There is nothing quite so delightful or rewarding in the dead of winter than to step into a greenhouse filled with golden daffodils, brilliant tulips of many colors, and fragrant white or blue hyacinths. Literally, you find yourself in springtime, not merely for a day or two, but continuously, for weeks on end all this with a few days of effort!
Most of the hardy bulbs are the spring-flowering kinds, and they are true bulbs, that is, they are composed of fleshy scales that protect a bud at the base, the embryo flower forms inside the bulb, and the bulbs are usually encased in a very thin, dry skin. "Forcing" these bulbs means only that you beguile them into blooming in your greenhouse weeks or months earlier than they would bloom naturally out-of-doors. They must be potted, placed in a cold, dark location while roots develop, and then brought into a cool greenhouse where foliage and flowers will mature.
Hardy bulbs may be potted any time from early October to the middle of December. Six to eight weeks must be allowed for root development, so plan to do your potting as early as possible, or according to the flowering schedule you desire.
Good bulbs are not inexpensive, but it is better to buy fewer good bulbs, if necessary, than to settle for a larger quantity of average-quality bulbs. Send for catalogues (usually they are profusely illustrated in color) from several bulb specialists. You will find many colors and types of tulips, hyacinths, and daffodils to choose from, but you will be better satisfied if you select bulbs described as good forcing varieties. Select carefully, and you will have a brilliant display for several weeks in spring, before your other plants are in flower. Do not overlook the white-flowered plants nothing is more pleasing than a burst of white against a background of color. Bulb dealers fill orders for top-quality bulbs in the sequence in which they are received, so assure yourself of getting what you want by placing your order early.
Most bulb dealers offer a few pre-cooled forcing varieties for very early greenhouse flowering. If you cannot obtain pre-cooled kinds, you can condition them yourself by placing the unpotted bulbs in an un-heated garage or other building for several weeks. My first bulbs, acquired late in the season because my greenhouse was not in operation until January, were purchased in the local variety store and kept in the refrigerator for three weeks. Pre-cooled bulbs must be potted immediately upon receipt or upon being removed from their cool treatment, and of course they require the same cold rooting period that all hardy bulbs must have.
A soil mixture consisting of two parts garden loam, one part clean sharp sand, and one part peat moss is fine. Little if any fertilizer is needed, since high-quality bulbs store their own supply of food. If you think your soil might be inadequate in nutrients, add bone meal to your mixture it acts very slowly and provides nourishment without damaging roots.
Special pots called bulb-pans actually shallow flower pots that are wider than they are deep are best for potting tulips, hyacinths, and daffodils; 4-inch-deep flats are also adequate. For the most attractive display, plant three or four bulbs of one kind to a bulb-pan, depending on the size of bulbs and pan. If flats are used, bulbs can be spaced about 1 inch apart. A single bulb may be planted in a small pot, but the flowering display will not be very dramatic.
Place a layer of pot shards or gravel in the bottom of the pot, then a layer of charcoal to "sweeten" the soil, then a layer of soil mixture. At this point, you will have to predetermine the depth to which the bulbs will be set into the pot in order to leave about 1 inch of space above the top of the soil when potting has been completed. When you have the proper depth of soil, place the bulbs in position gently on this soil bed and fill in around them and over the tips with soil. Shake the pot to settle the soil around the bulbs. Water thoroughly; this one watering will be sufficient until growth begins. Label or mark the pots in some manner so they can be easily identified when you start to bring them into the greenhouse for forcing.
For the root-making period, the bulbs may be handled in any of several ways. Select the method most practical for you:
Trench. Dig a trench about a foot deep in a well-drained section of the garden. Cover the bottom with 3 to 4 inches of sand, cinders, or gravel to assure good drainage. Place the bulb-pans or flats on this base and cover with soil. Just before freezing temperatures are expected, cover the trench with a layer of straw, salt-hay, or leaves. This keeps the ground from freezing solid and enables you to remove the pots with ease, once the roots have developed.
Coldframe. If you have a coldframe, place the pots or flats in it and cover with salt-hay, leaves, or sand. Put on top sash when temperature goes to freezing.
Above ground. Along one side of your greenhouse outside, place a row of stakes about a foot out from the wall. Enclose the staked area with a temporary, makeshift retaining wall of screen or boards. Place a layer of sand on the ground in the enclosed area, and position the pots or flats on this sand base. Fill in, around, and over the pots to a depth of about a foot with leaves. This worked very well for me one winter.
Refrigerator. Fill a refrigerator (the old one in the garage or basement that you use for soft drinks during summer) with potted bulb-pans and set the temperature control at its lowest point. Open the door as seldom as possible, though you will have to check the pots about once a week for signs that the soil might be drying (because of efficient air circulation in a refrigerator); most likely you will need to water the pots several times during the eight-week rooting period.
Even though this is a period for root development, some plants will start top growth before they are ready to be removed to the greenhouse. Therefore, do not place one pot directly on top of another. In any of the four cold areas suggested above, however, you can make double tiers by inverting an empty pot over each one in the bottom layer and then stacking a second layer on top of the inverted pots.
Occasionally, a hardy bulb will develop sufficient roots in about six weeks to be ready for forcing, but usually eight full weeks should be allowed. By the end of the eight-week period, the pot should contain a mass of well-developed roots which have started to grow through the drainage hole in the bottom of the pot. That, in fact, is how you will know that a pot is ready to go to the greenhouse when you see roots growing through the drainage hole.
When this stage has been reached, you may start bringing the pots into the greenhouse for forcing into bloom. Bulbs are not injured by a longer cold storage, however, so you will not find it necessary to transfer all pots to the greenhouse at the same time. If you have marked the pots carefully, you will be able to select one or two of each kind for an interesting display. A succession of bloom is possible over a long period of time if you bring in only a few pots at a time.
It is important, when the bulbs come into the greenhouse, to raise their temperature gradually to assure development of foliage and stems before forcing the blooms. Water well and place the pots in a cool, dark place, as under a bench, for about 10 to 14 days. A 50 F temperature is ideal for this period of development. If stems do not seem to be stretching properly, place a section of cardboard tube (such as the core of a roll of waxed paper or aluminum foil) or a homemade paper cone over the top growth. Stems will "reach" for the small area of light at the top of the tube or cone. Inspect periodically, and when stem is of adequate length, remove the tube so as not to interfere with or deform the flower.
At the end of this two-week period, move the pots onto the bench in full sunlight, since more warmth is necessary to promote flowering. Water freely until flowers open. Then move the plants out of direct sun so flowers will last longer. If you maintain a temperature of 50 F, it will take about five weeks to force bulbs into bloom; if the temperature is raised to 60 F, blooms will appear in three to four weeks. But heed a word of caution: don't be carried away with the idea of raising the temperature simply to force bulbs there may be other plant material in your greenhouse that will not tolerate a 10-degree increase in temperature. Also, bulbs that are grown slowly and cool will reward you with stronger, healthier, longer-lasting blossoms.
Once a hardy bulb has been forced, it cannot be forced satisfactorily a second year. It should be discarded or transferred to the outdoor garden. If you plan to use it outdoors, continue to water and give plenty of sun until the foliage ripens, to guarantee the development of the new flower bud and stem within the bulb. Remove bulbs from pots and plant in their permanent outdoor location, any time after the ground is workable. They will not flower their first year outdoors, probably, but after that they should perform very satisfactorily.
The small hardy bulbs that we are so happy to see flowering outdoors in spring respond to greenhouse forcing as easily as the more spectacular tulips, hyacinths, and daffodils. You may not have time or space to devote to these small, modest flowers the first year in your greenhouse, but try to work in a few pots of crocus, grape-hyacinths (Muscari), scillas, and snowdrops (Galanthus) anyway.