Creating Spring

by Juergen Steininger

Guidelines for forcing bulbs with notes for the home grower

Longwood Gardens employs forcing methods for tulips and narcissi recommended by the Dutch bulb growers.

The first step in successfully forcing bulbs is the selection of suitable cultivars. Not every available tulip or narcissus does well as a pot crop. In general, many of the Triumph type tulips are suitable but one should spend a little time in studying a good bulb catalog. Many catalogs from companies specialized in forcing bulbs indicate the best use for each cultivar. The three major classes of use are cut flower, pot crop, or outdoor plant. A good catalog also provides information on the chilling requirement and greenhouse forcing time. Each cultivar has a different chilling requirement and the bench time requirement varies greatly among varieties. It takes a bit of planning to get a group of narcissus and tulips flowering on the same date.

People interested in forcing bulbs should be looking now for suitable varieties at flower shows, catalogs, display gardens and the California Pack Trial. Now is also a good time in placing early orders for bulb delivery in the fall, and to calculate the potting dates based on the desired bloom dates.

Bulbs offered in flower shops in the autumn may be satisfactory for outdoor planting in the garden but frequently are inadequate for pot forcing. For forcing use only top grade bulbs (the largest bulbs). Upon arrival, all bulbs should be carefully checked for disease and injury. Moldy or damaged bulbs should be discarded since bad bulbs most likely perish during forcing and may infect healthy plants.

Another aspect often overlooked in forcing tulips is the proper programming of the bulbs. Forcing bulbs should be in the â€œG-stageâ€ before initiating chilling. This term is used to describe a developmental stage of a bulb. G-stage bulbs have completed developing the entire flower shoot inside the bulb. In other words, a small tulip flower bud is present inside the bulb. Raw bulbs have not been programmed, and are unpredictable when forced; therefore, it is important to order the proper bulbs from the supplier.

It is time to start forcing tulip bulbs that were planted in October last year and have completed their chilling requirement. The chilling requirement for each cultivar is different and your supplier or a catalog should give you this information. For example Tulipa â€˜Rosalieâ€™ requires 16 weeks chilling and takes about 15 to 20 days to reach flower in the greenhouse. Bulbs that were planted in the third week of October were ready to be moved to the bench on February 9 for a projected flowering date of February 26.

POTTING MEDIA, POTS AND FORCING ENVIRONMENT

A good potting media for bulbs is very porous and rather heavy in order to wedge the bulbs into the pot. Longwood currently uses a mix containing 50% course sand, 30% peat moss, 10% Terra Green and 10% vermiculite. This is a very heavy mix and each pot weighs a â€œton.â€ Nonetheless, it is a very good mix with good drainage and it is heavy enough to hold the bulbs down in the pan.

Plastic bulb pans work well and are available in many sizes. The size of the pan essentially is dictated by the final size of the plants. Small cultivars such Narcissus â€˜Jetfireâ€™ should be planted in a 6-inch bulb pan with 7 bulbs per pan. Very large cultivars such as Tulipa â€˜Dordogneâ€™ do best in a large 8-inch pot with 7 bulbs per pan. As a rule, only odd numbers of bulbs are planted into a pot. It is also important to make sure that the flat side of a tulip bulb is facing out toward the wall of the container. This insures that the shoots grow toward the center of the pot and are not flopping outward.

The potted bulbs are watered and promptly moved into a cooler with a temperature set point of 48F. The pots stay in this cooler for a few weeks until roots start growing through the water holes on the bottom of the pot. At this time the pots are moved to a cooler with a temperature of 41F. They will remain in this cooler until shoots have grown to about one inch long. The pots are then moved to a cooler with a temperature of 33F in which they will complete the chilling requirement. Completed pots are moved to a greenhouse bench and are forced into flower.

The greenhouse temperature during the day is 62 to 67F and night temperature is 60 to 65F. This temperature setting is to optimize plant development and to minimize the bench requirement without significant reduction of overall crop quality. In fact, plants grown at lower temperatures may be sturdier, however, production time would increase. Tulips should be ffertilized with a 300ppm liquid fertilizer application every watering. The fertilizer of choice is Calcium Nitrate to prevent stem topple.

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