Photo: Thinkstockcom

In 2018, Easter Sunday falls on April Fools’ Day. Although this is considered an early date for Easter, the forcing schedule shouldn’t require a lot of fooling around. Barring any unexpected events or tricks from Mother Nature, growers should be able to manage this crop without too much complication.

To get on the recommended 23-week forcing schedule, growers will need to begin the process by Oct. 22, 2017, and maintain proper cooling and forcing temperatures throughout. This requires that bulbs arrive on time and in good shape and that you handle them immediately upon arrival. The key steps in the forcing program for pot-cooled bulbs include a 3-week rooting period (at 63oF), followed by 6 weeks of bulb cooling (at 40-45oF). Then plants are forced in the greenhouse at 60-62oF until bud initiation is complete (about 4 weeks). Once buds are set, higher temperatures are used to force the crop during the final 10 weeks. With case-cooled bulbs the process involves 6 weeks of bulb cooling at 40-45oF followed by a 17-week greenhouse production phase during which bud initiation occurs and the crop is forced to flower.

In both cases the entire process requires 23 weeks from start to finish. If bulbs arrive late or if your sales schedule calls for lilies earlier than 1 week before Easter, there are a couple of shortcuts you can take in the 23-week schedule. With pot-cooled bulbs you can reduce the length of time that pots are held at 63oF prior to the 6-week cooling period. If you are tempted to cheat here, allow enough time for bulbs to show some root development, 2 weeks if at all possible, but at least 1. As an alternative or if your schedule is still a little tight, you can substitute “insurance lighting” for a portion of the 1,000 hour (6 week) bulb-cooling period. The same insurance lighting rule applies to all forcing methods including naturally cooled, pot-cooled or controlled-temperature forced (CTF), and case-cooled bulbs.

Lilies exposed to these long photoperiod conditions immediately after shoots emerge, respond as if the bulbs were exposed to additional hours of cool (40oF) temperatures. In seasons when Easter falls on an early date, growers can extend the natural daylength with low-intensity light to “insure” that adequate vernalization occurs. You can also use insurance lighting in lieu of cooling if you are trying to reduce the length of the forcing schedule.

Use “insurance lighting” to directly substitute for lost bulb cooling time, one day of additional lighting for each day of lost cooling for up to 14 days (but no more). Lighting is most effective when started immediately at shoot emergence. Do not use “insurance lighting unless the crop is short of the 1,000-hour-bulb cooling threshold since excess days of insurance lighting, just like excessive bulb cooling, will reduce lily leaf number, reduce bud counts and shorten the time to flower. Reducing time to flower may be your goal but dramatically reducing bud count is not.

Insurance lighting is achieved by providing at least 10 foot-candles (measured at plant height) for four hours (10 p.m. to 2 a.m.) each night. Incandescent, fluorescent, LED or HID lamps can be used to provide the necessary night break.

Even though Easter 2018 is early, avoid the temptation to speed up lily growth in the first few weeks after emergence. Too often, growers run temperatures in the 70-75oF range during this critical period in a misguided effort to get ahead of schedule. The result is excessive lily height, poor bud counts and prolonged cold storage periods at the end of the crop. At emergence, hold a constant day and night temperature of 60-63oF until bud initiation is complete. Bud initiation is typically complete when shoots are about 3-5” tall, in mid- to late-January 2018. The development of stem roots coincides with flower bud initiation. During this period, it is imperative that temperatures not exceed 65oF.

Chart: Richard McAvoy

Leaf counting and crop timing

Start checking leaf counts in mid-January (week 11). If bud set is not yet complete, wait one week and try again. This will allow plenty of time to determine whether lily development is on schedule and make temperature adjustments as needed. Use average daily temperature (ADT) to control the rate of lily development for the remainder of the forcing period. The rates of both leaf and flower development can be modulated with temperature. By controlling the rate of development you can control when the crop reaches the saleable stage. For example, at an average daily temperature of 72oF leaves unfold at a rate of two per day on average, while at 63oF the rate decreases to 1.5 leaves per day. Likewise, a lily will go from visible bud to bloom in 24 days at 81oF, 31 days at 70oF, 35 days at 64oF and 42 days at 59oF. If you arrive at visible bud 5-7 weeks before Easter and you can control temperature within these limits you should be in good shape to finish on time. Finally, plants that bloom early can be held in a cooler for up to 2 weeks. Storing finished lilies for longer than two weeks is not recommended.

The leaf counting technique is based on the fact that once flower buds initiate, leaf number is set and will not change. However, the exact number of leaves varies from year to year, between bulb lots, and with bulbs exposed to different cooling conditions.

After bud initiation, select five lilies for every 1,000 plants in each lily group (per bulb source, emergence time, etc.). Select plants representative of the overall crop, and then remove, count and record the total number of leaves. Use a needle to remove and count the smallest, un-expanded leaves, and use magnification if necessary to determine if the shoot tip shows evidence of flower bud formation. If bud formation is not evident, wait a week and try again. Record the number of fully developed leaves (those at a 45o angle to the stem or greater) and the number of undeveloped leaves (those at an angle less than 45o to the stem). Now, divide the number of fully developed leaves by the number of days since shoot emergence. This is the “current rate of leaf development.” Divide the number of undeveloped leaves by the number of days remaining until visible bud. This is the “required rate of leaf development” or the rate you need to maintain as you move forward in the schedule. Determine a new current rate each week (the rate since last count) and, by subtracting the number of fully developed leaves from the average total number of leaves, determine a new required rate.


Controlling lily height

The 2018 schedule sets a targeted lily height for each week in the greenhouse. You can adjust these targets to fit your needs (e.g. increase plant height if you desire a taller finished product). This schedule is designed to produce a finished plant of about 16”. You can chart the height of your crop against these target heights. Monitor lily height on a regular basis (daily, bi-weekly or weekly) and compare the actual height to the idealized growth curve for the lily height you wish to produce. If average plant height is too short, run a positive DIF to increase stretch. If plant height is too tall, run a negative DIF or use a plant growth regulator to slow elongation. It is extremely important that you maintain the proper average daily temperature (ADT) so that crop timing is not adversely affected.

Lilies typically double in size in the 5-week period from visible bud to bloom but it takes 9 weeks of forcing prior to visible bud to reach the first 50 percent of final lily height. A-Rest, Abide, Chlormequat E-Pro, Concise, Cycocel and Sumagic are all labeled for use on Easter Lilies. PGR applications typically begin after buds are set is complete, when lilies reach 3-5” tall. However, with low concentration split applications, PGRs can be applied at any point in development beginning with emergence. A-Rest and Sumagic (or the generic equivalents) can also be used to pre-treat bulbs using bulb soaks. With sprays and drenches, split applications produce the best results. Reduce the concentrations of PGR used when combined with negative or zero DIF.

Leaf yellowing

Crowding, root disease and poor nutrition (especially low phosphorus and nitrogen) during the final stages of development and stress from unfavorable cultural and environmental conditions such as excessive shading and high temperatures all favor lower leaf yellowing. In some years, leaf yellowing can be a significant problem for growers especially when encountering unusual weather or when lilies are too crowded or too tall too early, or when plants are subjected to excessive stress and root damage. If you experience conditions that favor lower leaf yellowing during the final weeks of forcing, apply Fascination or Fresco. Applications in the weeks prior to visible bud or 2 weeks after visible bud should be applied only to the lower leaves to avoid stem stretching. Higher rates can be applied over the entire plants on mature lilies ready to go into cold storage.

We don’t know what climatic challenges will occur in 2018, but proper attention to detail and control of the production environment will go a long way toward mitigating extremes and preventing your crop from falling prey to an April Fools’ gag in 2018.