Production of Tulips as Cut Flowers

Bulb Selection

The major groups of tulip flower types are simple petal, parrot, fringed, double flowering, multi-flowering, and lily flowering. Fringed and parrot tulips are considered distinct flower types. The fringed types have a regular tulip bloom with a fringed edge. Parrot tulips, however, have a deeper cut in the bloom edge and are more ruffled overall. Double flowering tulips have a cluster of multiple petals forming the flower head. Multiflowering tulips have more than one flower per stem. Lily flowering tulips have more pointed petals and usually bloom later than any other type of tulip.

Bulbs are measured by cir-cumference (cm) in a horizontal plane at the middle height of the bulb. The ideal bulb size for tulips is top size 12/+ cm, although some growers use a smaller 11/12 cm size. However, for optimum flower production the 12/+ cm top size is highly recommended because smaller bulbs result in a poorer quality crop with smaller flowers and shorter stems. Bulb orders should be placed with suppliers after Mother’s Day, preferably no later than the Fourth of July weekend. Most suppliers provide the tulips in trays of 500 per variety for a 12/+ size and 750 per variety on the 11/12 size bulbs.

Precooling Requirements

It is highly recommended you follow the information provided in the Holland Bulb Forcer’s Guide–5th Edition. There are two main types of bulbs, precooled and non-precooled. The Guide defines precooling as “the dry storage of spring flowering bulbs at temperatures between 35-48°F after floral initiation and development is completed, but prior to planting.” Precooled bulbs need to be planted fairly quickly upon receipt. If precooled tulip bulbs must be stored for a short period, keep them at the temperature specified by the supplier (usually 40-45°F). The second type, non-precooled bulbs, are stored at non-chilling temperatures (typically 63°F) until cooling begins by the final forcer. Non-precooled tulip bulbs can be potted and stored in a dark, cool (33-40°F) barn or shed for 12-14 weeks. Monitor the stacked crates often to prevent the shoots from growing into the crate above.

Production Methods

Greenhouse—Hydroponic in Crates

Reusable water trays are placed in tulip bulb shipping crates. The trays float in a liner inserted in the bulb crates. Two kinds of water trays are commonly used—the egg crate type and the prong type. The prong-type water trays are able to accommodate bulbs of various sizes, whereas the egg crate type comes in two different sizes to hold a particular size bulb.

Precooled bulbs are required for greenhouse forcing. Precooled bulbs are planted into the hydroponic trays and typically placed back into a cooler at about 40°F for 1-3 weeks depending on cultivar and time of year. This temperature allows for some rooting to take place before the bulbs are placed in the greenhouse for forcing. The tulips can then be grown in a cool (40°F night) or warm (55°F night) greenhouse. The warmer the greenhouse, the faster the bulbs will bloom.

Level benches are needed so that the water level is the same for the entire crop. Water is added to the trays so that the level of the water just touches the bottom of the bulbs to initiate root growth. Some bulb suppliers recommend removing the paper skin from the base of the bulb to allow faster root growth, being careful not to damage the root area.

The water in the trays should be changed several times during the growing cycle, either by overflowing the crates when watering or completely changing the water. Be aware that dumping excess water on the greenhouse floor increases the humidity in the greenhouse, thereby increasing the chance of Botrytis problems.

Greenhouse—Soilless Substrate in Crates

The shorter tulip bulb crates are more suitable for growing tulips than the taller lily bulb crates. Place a sheet of newspaper in the bottom of the crate to prevent the substrate from falling out the bottom. Fill the crate with about three inches of moistened substrate. Tulips produce roots at the base of the bulb, so the amount of substrate under the bulb is more important than the depth the bulb is planted. Arrange the bulbs on the substrate in ten rows of six bulbs, or sixty bulbs per crate. The number per crate can vary by bulb size, with as many as 100-105 11/12 cm bulbs being used in some cases. Cover the bulbs with 2-3 inches of substrate and then water well. Tips of the bulbs should still be visible after watering the substrate.

If the bulbs previously received their entire precooling period, forcing may begin by moving the crates into a cool (40°F night) or warm (55°F night) greenhouse, or holding in a cool area several weeks to grow roots. If coolers are available, bulbs are planted then cooled at 40°F (or below to reduce shoot growth) for a total duration of 12-16+ weeks depending on the cultivar and time of year. After cooling, forcing may begin. Protect the bulbs from freezing. Keep the substrate moist at all times, while being careful to keep the foliage as dry as possible when watering.

Greenhouse—Raised Beds

An indoor raised bed can be constructed using pressure treated wood to create six-inch high sides. When constructing raised beds in a high tunnel or greenhouse, consider the space between the beds for maneuvering equipment. Raised beds are useful for tulip production because they help improve drainage and prevent disease problems caused by root rots. Do not replant in beds that have had tulips in the past unless able to steam sterilize the substrate at 160–180°F for 30 minutes.
During the heat of the summer when a greenhouse is not in use, plastic covers can be used to solarize (using the sun to heat the soil) the beds for 4-6 weeks.

Growing tulips in raised beds is very similar to greenhouse production of tulips in crates of soilless substrate. Programmed bulbs are used and planted in steam-sterilized soil. Keep the beds well watered and the greenhouse nighttime temperatures between 40 and 55°F. The warmer the house is kept, the shorter the crop time, but the higher the heating cost.

Field Production

It is easier to plant tulips by digging a trench than it is to plant individual bulbs. To form a trench, cultivate the planting area 12 inches deep and shovel the soil to one side. In field production, tulip bulbs can be planted up to eight inches deep. The deeper the bulb is, the longer the stem will be when pulled at harvesting. If a deeper trench is needed, cultivate the area again and remove more soil. Place the bulbs in the bottom of the trench 6-8 inches deep, leaving about as much space between each bulb as the size of the bulb itself. Cover with soil, being careful not to overturn the bulbs in the process. The loosened soil can be used to create a raised bed over the tulip bulbs. Water the bed well. A pre-emergent herbicide can also be applied at this time to prevent winter and early spring weed germination.

Low tunnels can be constructed over field-grown tulips in early February to force the bulbs into flower in late March, three to four weeks before other field tulips. For information on how to construct a low tunnel, contact the University of Maryland Cooperative Extension or visit the Pennsylvania State University’s Center for Plasticulture website at http://plasticulture.cas.psu.edu/.

Low tunnels will also help to protect early crops from deer. Later crops can be protected by placing posts along the sides of the bed and using row cover vertically along each side of the row. Rope zigzagged down the row will support the row cover above the tulips. If the row cover rubs the flowers, the buds will be damaged.

Fertilization

Proper fertilization for tulips used as cut flowers is important. Tulips are not  considered high feeders, and the bulbs themselves store many nutrients for the plants’ initial growth. Excessive fertilization can lead to reduced plant height, which may affect marketability. The use of slow or controlled release fertilizers is not recommended as the plant will be harvested before most of the nutrients are released. For field production,
use 1.5 pounds of nitrogen per 1,000 square feet at planting, making sure not to allow the fertilizer to come in contact with the bulbs.

After bulb emergence in the spring, the application of one pound of actual  nitrogen per 1,000 square feet (1.6 oz per 100 sq ft) of row is recommended. The substrate should have a pH of 6.2 to 6.8 for the greatest nutrient availability and plant growth. Adjust phosphorus and potassium to the optimum range based on soil tests.

In greenhouse and high tunnel production fertilization should begin after shoot emergence. When the shoots are 2 to 2.5 inches tall use a fertilizer with a 2:1 ratio of calcium nitrate to potassium nitrate after amending the substrate for phosphorus and potassium. Weekly application of this ratio can be used, or a fertilizer injector can be utilized to supply 200 to 250 ppm of N on a constant basis using a well-balanced fertilizer (either 2:1:1 or 3:1:1 nitrogen to phosphorus to potassium ratio) that includes a small amount of phosphorus and potassium. With a soluble salt meter, use the 1:2 dilution method to monitor the electrical conductivity. Acceptable readings should be between 1 and 1.5 mS/cm (mmhos/cm).

Harvesting and Postharvest Handling

Tulips are harvested when petals show color, but are not completely colored or open. When harvesting, tulips can be cut or pulled. Crate-grown tulip stems are cut at harvest time. Tulips grown in the field are pulled at harvesting to increase the stem length. If the soil is soft, the bulb usually comes up when the stem is pulled. When pulling tulips, grasp the stem at the soil line and pull straight up. The stem should snap off at the bulb, producing a longer stem. The used tulip bulbs should be discarded after harvest. This pulling method removes the used bulbs from the bed, making bed preparation for the next cut flower species easier. Rotating tulips