Commercial Greenhouse Production
Scientific Name: Lilium longiforum
Common Name: Easter Lily
Dr. J. Raymond Kessler, Jr.
Easter lilies are indigenous to the southern islands of the Liu-Chiu Archipelago of southern Japan. The area where Easter Lilies grow naturally is a rocky sea coast with alkaline soils and weather conditions similar to the southeastern U.S. Easter lilies were first collected by Carl Peter Thunberg in 1777, and bulbs were sent to England is 1819. Sailors and missionaries introduced bulbs into gardens of Bermuda. By 1930, bulbs were being imported from Japan and Bermuda into the Pacific Northwest of the U.S.
How the Easter Lily became associated with the Christian Holiday of Easter is uncertain. Most of the lilies grown in Western Europe and Japan are forced over an extended period as cut flowers. Only in North American is there the tradition for marketing Lilium species for Easter. This tradition, which began before 1930, is unusual because lilies naturally bloom in mid- to late summer, not at Easter time. Therefore, it is a tradition that depends on greenhouse forcing technology.
Potted Easter Lily ranks number three among pot crops in value and over 8-10 million are grown annually in the U.S. Most of U.S. bulb production occurs in the Pacific Northwest coastal area of Northern California and Oregon), though large numbers are imported The Netherlands.
Part of the challenge in producing Easter Lilies for Easter is that the Holiday does not fall on the same day each year. Easter is the first Sunday following a full moon, on or after the vernal equinox (March 21). Therefore, Easter Lily production schedules are slightly different each year. This is complicated be the fact that the product has little value after Easter. Easter date classification: Early - Mar 22 to Apr 2, Medium - Apr 3 to Apr 15, Late - Apr 12 to Apr 25.
|Easter Sunday Dates|
A number of cultivar have been developed over the last century, mainly from breeding programs in the USDA and the Pacific Flower Buld Growers' Association. However, the two most important by far are 'Nellie White' (80% of production) and 'Ace' (20% of production). Production of 'Nellie White' is increasing in importance, while 'Ace' is declining. 'Nellie White' is a shorter plant, has fewer but wider leaves, more basal leaves, fewer flowers, and does not suffer from leaf tip burn as much as 'Ace.
Easter Lily Cultivars & Case-Cooled Forcing Time
|Cultivar||Origin/year||Forcing time (days)|
|'White American'||The Netherlands/1987||80-102|
Choices of bulb grade (size- inches in diameter), single or double-nosed bulbs, and number of bulbs per pot are based on market demand. However, one single-nose, 6- to 10-inch bulb per 5- to 6-inch pot is the most common. One 9" or larger bulb to a 7" pot or multiple bulbs to 8- or 10-inch pots may be grown. In general, the larger the bulb size, the greater the number of flowers and leaves. Larger bulbs also tend to force faster than smaller bulbs.
Bulb Sizes Marketed
|'Ace'||'Nellie White'||Days to flower||Flower number||Height (in)||Bulbs/case|
The Easter Lily bulb is a compressed shoot that functions as a storage and reproductive organ. The bulb is composed of basal plate to which is attached numerous fleshy, modified leaves called bulb scales. Bulb scales and roots are initiated from the basal plate. Under the right environmental conditions, a central shoot arises from the center of the bulb producing leaves and flowers. 'Baby' bulbs called bulblets may be initiated on the stem close to or just above the ground. The bulb is non-tunicate, meaning there is no papery protective covering around the scale as in onion and garlic.
Vernalization and Floral Induction
Unlike many bulbs, the Easter Lily bulb is never dormant, new scales or leaves are formed year-round. However, when exposed to the proper environment, the bulb will sprout, form leaves, and flower. The primary means of inducing flower is vernalization, or cold, moist treatment before greenhouse forcing. Note that the bulbs must be in moist medium to perceive the cold treatment. Optimum vernalization temperatures are as follows: 'Ace', 38-40F and 'Nellie White', 40-45F vernalization:
1) Natural Cooling - Non-cooled bulbs are potted upon arrival and placed in a minimally heated area (shed, cold frame, etc.) with temperatures as close to 35-45F as possible. Records must be kept of minimum and maximum temperatures and duration to ensure that bulbs receive at least 1000 hours near 40F. Cooling can usually be accomplished in 9 weeks during October, November, and December. Bulbs must not be allowed to freeze. Bulbs are moved into the greenhouse at least 14 weeks before Easter for forcing. The main drawback to this method is inadequate control. Growers use this method where little or no cooler space is available.
2) Case-Cooling - Using this method, the bulbs are cooled in the shipping crates wrapped in moist peat moss. Potting of the bulbs occurs after cooling. There are two options:
a) Commercial Case-cooling - the commercial bulb supplier cools the bulbs in a commercial cold storage facility and then ships them to the grower for forcing. The vernalized bulbs are potted by the grower upon arrival, watered in, and placed in the greenhouse for forcing about 17 weeks before Easter. Case-cooled bulbs should not be exposed to temperatures above 70F before potting either by the grower or during shipping. The main advantage to commercial case-cooling is that the grower does not need a cooler facility. However, the grower has no control over how much cooling was received by the bulbs or at what temperatures. High temperatures (>70F) can erase vernalization.
b) In-house Case-cooling - The grower receives non-cooled bulbs and cools them in his own cooler facilities. This method allows the grower complete control over the vernalization process but is possible only if the grower has well-maintained, accurate cooler facilities. Cases of non-cooled bulbs are placed in cooler upon receipt at 'Ace', 38-40F and 'Nellie White', 40-45F. Most growers monitor temperature by inserting thermometers at the center of several cases. It may take as long as a 7-10 days for the temperature to reach 40F at the centers of the cases. Once at 40F, the bulbs should be cooled for 6 weeks. Once vernalized, the bulbs are potted, watered in, and placed in the greenhouse for forcing about 17 weeks before Easter.
In the case of commercial and in-house cooling, the bulbs are maintained at 63-65F for three weeks (to 14 weeks before Easter) to get root growth to the bottom and sides of the pot. Night temperatures are then lowered to 60F. The main disadvantage to case-cooling is that the plants will not have leaves all the way to the soil line like those cooled in pots.
3) Controlled Temperature Forcing (CTF) - Non-cooled bulbs are potted, watered in, and given three weeks at 63F for root growth prior to cooling. Afterward, pots are moved into the cooler for 7 weeks cooling at 'Ace', 38-40F and 'Nellie White', 40-45F. This method allows the grower complete control over the entire production process but is possible only if the grower has a large well-maintained, accurate cooler facilities. Typically, CTF results in plants with a higher flower count and more lower foliage. This method undoubtably has the greatest potential for the highest quality plants. However, it is the most labor-intensive and facilities-intensive, and requires a greater attention to detail on the part of the grower.
Long days can substitute of at least part of the cold requirement in Easter Lily and is often called "insurance lighting". In years when Easter is early in the season and six weeks of cooling may not be possible, long days can substitute for up to two weeks of cold. Plants are most sensitive if night-interruption lighting (10 ft.ca. incandescent light from 10:00 PM to 2:00 AM) is applied for up to two weeks at or just after shoot emergence. Light should be applied only for the amount of time to make up for cold treatment, up to two weeks. Lighting should not be used in late Easter year unless a major disruption occurs in the cooler. Combining a full six weeks of cold and two weeks lighting results in premature flowering.
Forcing Easter lilies can be divided into three phases:
I. Planting to flower bud initiation.
a. Planting to shoot emergence.
b. Emergence to flower bud initiation.
II. Flower bud initiation to visible bud.
III. Visible bud to open flower.
Range of Easter Dates and Key Forcing Dates
|Easter Classification||First Sunday of Lent||Palm Sunday|
|Early: Mar 26||Feb 13||Mar 19|
|Early: Apr 1||Feb 19||Mar 25|
|Medium: Apr 7||Feb 25||Mar 31|
|Medium: Apr 13||Mar 2||Apr 6|
|Late: Apr 18||Mar 7||Apr 11|
|Late: Apr 23||Mar 11||Apr 15|
Planting to Shoot Emergence
During this period, the bulb is covered by medium so the major factor controlling growth is temperature. The bulb is developing a root system and shoot growth begins. A minimum soil temperature of 60-63F should be maintained, but not exceeding 70F. Shoot emergence should occur in about 2 weeks. This should occur in early January.
Emergence to Flower Bud Initiation
During this period, leaves are unfolding and the terminal apex initiates flower buds. The night temperature should be 60-63F. Keep day temperatures cool, high temperatures can delay flower initiation. Flower initiation (microscopic) normally occurs around January 19.
Flower Bud Initiation to Visible Bud
During this period, about half of the total stem length will occur and all the leaves initiated below the flower buds will unfold, exposing the flower buds themselves. Flower buds are considered visible when the largest is 0.5-1.0 inches long. This stage should be accomplished by the first Sunday of Lent. Temperature adjustments to accomplish visible bud by the first Sunday of Lent is determined by the leaf counting method (see below).
|Average Temperature effect on VB to Flower|
|Temp F||VB-Flower||Decrease in days from VB-F
due to 5F increase
Visible Bud to Open Flower
The period from visible bud to first open flower is a time to fine-tune development to target sales for the week or so before Easter. More advanced plants should be separated into cooler greenhouses and slower plants into warmer greenhouses. Easter lilies are normally marketed when the older flower buds are at the "puffy white stage", about one week before Easter. The time from visible bud to the puffy white stage usually takes 35 days at 63F. This stage should be reached by Palm Sunday. Ideally, temperatures between 60 and 70F can be used. Above 70F, plant quality suffers. Based on a 63F night temperature, the average number of days to flower for different sized flower buds are: 1 inch = 20 days, 3 inch = 11 days, 5 inch = 5 days, 2 inch = 15 days, 4 inch = 9 days.
The Easter Lily is an determinate plant in that once the growing apex initiates flower buds, no new leaves will be formed. The total number of leaves on the plant compared to the number of leaves that have and have not unfolded from the apex is used to determine the stage of development up to visible bud. Leaf unfolding rates can be modified by altering the temperature.
Leaf counting should ideally begin at the time of floral initiation and continue weekly until visible bud. Leaf counting is usually started around January 15-20. Easter lilies begin to form stem roots about the time of floral initiation. Scrape soil away from the stem on several plant to see if stem roots have formed. Then dissect and examine the shoot apex from several plants. A vegetative apex will be round and smooth, a floral apex will have a distinctive mound in the middle. After floral initiation, the number of leaves on the plant is fixed, and dissection and counting leaves on several plants is used to determine how many leaves are yet to unfold from the apex before visible bud. Proceed as follows:
1) Randomly select 5-10 plants from different sampling groups: different cultivars, bulb sizes, bulb sources, or growing areas.
2) Mark with a pen or hole punch the topmost leaf on the plant that is "unfolded". A leaf is considered unfolded if the tip of the leaf is angled at least 45 from the main stem.
3) Start from the bottom-most leaf on the plant and count all the unfolded leaves including the one marked. Record the number of unfolded leaves.
4) Now count all the remaining leaves all the way to the shoot apex. This will require a needle and dissecting microscope or strong hand-lens. Record the number of leaves yet to unfold.
5) Add the number of unfolded leaves to the number of leaves yet to unfold to get a total. Repeat for all 5-10 plants in a sampling area and calculate an average.
The number of leaves yet to unfold must be unfolded between the sampling date and the desired visible bud date (First Sunday of Lent). With many leaves yet to unfold, the temperature must be raised to speed the rate of leaf unfolding, with few leaves yet to unfold, the temperature is lowered to slow the rate of leaf unfolding. To determine the correct temperature:
1) Count the number of days remaining between the sampling date and the desired date of visible bud. Suppose the sampling date is January 20 and desired VB is March 1, allowing 40 days to unfold all the leaves.
2) Determine the number of leaves using the method above: Leaves unfolded=34, Leaves yet to unfold=58, Total leaves=92.
3) Dividing the number of leaves yet to unfold by the number of days remaining until the desired VB date gives the desired leaf unfolding rate per day. Set the average daily temperature according to the table. Example: 58 leaves / 40 days = 1.45 leaves per day.
4) These are Average Daily Temperatures (ADT) based on the following: ADT = (day temp × hr/day) + (night temp. × hr/day) /24.
The average daily temperature for the example should be about 62F. Ideally, the crop should not be so far off schedule as to require ADT below 60 or above 70F. (See general schedule)
|Effect of Average Daily Temperature on Leaf Unfolding Rate|
|Leaves unfolded / day||Average Daily Temp (F)|
A growing medium for Easter lilies should be well-drained, well-aerated, and with exceptional water and nutrient holding capacity. Most growers prefer a heaver media for growing Easter Lilies and many still use a certain percentage of field soil. This is in part due to the high CEC required by lilies and because, being tall plant, it keeps them from tipping over on the bench.
Easter Lilies are heavier feeders than many other greenhouse crops and require a higher pH, 6.5-7.0. They also require large amounts of calcium both for pH adjustment and to prevent leaf scorch (see table). Leaf scorch appears as necrosis at the leaf tips on the lower foliage that can destroy most of the foliage on the plant in a day or two. It is associated with even modest amounts of fluoride in the medium commonly from treble or superphosphate or fluoridated municipal water. Some growers will add monocalcium phosphate which does not contain fluoride at 0.67 lbs/yd3 as a pre-plant source of phosphorus. Perlite is also a small source of fluoride and may not be used, especially for 'Ace'. If perlite is used as a medium component, adjust the pH to close to 7.0. Calcined clay has become a popular Easter Lily media component as a perlite substitute. Susceptibility is genetic and 'Ace' has more problems than 'Nellie White'. Clearly, Easter Lilies require phosphorus so it must be supplied in the liquid fertilization program.
Full height, standard pots (6" top diameter, 6" tall) are strongly recommended (not ½ or ¾ height pots) because it provides better drainage. Bulbs should be planted so that the nose of the bulb is 2" below the soil surface with at least 1" of soil below the bulb. This ensures that the bulb is above the water saturation zone at the bottom of the pot and adequate soil above the bulb for stem root development. Potted bulbs should be watered once then apply a broad spectrum fungicide. It is crucial to water in the bulbs immediately and thoroughly after potting.
Easter Lilies should be maintained constantly moist and never allowed to reach the wilting point. Most large growers use some form of automatic watering. The most popular are microtube irrigation. Microtube irrigation often results in taller plants than hand watering.
Lilies are heavy feeders and the best growth is obtained using both pre-plant incorporated fertility and a constant fertilization program. Fertilizer is usually applied at 200-250 ppm nitrogen. Many growers will alternate calcium/potassium nitrate with a balanced fertilizer such as 20-10-20. Commercial fertilizers such as 20-10-20 do not contain damaging levels of fluoride. Tank mixes with calcium/potassium nitrate often include injection of food grade phosphoric acid at 50 ppm P to provide phosphorus when it is not added as treble or superphosphate. High ammonium levels in the fertility program should be avoided to keep from reducing the soil pH. Medium soluble salts should be about 1.5 mmhos/cm (2:1) but should not exceed 2.0 mmhos/cm (2:1) or 3.5 (saturated paste). Soil testing should be performed monthly and pH and soluble salts should be tested ever 10-14 days and tracked. Many growers will limit fertility to calcium nitrate only at 200 ppm nitrogen after the flower buds are 0.5" long. Do not terminate fertilization in the last 2-3 weeks before the shipping the crop. Though this may be practiced with many crops, it can result in loss of foliage in the post-production phase for Easter Lily.
Studies have shown that the ideal height for an Easter Lily is 2.6:1, plant to pot. This would a 15.6" plant in a 6" pot. However, throw this aesthetics ratio out when considering the type of market. Mass markets have attempted to set height standards so the grower must grow to the demand by the market. The trend, however, is toward shorter, more compact plants. The following factors affect Easter Lily height:
1) Long day lengths increase height, short days decrease height.
2) Low light intensity increases height.
3) Low fertilization (especially nitrogen) increases height.
4) High temperature increases height.
5) Close plant spacing increases height.
Plants may be maintained pot-to-pot until about the time of flower initiation to conserve greenhouse space. Afterward, plants (6" pots) are usually placed at a final spacing of 2.5 pots/ft2 or 8" × 7". For better uniformity, many growers place the taller plants toward the edge of the bench and the shorter plants in the center, producing a "dish" when viewed for the end of the bench. This practice tends to stretch the plants toward the center.
About half of the height of an Easter Lily occurs between visible bud and flowering. Artificial long days (16 hr/day) applied during January and February increased height by as much as 19-21% compared to natural photoperiods. Of course, photoperiods during March, April, and May are naturally longer. However, short-days throughout the forcing period results in a 29% reduction in height compared to natural photoperiods. To summarize, if black cloth is already available, it can be considered an option for reducing plant height.
Easter Lilies are high light plants and require the maximum light possible for forcing. The grower should be keenly aware of overhead structures and glazing characteristics that influence light intensity. This includes old glazing material, pipes, shade cloth bundles, equipment, and old shading compound.
Graphic tracking is a systematic means to manage both the timing and height of Easter Lily crops. The key to understanding the system is to understand the crop's reaction to temperature. Before visible bud, the rate of leaf unfolding is directly proportional to average daily temperature. While average daily temperature influences the rate of leaf unfolding, the difference between the day and night temperature (DIF) influences plant height. Warm days and cool nights (+DIF) promote internode elongation, while cool days and warm nights (-DIF) reduce internode elongation.
The graph shows the relationship between day and night temperature and the rate of leaf unfolding. The first step toward using this graph is to perform leaf counting to determine how many leaves per day need to unfold before visible bud. Different combinations of day and night temperatures can be used to accomplish the required leaf unfolding. However, the choice of which temperature combination to use depends on the plant height desired.
Graphical tracking involves constructing a graph of the projected progress in plant height based on desired final height and plotting actual measured height. If the actual heights do not fall within the "tracking window" corrective action must be taken either by changing the temperature or applying a growth retardant. The graph is constructed as follows:
1) Establish the final desired height. Ex: Total pot=19-22", minus pot height=13-16" plant.
2) Establish the desired visible bud date, usually 5-6 weeks before Easter. Ex: 0
3) Draw a graph with time on the X-axis and plant height on the Y-axis with a horizontal line at 6" for the pot.
4) Mark the maximum and minimum total heights at flowering.
5) Calculate the maximum and minimum height at visible bud using the rule that the plant doubles in height between visible bud and flower. Ex: half final height=6.5-8" plus 6" for the pot=12.5-14". Mark these heights 6 weeks before flower.
6) Connect the flowering heights to visible bud heights with straight lines and the visible bud heights to the 6" mark on the Y-axis.
Average actual height can be determined by measuring 5-10 plants weekly and plotting the values. This is often done at the same time as leaf counting. If the trend is toward plants that are too tall, day/night temperatures toward a more negative DIF can be chosen but still maintain the desired rate of leaf unfolding. Conversely, if the trend is toward plants that are too short, day/night temperatures toward a more positive DIF can be chosen but still maintain the desired rate of leaf unfolding.
|Typical Easter Lily Forcing Schedule|
|Weeks before Easter||Comments|
|14||Place in greenhouse at 60-65F|
|12||Shoot 1-2 inches long|
|11||Shoot 3-5 inches long|
|10||Shoot 5-9", 47 leaves to unfold, floral initiation|
|9||Shoot 9-12", 35 leaves to unfold|
|8||Shoot 9-12", 25 leaves to unfold|
|7||Shoot 9-15", 12leaves to unfold|
|5||Bud ½-1" long|
|4||Bud 1-2" and bending|
|3||Bud 2-3 long|
|2||Bud 3-4" long|
Growth retardants have been used on Easter Lilies for over 30 years. The first was Phosphon-D which was only effect on Easter Lily and is no longer available.
A-Rest is currently the only growth retardant labeled for Easter Lily. It may be used as either a spray or drench. No single recommendation can be provided for A-Rest on lilies because it depends on the cultivar and environmental conditions. Only through grower experience can optimum rates be determined. In areas where lilies grow taller, 0.5-1.0 mg active ingredient may be applied per 6" pot as a drench. This is applied as 6 fl.oz. per pot. The total amount of active ingredient may be split and applied in 2-3 applications. In areas where Easter Lilies grow shorter as little as 0.1 mg active ingredient may be applied as a drench per 6" pot. Growing medium with a significant bark component will absorb and tie-up A-Rest. Drench A-Rest should not be used in bark media. Other growers prefer to apply A-Rest as a spray using multiple applications at 25-50 ppm.
Storage of Budded Plants
Because of the narrow market window and the difficulty in forcing Easter Lilies on time, many growers will remove advanced plants from the greenhouse a week or so before marketing to delay the progress of flowering. Even with perfect timing, 5-15% of the crop can be expected to be earlier than desired. Advanced plants can be placed in dark coolers at 34-40F for a week or less at the "puffy white" bud stage with no adverse affect. Some growers with large coolers will actually schedule a week early to reduce the chance of missing Easter. Beyond a week of storage, however, problems start to develop including bud abortion, foliar chlorosis, and reduced flower longevity.
Insect that can be a problem on Easter Lilies are few but must be addressed. They include aphids, fungus gnats, shore flies, and bulb mites. Bulb mites are a particular problem that is often shipped on the bulbs themselves. Feeding by the mites not only weakens the bulb but also opens the plant to diseases by wounding. All bulbs received from the supplier should be inspected upon arrival for these pests. Pesticide bulb dips can be used prior to planting.
Easter lilies may be infected by a number of bacterial, viral, and fungal diseases. Viruses include cucumber mosaic virus, lily symptomless virus, and tulip breaking virus. All are transmitted by aphids! Diseases include Botrytis, Pythium, Rhizoctonia, Fusarium, and Cylindrocarpon. Lilies have also have problems with foliar and root lesion nematodes.
Leaf Scorch - brown necrotic areas on the leaves due to fluoride toxicity (see earlier).
Lower Leaf Yellow - Often due to low fertility, especially nitrogen during cool, cloudy weather. May also be caused by an interesting interaction between A-Rest and low phosphorus. High A-Rest combined with low phosphorus levels in the leaves can cause early leaf senescence. The two alone do not cause a problem.
Flower Bud Abortion - May be caused by high forcing temperatures, low light, or ethylene. High ethylene accumulation in the greenhouse have been associated with a number of problems including bud blasting, premature bud opening, and leaf epinasty.
Subdue Fungicide - this fungicide can cause a white tip burn of the leaves. Use the low end of the recommended rate or choose a different fungicide.