Forcing strategies for a late Easter

Easter 2011 falls on April 24, one of the latest dates possible. The date of Easter varies annually based on specific astronomical events. Easter can be as early as March 22 or as late as April 25. You will have to wait until the year 2038 for Easter to fall on a date as late as this year.

Under ideal circumstances, programming of the Easter lily crop begins 23 weeks before Easter to allow adequate time to fully cool and force the crop for maximum quality. This season, growers receiving bulb shipments in mid- to late-October will find that they have an extra two to four weeks in their production schedule.

The key question for 2011: How should growers handle the extra time and still produce a quality crop? Another concern with a late Easter is that the lily crop will be competing for production space with spring bedding plants.

Using the extra crop time
There are several ways growers can use the extra production time. The 2011 Easter lily schedule projects lilies for sale one week before Easter. However, most growers begin shipping lilies two weeks before Easter. Natural variation in date of lily maturity usually produces enough early blooming plants to meet this demand without changing the basic schedule. If a grower wants to bring the bulk of his crop in earlier he needs only to adjust each step in the production schedule back one week.

Another opportunity to use some of the extra time is during the six-week cooling or vernalization period. The lily bulbs need to accumulate six weeks (1,000 hours) of cooling at 40°F-45°F to be properly vernalized. Too much cooling (more than six weeks) hastens finishing time, and reduces both leaf number and bud count.

A grower can prolong the cooling period without encountering these adverse effects. Start cooling the bulbs at 40°F-45°F as normal. After four to five weeks of normal cooling, drop the temperature to freezing or slightly above (32°F-34°F). The near freezing temperatures suspend normal metabolism, placing the bulbs in a near dormant phase during which additional vernalization won’t occur. The bulbs can be held at this temperature for two to four weeks if necessary. Once enough time has passed to put production back on schedule, the temperature can be raised to 40°F-45°F for the remainder of the six-week vernalization period. A word of caution when using this technique: the temperature must be carefully monitored and controlled. A light freeze of 32°F won’t harm the bulbs but a hard freeze will.

Finding adequate production space will be more of concern in 2011 since the late Easter date will cut into the peak spring bedding plant season. Suspending baskets over budded lilies is not the ideal production setup.Increasing bud count
Another opportunity to use the extra production time will come during initial bud set. Growers can increase bud count by lowering the greenhouse forcing temperatures for seven to 14 days following primary bud initiation. A cooler forcing temperature has the added benefit of slowing the rate of crop development.

If a grower plans to lower the forcing temperature, greenhouse forcing of case-cooled lilies needs to begin 19 weeks before Easter rather than the normal 17 weeks. For pot-cooled lilies, greenhouse forcing needs to begin 16 weeks before Easter rather than the normal 14 weeks.

The greenhouses should be maintained at 60°F-62oF until the primary buds initiate (e.g., about week 16 for case-cooled bulbs). Once primary buds are set, lower the greenhouse temperature to 46°F for up to 14 days to stimulate secondary bud formation. After this period, raise the temperature to 60°F-62°F until bud initiation is complete. If a grower starts greenhouse forcing just one week ahead of schedule, the duration of the 46°F cool period should be limited to seven days.

Timing of the 46°F treatment is critical. It must start with primary bud initiation, which coincides with stem root initiation (early to mid-January 2011).

The best way to tell when bud initiation has started is to dissect some lily plants and examine them. This requires strong magnification to view the anatomical changes in the shoot tip. By comparison, stem roots are easy to view and bud initiation typically starts just as stem roots begin to emerge. When primary bud initiation occurs must be determined to achieve the desired effect.

Flower bud initiation may be adversely affected or delayed by temperatures that are lowered before bud initiation starts. In contrast, temperatures lowered after bud initiation is complete will have no effect on bud count but will slow lily growth.

Controlling lily development
After bud initiation, temperature can be used to control the rate of lily development at any stage of greenhouse forcing. This year growers will want to run cool temperatures (63°F-65°F) early in the season since outdoor temperatures will probably force development very fast during the last month of the season.

Both the leaf unfolding rate and the rate of flower bud elongation can be increased or decreased with temperature. For example, at 72°F the typical leaf unfolding rate is two leaves per day, but at an average daily temperature of 63°F the rate decreases to 1½ leaves per day. Likewise, a lily will go from visible bud to bloom in 31 days at 70°F, but it will take an additional week at 60°F. If the temperature can be controlled within these limits, finishing can be delayed without sacrificing quality.

Click here to see the 2011 Easter Lily Schedule

Plants that reach the puffy white bloom stage early can be held in a cooler (35°F-45°F) for up to two weeks. Growers with adequate cooler capacity may want to bring plants in early just to free up bench space for other spring crops. A Fascination or Fresco spray can be applied to lilies that are held in cool storage for seven days or more. Apply 100 parts per million over the top of the crop just before starting storage.

With naturally cooled lilies follow the controlled-temperature forcing schedule, but remember the ability to control temperature during vernalization is limited. Therefore plants may not be able to be held at 32°F-34°F to delay vernalization. However, if these temperatures occur naturally they need to be factored into the production schedule. Once vernalization is complete, reassess the schedule. If there is extra time, use temperature to increase bud count and to control the rate of crop development as previously described.

Richard McAvoy is professor and extension specialist greenhouse crops, University of Connecticut, Department of Plant Science and Landscape Architecture, (860) 486-0627; richard.mcavoy@uconn.edu.