Easter Lilies: A Challenge You Can Master By Theo J. Blom and Paul Fisher

When it comes to controlling flowering and height of your Easter lily crop, precision is key. Temperature manipulation, graphical tracking and light monitoring are just a few of the tools you will need to make sure your plants are on time and in perfect c

By the time you read this article, Easter lilies will havebeen planted, and flowering and height control are likely the major issuesremaining for a successful conclusion of this crop. This year, Easter will beon March 31, 2002, and most plants will be ready to ship (puffy bud stage)between March 11-26, 2002. This early Easter makes for a very tight scheduleand means that it is especially important to use the well-proven tools wedescribe in this article to help you meet flower date and final heightspecifications.

Hitting your target flowering date

There are several key dates in the Easter lily crop cycle— if you hit these intermediate target dates, you will be on track forflowering in time for Easter. The target dates for 2002 are shown in Table 1below.

Average temperature is the key accelerator for pushingflowering date forward or back (faster flowering occurs as temperature isincreased). Some very effective tools can be used to identify the optimumtemperature to control timing of each stage:

1. Emergence occurs when the tip of the bulb emerges abovethe soil line, which should have happened around December 20-25, 2001. Plantsshould have received six weeks of cooling before emergence to ensure reliableflower initiation and acceptable flower bud count. Maintain 62-65° F soiltemperature until flower initiation. Excessive temperature will cause too muchunwanted initial stretching and more leaves, while very low temperaturesprevent the initiation of a healthy root system and can cause overcooling.

2. Flower initiation is the stage when flower buds are firstvisible on a dissected plant using a magnifying glass. The process ofinitiation starts when shoots emerge above the soil line and continues untilthe buds have formed microscopically (approximately January 12-20, 2002). Atthis point, no more leaves are being initiated, and you can start to estimatehow many leaves are on each plant. Checking for flower initiation is importantbecause you can set greenhouse temperatures and control crop timing based onhow quickly leaves unfold.

3. Visible Bud occurs when the immature flower buds canfirst be seen (overhead view) on the plant without removing Á leaves.The key tool to control timing of visible bud is leaf counting. To use the leafcounting method:

On plants that have initiated flowers, use leaf dissectionto count (a) the number of leaves already unfolded, (b) the number of immatureleaves that have yet to unfold and (c) the total leaf number (a) plus (b)).

Set a target visible bud date 30-35 days before theflowering date (see Table 1 on page 10, which assumes 32 days), and calculatehow many days remain from the date you counted leaves until the target visiblebud date.

The warmer the air temperature, the faster the leaves willunfold. Estimate the temperature you will need to reach the visible bud date byunfolding all leaves using this formula:

 

24-hour average temperature (°C) = target leaf unfoldingrate x 10.64 + 1.12

 

(with the target unfolding rate expressed in number ofleaves per day)

 

For example, if you want to unfold 50 leaves to reachvisible bud in 25 days, the target leaf unfolding rate would be two leaves perday, and the target temperature = 2 x 10.64 + 1.12 = 72° F (22.4°C) for an 8/9 case-cooled ‘Nellie White’. For larger bulbs orCTF-bulbs, the natural unfolding rate is slightly higher, and thus, a slightlylower temperature can be maintained.

While it is possible to predict in advance the average dailytemperature necessary to reach visible bud on a desired date, actual greenhouseand plant temperatures seldom exactly equal that desired. Therefore, it isimportant to repeat the leaf counting process on at least five plants randomlychosen from the crop twice per week until visible bud.

Each time the number of unfolded leaves is counted, one getsa new update on crop development. Temperatures can then be adjusted up or downto keep the crop developing properly. Reaching visible bud at the proper timeavoids the problem of having to run excessively cool or warm temperaturesduring the next stage of lily development, the time between the visible bud andopen flower stages.

4. The Open Flower stage occurs when the plant has an openflower in the retail environment (around Palm Sunday). Most plants are shippedat the puffy bud stage, 1-2 days before the first flower will open. Optimumtime for the puffy bud stage is earlier for wholesale/big box suppliers, whowill hold plants in a cooler until ready, and later (near Palm Sunday) forlocal sales, where a “ripe” plant is delivered directly to themarket (see Table 1 on page 10). Remember that there is usually a span of 14days between when the first lily in a crop is ready to ship until the lastplant can be shipped, because of variability in flowering. Assuming you havecooler space to hold early plants, shoot for 50 percent of the crop being readyto ship seven days before your final ship date to bring all of the crop intoflower on time.

Easter lilies open their first primary flower bud after itis just over six inches in length. The time it takes for an immature flower budto reach that length (and an open flower) can be predicted using a model thathas an accuracy of ± 2-3 days. You can download this model, called theflower bud meter, from Dr. Heiner Lieth’s University of California DavisWeb site at http://lieth.ucdavis.edu/Research/dss/bud.htm.

Plant Height Tracking

Height control is also crucial to ensure that plants fitinto suitable containers for shipping and are balanced, attractive products.Graphical tracking is a technique whereby plant height is measured on 10 plantsper crop twice each week, and the average height is plotted onto a graph thatalso shows target elongation curves. If actual height is below or above the”window” between maximum and minimum curves, then height controlmeasures (e.g., growth regulators or DIF temperature) are necessary.

The target stem-elongation curve for Easter lilies is simpleto calculate. The easiest way to develop the curve is to assume that plantsdouble in height from visible bud to open flower. This means the plant heightat visible bud, not including the pot, is half the height at flowering (seeFigure 1 to the right).

Considering a target total (plant plus pot) height of 21inches at open flower, plant height at flower will be 15 inches (21 inchesminus 6 inches for the pot size). Half of 15 inches is 7.5 inches. Total heightat visible bud should therefore be no greater than 13.5 inches (7.5 inches plus6 inches for the pot equals 13.5 inches).

We now know the height at emergence (pot height of sixinches), the total height at visible bud (13.5 inches) and open flower (21inches). Plot these three points (height at emergence, visible bud and openflower) and connect with two straight lines. Two lines are normally plotted toreflect the desired minimum and maximum final plant height (19 and 21 inches,respectively, in Figure 1 below). Á

While creating an Easter lily graphical tracking curve byhand is easy, many growers prefer to use a computer spreadsheet. The computerprogram UNH FloraTrack for Lilies that includes curves for Easter, Oriental andAsiflorum (LA hybrids) lily species can be obtained from the University of NewHampshire for $125 by contacting Paul Fisher at (603) 862-4525 orPaul.Fisher@unh.edu.

Another approach to height control is to take a yardstickand write onto it the dates through the production cycle. The target heightsfor each date (see Table 2, to the right) represent the middle of the targetgraphical tracking curve from Figure 1, page 13. For example, 13 inches up theruler write the date “12-Feb.” You can take this ruler into thegreenhouse each week and compare the ruler against plants on the bench. Ifplants are above or below the target height for the date, growth regulators orother height control actions are needed.

Controlling height and timing

Several factors affect elongation and development rate forEaster lily. Understanding each of these options will help you avoid”fighting fires” with a crop that is behind or ahead of scheduleand provide you with corrective strategies:

Air temperature. Regulating the difference between daytemperature and night temperature (DIF) is a well-established tool for heightcontrol. A negative DIF (cooler day than night) produces shorter plants than apositive DIF (warmer day than night). A negative DIF can be achieved bydropping the temperature to 50-59° F starting about 1-2 hours beforesunrise and maintaining this cool temperature for 4-6 hours (the”drop” or “dip” method) or for the duration of theentire daylight period. It has been shown that the DIF effect is greatest whenlow daytime temperatures are maintained during the early morning rather thanduring the afternoon. DIF temperature affects stem elongation but does notaffect time to flowering, which is determined by the average 24-hourtemperature over both day and night.

Growth regulators. Growth regulators such as A-Rest orSumagic are effective as a drench or spray when plants are 3-6 inches tall oras a spray at a later date until visible bud. These products have been proveneffective, but a side effect can be premature senescence (yellowing) of thelower leaves, especially with high label rates. Multiple applications at a lowlabel rate are most effective.

Irrigation frequency. Growing a high-quality lily that willperform well for the customer is largely dependent on root health. Most rootproblems in lilies result from overwatering, which can result in stunted andstressed plants. Although limiting the amount of available water may reduceelongation, running the plant overly dry is also likely to damage roots —a middle path that aims for healthy root growth is the best approach.

Light intensity. Easter lilies are very sensitive to totallight energy and will elongate much more under shade or cloudy conditions.Research found that Easter lilies grown in a glass greenhouse under differentlevels of shade (0-75 percent ) from emergence to flowering increased plantheight by about one inch for every 10 percent light reduction. Thisconsideration is important when deciding to grow lilies in a glasshouse or adouble polyhouse; high light is preferable to grow a compact, high-qualityplant.

Light quality. During the 1990s at Vineland Station inOntario, Canada, we experimented with using black cloth to eliminate twilightas well as Á trying to determine whether one hour of artificial twilight(i.e., far-red light) at the beginning of the dark period (end of the day) orat the end of the dark period (beginning of the day) would affect height. Theuse of blackout resulted in a 20-percent reduction in height for’Ace’ and Nellie White compared with ambient (natural photoperiod)conditions, when grown under 0° F DIF (See Figure 2 above). Plants grownunder an 8-hour photoperiod using blackout between 4 p.m. and 8 a.m. with onehour of low-intensity, far-red lighting at the beginning of the dark periodwere double the height of those grown under short days.

When these treatments were combined with either +9° F or-9° F DIF, it was found that the effects of DIF and that of twilight wereadditive. The shortest plants were obtained under -9° F DIF and short days,and the tallest plants were +9° F DIF under ambient conditions (excludingthe one hour of far-red lighting treatments).

Another question that needed to be answered was whetherelimination of twilight at the end of the day (sunset) is more effective thanat the start of the day (sunrise). This work was done with LA-lilies (orAsiflorums). The results are shown in Figure 2 above. The height of plantsgrown under blackout (4 p.m.-8 a.m. or 8-hour photoperiod) were indexed to 100.Artificial sunrise (one-hour FR plus eight hours PP) increased plant height by7 percent, but artificial sunset (eight hours PP plus one hour FR) increasedplant height by 39 percent compared to eight hours PP. This means that theeffect of sunrise is relatively small compared to that of sunset.

For commercial production, using blackout at or just beforesunset in combination with negative DIF are complementary tools. Opening blackcloth prior to sunrise provides further height control.

Water temperature. This year, an experiment with threedifferent irrigation temperatures, namely 41, 59 and 77° F, was undertakenat Vineland Station. We applied the irrigation solution (nutrients inclusive)either on the growing point (meristem) or the soil surface. In addition to theabove combinations, we treated some plants in the morning and others in theafternoon. Plants were irrigated every 4-5 days, whenever irrigation was neededfrom emergence until flowering. Although both 59° F and 77° F did notshow any effects, plants grown with 41° F irrigation water applied overheadwere 50-percent shorter than those grown with 41° F water on the substrateor the 59° or 77° F treatments. There was no difference in plant heightwhen irrigation took place either in the morning or the afternoon. Plant heightwas about one-half compared to any other treatment, while forcing time wassimilar to the other treatments. During the coming year, we will determine howhigh the temperature can be while still being effective or how low it can bewithout causing any bud abortion. In addition, we would like to determine howlong “cold water shock” remains effective in controlling plantheight.

In Summary

Plant dissections, leaf counting, bud meters and heightgraphical tracking are very useful tools for Easter lily growers. The targetdates that we have used for specific stages of development may have to beadjusted depending on cooling technique, size of bulb, latitude and market. Anumber of options have been given to control the plant height of Easter lilies.The use of negative DIF either with or without the use of blackout at the endof the day (remove twilight at sunset) and/or the use of cold water (41-44°F) are management tools to limit the height of Easter lilies.



Theo J. Blom and Paul Fisher

Theo J. Blom is associate professor of greenhouse floriculture at the University of Guelph, Vineland Station, Ontario, Canada, and Paul Fisher is assistant professor of plant biology at the University of New Hampshire, Durham, N.H. They may be reached by phone at (905) 562-4141 ext. 160 or via E-mail at tblom@uoguelph.ca.



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