Gerald Klingaman, Ph.D., is an emeritus professor of horticulture at the University of Arkansas. He is now working full time as Operations Director at the Botanical Garden of the Ozarks.

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A Show of Force
by Gerald Klingaman       #Bulbs

Tulips are a bit more challenging to force, but well worth the effort. Stock photo.

Bulbs have always intrigued me. Their much-appreciated splash of color during a generally bleak time of year brightens our lives and reminds us that warmer days are ahead. Forcing bulbs is just another way of enjoying the jewels of the late winter and spring garden, but you get to schedule the show. Let’s explore the mystery of bulbs and discuss the techniques involved in forcing them into flower.     

Basic Bulb Botany

The accompanying illustration shows a comparison of the internal anatomy of a hyacinth, an amaryllis and a lily bulb in the fall. The hyacinth bulb clearly shows the well-formed florets for next spring’s blossoms. The amaryllis bulb shows the extending floral scape on the left and the sharply pointed vegetative meristem that will form new leaves. The lily bulb shows no flower bud formations because they develop at about the time the stem begins to emerge in the spring.

Forcing spring blooming bulbs is pretty straightforward, because the flower is already formed in most situations. All we’ve got to do is grow some roots, provide a bit of chilling and then force the bulb into bloom. Generally speaking, the earlier in the year that a bulb blooms, the easier it is to force. Crocus are easier than narcissus which are easier than tulips.

The hyacinth bulb clearly shows the well-formed flower (left) and the amaryllis meristem shows a floral scape ready to grow (center), but the summer blooming lily (right) doesn't form flowers until it begins growing after chilling, so it is a bit tricky to force. Photo by Gerald Klingaman.

Selecting the Best Test

When buying bulbs to force, select those with flowers that attract you, but stay away from tall tulips or late-season alliums. If you are new to the forcing game, hyacinths, crocus and grape hyacinths are almost foolproof. Tulips and narcissus are a bit more challenging, but not by much.

Top size bulbs are best for forcing. The accompanying table provides some guidelines on the number of bulbs to plant per pot. When selecting a pot, keep in mind the concept of proportionality. A standard pot is as tall as it is wide, whereas a “bulb pan” is half as tall as it is wide. Ideally, the plant should be 1 ½  to 2 times as tall as the pot it is growing in. So, if the tulips grow to 16 inches tall, a 6- to 8-inch standard pot will work fine. But, if you are forcing crocus and put them in a 6-inch pot, they look odd. Instead, use the shorter bulb pan or even a 4-inch pot. Sometimes I force bulbs in 12-inch pots, but to get a better effect I double layer them, so the number of bulbs needed almost doubles. The more bulbs planted, the more flowers you’ll have. Use a variety of pot sizes and species to increase the visual effect of the project.

Table 1.  Guidelines for forcing spring blooming bulbs

Name Pot Size Bulbs/Pot Chilling Temp. Weeks of Chilling* Comments
Crocus 4 inch 6 to 10 45 6 Probably the easiest to force of all spring bloomers
Tulips 6 to 8 inch 6 to 10 45 12 The single and double early classes and the Triumph types are best for forcing
6 to 12 inch 4 to 10 45 8 to 10 Generally easy to force with early blooming types the easiest
6 to 8 inch 6 to 10 N/A None Easy to grow in soil or in shallow pots of gravel, the traditional Asian approach to forcing; can overpower a room with their fragrance
Amaryllis crowd 6 to 8 inch 1 to 3 55 8 Only chill those you have been growing for several years; several amaryllis bulbs will fit in a shallow bulb pan; it doesn't seem to hurt blooming
Hyacinth 4 to 6 inch 1 to 3 45 6 Very easy to force and always nice to have a few around in the spring to perfume the house
Grape hyacinth 4 inch 6 to 10 45 6 Easy to force and makes a nice tabletop decoration

*If you re-forcing outdoors, add two weeks to the chilling time.

Crowd several amaryllis bulbs in a large pot and grow them in a tight clump. They don't seem to resent being crowded.  Photo by Gerald Klingaman.

Planting Pointers

Any good potting soil will work as a rooting medium. When you pot the bulbs, remember that the roots may come out of the bottom of the container, so plant them high. Generally, the soil should come up to the shoulder of the bulb. As soon as the bulbs are potted, water them thoroughly and make sure all of the soil is wet. It is critical that the soil be kept moist during the chilling phase so the roots can develop.

If you are planting tulips, look for the flat side of the bulb and place it next to the pot rim. The first leaf will emerge from that side so the resultant plant will be more symmetrical. Don't be afraid to let the bulbs touch, but if you spy any with gray mold eruptions on the surface, discard them.

Chilling Criterion

Once potted, most bulbs need to be chilled. The ideal chilling temperature is between 40 and 45 F. The beer refrigerator in the garage set at medium to high temperature is ideal, but you can also use natural outdoor cooling. If you use a refrigerator, make sure the pots are inspected every couple weeks and water if the soil becomes dry.

Two exceptions to the temperature rule are worth noting. Paperwhite narcissus need no chilling and can be planted as soon as they are available at the nurseries. If you are buying new amaryllis bulbs, they too can be forced without any special treatment, because many of these bulbs come from production in the southern hemisphere, and to them, it’s spring. In contrast, with amaryllis that have already been growing for several years, I’ve found that chilling the bulbs at 55 F for eight weeks breaks flower bud dormancy. If they receive this chilling treatment they will bloom in just over a month, but without it, they don’t want to bloom until late March or April, regardless of how long you store them dry.

The crocus (left) and hyacinths (center) have had sufficient chilling to force, as judged by the amount of rooting and the fact that the stems are beginning to elongate. The daffodils (right) need another couple of weeks of chilling as they haven’t made enough stem growth yet. Photo by Gerald Klingaman.

Cooling Naturally

Outdoor cooling is a bit more unpredictable given the odd weather we have been experiencing, but most areas of the Southeast have ideal soil temperature conditions for natural chilling. Soil temperatures fall below 50 F by early November in most areas of the South and stay there until early March. While the accumulation of chilling units is not as fast at 50 F as it is at 45, it will still work. Select a shady area on the north side of a building where the soil temperature will be cooler and experience less fluctuation. If the bulbs freeze any time during the chilling period, they will almost certainly be killed.     

You could bury the pots in a thick layer of mulch on the north side of the house, or to make it a bit less messy, you can dig a rooting pit. Dig a shallow pit 12 inches or so deep, and as large as needed to accommodate your pots, in a shaded area with good drainage; sit the pots in the hole and then cover them with a sheet of Styrofoam insulation (it comes in 1- or 2-inch thicknesses). Be sure to leave some “head room” for new stem growth. The new growth is an indicator that the bulbs have had sufficient chilling hours. And remember, don’t let the pots get dry.

The chilling time varies from six weeks for crocus and grape hyacinths to 12 weeks for most tulips. Hyacinths need about eight weeks. Most narcissus require a minimum of eight weeks, but 10 weeks is probably better. Because of the unpredictability of outdoor weather conditions, give bulbs that have been chilled outdoors two more weeks than those chilled in a refrigerator. The bulbs will tell you when they have had sufficient chilling –– roots will begin to grow from the bottom of the pot and the stems will begin to elongate. I like to have an inch or two of shoot growth before I bring them indoors to force. Don’t rush chilling, because if they haven’t had enough chilling time, the stems will be very slow growing and flowering will be delayed.

Hyacinths make a cheery and sweet-scented addition to the home in winter. Photo by Gerald Klingaman.

Paperwhite narcissus are ready to plant when purchased and require no special treatment. Crowd lots of bulbs into the pot and allow them to grow in a cool, bright environment to prevent stem stretching.
Photo by Gerald Klingaman.

Scheduling Your Crop

If you want to aim for a specific blooming date, you can try to schedule your crop. Say for example you want to have tulips ready for St. Valentine’s Day – it takes about three weeks for the plants to flower once they are brought indoors, so that means you will need to count backwards from February 14 to January 24. If you are using the refrigerator for chilling, begin the treatment November 1 (12 weeks). If you are planning on outdoor chilling, I would back that up two more weeks and plant the bulbs and begin the chilling on October 18.

The Growing Enivroment

The trickiest part of producing high-quality flowering bulbs indoors is to provide the right temperature during the growing phase. Ideally, a bright sunroom with an average daily temperature of 55 F is best. Few of us have the perfect temperature and are forced to make do with a windowsill or porch. The warmer the average daily temperature is, the faster the plants will grow and become leggier. Cool temperatures and bright light during forcing is the secret to preventing excessive stretching.

After the plants have finished flowering, set the pot outside and let the foliage ripen naturally. Except for tulips, most other bulbs can be planted in the garden and should bloom again the following spring.

If you’ve never tried forcing bulbs, you are missing one of the most entertaining aspects of these springtime jewels. Give it a try – you’ll like it!


From Sate-by-State Gardening October 2005. Photos by Gearld Klingman and courtesy of Stock photo.


Posted: 10/31/12   RSS | Print


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