Forcing Flowering Bulbs

Many gardeners delight in producing a breath of spring during the cold winter months. The breath of spring I'm referring to is forcing spring-flowering bulbs. Bulbs can be purchased at garden centers, flower shops, or through mail-order suppliers in the fall. Select bulbs that are large, firm, blemish-free, and of uniform size. Forcing bulbs indoors requires forethought. Most bulbs require at least 12 to 16 weeks of cold treatment to initiate a well developed root system, stems, and flower buds. Giving the bulbs a cold treatment does not mean putting the bulbs in the freezer for part of the winter. Rather, potted bulbs are watered and placed in the refrigerator or another location that is cool but does not freeze. Besides the refrigerator, bulbs can be chilled by placing the containers in a 1 foot deep outdoor trench or cold frame and covering with dry straw or leaves. Other possibilities include an unheated garage or root cellar. Ideal rooting temperatures are between 40 and 45 F.

Bulb forcing requires good quality bulbs, a well-drained potting soil, and containers with drainage holes in the bottom. At planting, fill the container 1/2 to 3/4 full with potting soil, set the bulbs close together on top of the mix and adjust the soil level until the bulb tips are even with the rim of the container. Most bulbs are most attractive when planted in odd numbered groups like 3 or 5. Fill the remainder of the container until the bulb tips show just above the soil surface. Water containers thoroughly before putting them into cold storage and water regularly during the storage period. Placing containers inside plastic bags will help to conserve moisture. When working with flat sided bulbs such as tulips, plant the flat side of the bulb toward the sides of the container.

Many plants begin sending up shoots before the cold treatment is complete. Be careful not to damage these shoots. If containers are stacked on top of one another, this is the time to arrange them in a single layer to prevent shoot damage. Following the appropriate length of cold storage, the potted bulbs are ready to force indoors. Place the containers in a semi-dark location with a temperature of about 60 F. After a few days, place containers in a well-lighted location at normal room temperature to flower. Flowering usually occurs within a few weeks. To enjoy a succession of bloom, remove a few pots every 2 weeks. For extended flowering, keep plants in a moderately lighted location at fairly cool temperatures (65 F) and away from hot and cold drafts. Water plants when the soil surface is dry to the touch.

Some excellent choices for bulb forcing include:

Plant Cold Treatment (weeks) Weeks to Bloom
Crocus (Crocus spp.) 15 2-3
Daffodils (Narcissus spp.) 15-17 2-3
Fritillary (Fritillaria meleagris) 15 3
Glory-of-the-snow (Chionodoxa luciliae) 15 2-3
Grape hyacinth (Muscari spp.) 13-15 2-3
Hyacinth (Hyacinthus) 11-14 2-3
Iris (Iris danfordiae and I. reticulata) 15 2-3
Paperwhites (Narcissus tazetta) none 5-6
Puschkinia (Puschkinia libanotica) 15 2-3
Scilla (Scilla spp.) 15 2-3
Snowdrop (Galanthus nivalis) 15 2
Tulip (Tulipa hybrids) 14-20 2-3
Winter aconite (Eranthis hyemalis) 15 2

Through proper scheduling (planting and bringing out of storage), spring flowering bulbs can be blooming for you from December through spring. Once bulbs have been forced, many can be planted outside where they will flower again in a year or two. Most tulip bulbs should be discarded after forcing. Most tulip bulbs are quite small and weak after forcing and don't perform well when planted outdoors.

This article originally appeared in the November 11, 1994 issue, p. 153.


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on November 11, 1994. The information contained within may not be the most current and accurate depending on when it is accessed.