Gardening How-to Articles

How to Grow Hardy Bulbs Indoors

As each March approaches, I eagerly look for some early sign of spring. For me it is seeing the crocus push their way up through the partially frozen ground. I watch for them for days, until the one sunny day they burst into bloom and I know that spring is truly on its way. That day can seem to take forever, but with a little planning in the fall, you can enjoy a whole spring garden of colorful tulips, fragrant hyacinths and of course crocus—all indoors in February.

To get this spring bloom, you will need to "force" the bulbs. Standard forcing involves creating the conditions for hardy spring flowering bulbs—tulips, hyacinths, narcissus, crocus and other temperate zone bulbs—to flower when they wouldn't normally bloom. You can encourage them to do so by creating an abbreviated facsimile of their natural winter environment—cold. The cold that you supply will be of shorter duration than a natural fall and winter, but long enough to induce the bulb to send out roots.

Commercial bulb growers have been forcing hardy bulbs for many, many years, and forcing has evolved into an exact science with guidelines and schedules that ensure success. By applying the rules of this science, you can decide when you want the plants to bloom and work backward to determine when you should start the process.

Getting Started

To begin, select the flowers that you want on your windowsill to brighten your winter days. Choose named varieties that have been forced successfully over the years; these most likely will present the fewest problems. Among tulips, traditionally forced cultivars include 'Apricot Beauty', 'Christmas Marvel' (rose or red), 'Attila' (purple with a white edge), 'Paul Richter' (red), 'Page Polka' (pink with a white edge) and 'Golden Melody' and 'Hibernia' (white). Tried and true narcissus cultivars include 'Ice Follies' (white), 'February Gold' (yellow), 'Dutch Master' (yellow), 'Mount Hood' (white) and some of the shorter-growing cultivars like 'Tete-a-Tete' (yellow) and 'Jack Snipe' (white petals with a yellow trumpet). Almost all hyacinth cultivars force nicely, including 'Delft Blue', 'Ostara' (deep violet-blue), 'Pink Pearl', 'L'Innocence', (white) 'City of Haarlem' (yellow) and 'Hollyhock' (double pink). Among the best crocuses to try are Dutch hybrid cultivars such as 'Remembrance' (purple), 'Joan of Arc' (white) and 'Yellow Mammoth'.

When selecting any bulbs for forcing, look not only for tried and true cultivars but also for the largest, healthiest bulbs, and make sure they are firm, clean and unbruised. Tulips and hyacinths have a tunic, an outer paper-like covering that protects the bulb, which should be intact.


Next you will need containers, labels, planting media and a cool place to keep your bulbs during their cold treatment. Ideal containers are about 6 inches deep and 6 to 8 inches across, with holes in the bottom to allow for drainage. Each 6-inch pot will hold five tulips, four hyacinths, three to five narcissus (depending on the varieties), or ten to 12 crocus. Clay and plastic are the most popular materials for pots, but remember that clay pots will dry out faster than the plastic ones—and adequate moisture during the cold treatment is essential for success. If you choose to plant in clay pots, monitor the soil carefully, as it will need water more often than if you use plastic.

Various potting media are available. The ideal forcing medium is well drained, yet able to hold enough moisture for adequate root production and to support the flowering bulbs. I use soil, sand and compost in equal parts.

Next, think ahead and decide when you want your flowers to bloom. If that's early February to mid-March, plant your bulbs from early October to mid-November to give them the 13 to 15 weeks of cold treatment hardy bulbs require to bloom. Plant them with their noses (tips) just above the soil level in the pot, or to about two-thirds their length. Tulip bulbs have a curved side and a flat side—not to be confused with the basal plate where the roots will emerge. Plant your tulip bulbs so that the flat side faces the outside of the pot. This assures that the plant will unfurl its first leaf toward the perimeter of the pot, and will have a fuller look. Cover crocus and grape hyacinth with no more than an inch of soil.

After planting, all bulbs need to be thoroughly watered. Remember to label each pot with the name of the cultivar you plant, the planting date and the date it should come into the house.

Out in the Cold

Now place the bulbs where they will get a minimum of 13 to 15 weeks of 35 to 48 degrees F cold. Some tulip and narcissus cultivars require an average of 17 weeks, but don't keep any bulbs in cold treatment for more than two weeks longer than maximum recommended cold time, as you will end up with flowers of poor quality. You're also likely to get low-quality flowers if you don't give bulbs the minimum cold treatment.

Look around your house for a spot that's sure to be cold throughout the winter—an unheated garage, patio or basement, for example. Old refrigerators can also work very well, but don't store any fruits— especially apples—in the refrigerator with the bulbs, because the ethylene gas produced from the ripening fruit can cause the flowers to "blast," or develop improperly. Frost-free refrigerators can make the plants dry out quickly, so pay special attention to bulbs in modern fridges and make sure they have enough water.

For the next several weeks it is vital that you keep the soil evenly moist and watch the temperatures, because it's during this time that the bulbs are producing roots. Without an adequate root system your plant won't develop properly and you're likely to get poor-quality flowers. Fully developed roots may take between four to eight weeks to grow—be patient. Bulbs root best at temperatures around 45 to 48 degrees F. After a few weeks at this temperature, you may see roots by looking at the hole in the bottom of the pot. If the roots have grown down that far, it's a good indication that the bulbs are well established.

Once well rooted, bulbs prefer an even lower temperature for shoot development—38 to 42 degrees F—the temperature range they experience outdoors. If your bulbs are in a controlled spot like a refrigerator or greenhouse, lower the temperature to within this range. In the northern tier of states, bulbs in an unheated garage or patio will also get enough cold. Your bulbs' emerging leaves will be white because they lack chlorophyll, but they'll green up when you bring them into the light. When the leaves of hyacinths, tulips and narcissus are 1 to 2 inches tall, or the sheaths of crocus and other minor bulbs are an inch tall, the plants are ready for forcing.

If they achieve the heights described above and are still growing but have not met their minimum cold requirement, put the bulbs in an even colder spot (33 to 35 degrees F) for the duration. By 15 to 17 weeks the bulbs should be well rooted and have leaves about 1 to 2 inches tall.

For those seeking more instantaneous results, look for bulbs that have been pre-cooled; these will have gone through some variation of the cold treatment described above, depending on the bulb and the supplier. You should receive instructions from the nursery on how to proceed. Windowsill gardeners even more impatient will probably find potted bulbs ready to burst into bloom in late winter at their florist, farmer's market or supermarket.

Forcing Your Way to Flowers

Next comes the forcing. If you bring your plants in from the cold a few at a time, you can stretch the flowering season over many weeks. Bring the pots into the house and put them in a 55 to 60 degrees F sunny area such as a windowsill or a table near a window where they will get at least eight hours of light but temperatures no higher than 60 degrees F. Nighttime temperatures should stay within a 5 to 10 degree F range of daytime temperatures. Make sure your plants get enough moisture.

Because each bulb has everything it needs to produce its flowers, you shouldn't need to fertilize. After a couple of days in the sun, the leaves will have greened up and will start to grow. In two to three weeks, depending on the cultivar, your plants will bloom. The cooler the spot where you keep the plants, the longer the flowers will last—if you keep them at 50 to 55 degrees F, some plants will keep their blooms for up to 10 days.

After the Show

Because of the energy plants expend during the forcing process, most bulbs (an exception is narcissus) are usually not strong enough to produce flowers the following year and gardeners often compost or discard them. To save the bulbs, add some low-nitrogen fertilizer to the soil immediately after flowering and weekly thereafter (follow the package directions) and keep the plants in a sunny location. The bulb can then build up the energy it will need to develop flowers the following year. When weather permits in the spring, plant the bulbs outdoors in the garden. Keep the foliage intact, as it will continue photosynthesizing. These bulbs won't be strong enough for another season of forcing but may do fine in the garden.

Mark Fisher is director of Horticulture at Brooklyn Botanic Garden.

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