Berks County Cooperative Extension - Forcing Bulbs for Indoor Bloom

Forcing Bulbs for Indoor Bloom

What is "forcing?"

Forcing allows gardeners to give spring an early start. Starting blooms indoors over the winter months can fill your home with fragrance, freshness, and bright colors-just when you need it most.

Forcing is a term used to describe the process of manipulating a plant's growing environment, allowing it to bloom earlier than usual. All hardy bulbs require a cold or chilling period and then an increase in temperature in order to bloom. A bulb planted outside in the fall and winter gets this cold period naturally and has to wait for warmer spring temperatures to bloom. The forcing process mimics the outdoor winter climate, but brings it into warmer temperatures before nature is able to provide it. This premature dose of warm weather is what "forces" the blooms to emerge earlier than naturally possible.

Almost any bulb can be forced indoors, but some are easier to do than others. Hardy bulbs are best forced in a soil media, while some are capable of being forced in water only.

Forcing is easy to do. All that is needed are the proper supplies, location, and a little planning and patience.

How do I choose the best bulbs for forcing?

Early blooming type bulbs usually work the best. Hyacinth is quite reliable, where a tulip may be more challenging. Quality bulbs can be purchased from garden centers and mail order catalogs. If you have the opportunity to choose the bulbs yourself, choose large, firm bulbs without blemishes, spots, or bruises. For indoor forcing, the bigger the bulb, the better. Many catalogs will label bulbs as "good for forcing."

How can I force bulbs in just water?

A few bulbs are able to bloom by being placed in a container filled with water instead of a soil-filled pot. The easiest bulbs to force using this method are hyacinths and narcissus.

Hyacinths do need a period of cold treatment. This cold period usually takes about eight weeks or until the hyacinth is well rooted. This is most easily done by placing the bulb in a special hyacinth glass that has an hourglass shape. Fill the bottom section of the glass with water so that it touches the bottom of the bulb. The bulb is placed in the top portion of the glass. Place the glass in a dark place at 40-45oF for about eight weeks. A cellar, shed, or garage during the months of October and November can provide the right environment. So can a refrigerator. It is important that the bulb remains dark. Placing a black plastic bag over the bulb glass can keep the light out. Periodically add water to the glass as needed to keep the roots growing.

After eight weeks, the roots of the bulb should be well formed and a flower bud should be extending two to three inches above the neck of the bulb. The hyacinth should then be moved to a warmer room in your home that receives full daylight. This warming temperature will encourage spring-like growth and a fragrant bloom within four to five weeks.

Specially prepared hyacinths can be purchased from catalogs or garden centers. These prepared bulbs have already received their cold treatment and are ready to bloom in warm temperatures. Purchasing these bulbs can make the forcing process very easy. Just place in a glass and enjoy!

Paperwhite Narcissus are the easiest bulbs to force because they do not require a cold treatment in order to bloom. Select a container, dish, or shallow bowl. Fill the container halfway with pebbles, stones, or marbles. Settle each bulb into the pebbles. Fill the container with water until the bottoms of the bulbs are touching. Even though paperwhites do not need a cold storage, they should be started on the cool side and then moved to a sunny window in your kitchen or living room. This gradual temperature change will discourage the paperwhites from becoming stretched or too leggy. They will flower in three to five weeks and their fragrance will permeate the air.

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How do I force bulbs in soil?

Some of the best bulbs to use for forcing in soil are hyacinths, crocus, daffodils, tulips, and amaryllis. An amaryllis can be purchased, potted up, and placed in a sunny window without cold treatment. All others need cold treatment.

Containers. First, choose a clean container. A standard four- to six-inch pot or a shallow bulb pan works well. There are also lots of decorative containers as well. A low, wide container allows for more planting space of bulb clusters. As long as it has good drainage, it should work. Place a few crock pieces or flat stones in the bottom of the pot to prevent clogging and loss of soil.

Soil. Fill the container 1/4 full with a good potting soil. Use a mix of 1/3 garden soil, 1/3 sand, and 1/3 peat-or purchase a prepared bag of a similar mix. Purchasing a sterilized potting soil is wise to be sure it is free of insects, disease, or weed seeds. Fertilizer is not necessary since the bulb has the food reserves it needs to grow.

Planting. Place as many bulbs of one kind that will fit in the container, leaving 1/2 inch between each bulb. Do not let the bulbs touch each other or the side of the container. A six-inch pot will usually hold six tulips or 15 crocuses. Plant hyacinths singly or in groups of three. Cover and firm the soil so that the tip of each bulb is above the soil line. Leave a half inch between the soil line and the top of the container for watering. Soak well with water immediately after planting.

Cold Treatment. Once the bulbs are planted, they need a cold treatment in order to bloom. Most bulbs need eight to ten weeks of cold treatment. All bulbs need sufficient root growth and about one to two inches of top growth before they can be removed from cold storage. The best places for cold storage are a cool cellar, cold frame, or a pit or trench outside. Wherever you choose, the location needs to be a fairly constant 40-50oF temperature. It should be dark and should not reach freezing temperatures. You may need to place boxes or burlap over the pots to keep them dark.

To make a trench outside, select a well-drained, cool spot. Dig a hole or trench, 12-18 inches deep. Fill the trench with three inches of small gravel. Put the planted bulb pots on top and sprinkle a half inch of sand on top. Then fill the trench with soil and apply six inches of straw after the ground freezes.

After 8-10 weeks, check the pots for masses of white root growth and a visible flower bud. Do not remove from the cold storage before this stage. Timing of all processes does vary depending on type and variety of bulb grown. If it is ready, move it to a warm sunny location. Keep the pot well watered and turn the pots from time to time to insure even growth. In a few more weeks, you should be enjoying colorful blooms, even though there may be snow outside! Keeping the blooming plants away from a direct heat source will prolong their bloom time.

What should I do after it blooms?

Once the blooms have faded, cut back the flower stalks. If you intend to keep the bulb, allow the leaves to remain and continue watering until they have died back on their own. Store the bulbs in their pots in a cool, dry place until autumn, and then plant them outdoors. Do not try forcing the same bulb over and over. A forced bulb will only bloom once indoors without another cold storage treatment. Forcing uses a lot of the bulb's energy and food reserves. Next year's garden display of this previously forced bulb will no doubt be less. But after more food reserves have been built up, it will surely show off again. Paperwhite narcissus are not hardy and will not bloom again, indoors or out.

Prepared by Laura McNutt, Extension Agent. August 2001
Document posted: August 2004

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