs. Although the number of bulbs per case ranges from 100 bulbs (grade 10+) to 300 bulbs per case (grade 6/7), there is no space requirement for pots filled with substrate. In addition to cooler space efficiency, case-cooled Easter lilies also reduces space conflicts with poinsettia crops, unlike CTF. An additional opportunity to reduce steps in production is possible when purchasing pre-cooled bulbs from suppliers. These bulbs are provided with a case cooling, then sent to growers for finishing. While there is an additional cost associated with this, it further simplifies production.

Weeds serve as a refuge for insects, supporting pests that can infest crops and transmit viruses. Weeds can also serve as host plants and carry diseases.

Although case cooling can be an extremely space efficient method of production, it can affect the quality of the final product. Compared to lilies cooled using CTF, case-cooled bulbs may not have quite as high a flower count and the bottom or basal leaves do not elongate as much (sometimes referred to as a “palm tree” shape).

Insurance lighting for insufficient cooling

For a variety of reasons, Easter lilies may not receive the full 6 weeks of cooling at 41° F, in both case cooling or CTF forcing methods. Fortunately, lighting can be used to compensate for insufficient cooling. “Insurance lighting” is the use of photoperiodic lighting to create long day conditions for Easter lilies until flower initiation is complete at the end of January. Either low-intensity (=2 µmol·m–2·s–1 at plant height) day-extension lighting or night-interruption lighting are effective in creating long days for lilies. A general recommendation is that one week of insurance lighting can make up for one week of missed cooling. Note that insurance lighting does not act as a complete substitute for cooling; it can compensate for only a couple missed weeks at most.

Take-home message

With the arrival of the harvest of field-grown Easter lily bulbs, take the time to review your Easter lily production plans to review your cooling strategy. Whether CTF or case cooling, successful cooling is one of the most important steps for programming and forcing Easter lilies.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu