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While our North State Gardens have not had much of a winter just yet, one of winter’s particular pleasures is that of bringing spring in. Having bright, fragrant spring bulbs or dramatic branches of spring flowering trees and shrubs bloom inside our homes during the cold dark winter months does just this. This technique – commonly referred to as forcing – is one of the gardener’s great tricks for getting through the winter with minimal garden-variety seasonal affective disorder.
The history of forcing plants to grow and produce out of their normal seasons dates back to the Roman Empire when records show that cucumbers were grown in a winter greenhouse for the Emperor. The forcing of bulbs out of season for their ornamental flowers is recorded at least as early as the mid-1700s.
Paperwhite Narcissus (Narcissus tazetta “Paper White”) and Amaryllis are readily available in shops and catalogues from early fall through Christmas, but why stop the fun there? You can force all kinds of bulbs from October all the way till whenever real spring arrives in your garden. Furthermore, forcing bulbs is a technique that allows you to see the whole – normally hidden in the dirt – life cycle of these plants – from shriveled brown orb, to a pale green nub letting you know they are alive, to a miraculous flower on your kitchen windowsill in mid-winter.
The trick to forcing bulbs lies in trying to mimic their normal outdoor season. The hardier a bulb is outdoors, or the later it is supposed to bloom, the longer you need to cool it while it is rooting.
Warm-climate natives Amaryllis and paperwhite Narcissus are easy to force indoors. Place the bulbs, pointy side up in a vase or bowl that will allow their rooting bottoms to sit just over or in water, you can nestle them in pebbles with water, or actually pot them up in just enough soil to surround an cover the bulb itself. Once placed in their growing medium, and positioned in a bright, warm place in your home, these bulbs will generally bloom in 4 – 8 weeks – paperwhites bloom more quickly than amaryllis.
For hyacinths, iris, snowdrops, tulips, and Narcissus other than paperwhites, a cooling period is necessary in order to get the bulb to bloom. The general rule of thumb is to cool your bulbs for forcing 10 to 18 weeks at 50 degrees or less (see below for a reference guide to a few kinds of bulbs). One way to do this, is to pot your bulbs up in unfertilized soil, water well and put them in a refrigerator for 8 to 18 weeks. Check on them every now and then to see if they need a little more water, but for the most part they need very little while they are cooling and rooting. Once you see roots popping out the bottom of the pots, or the green shoot of the plant above the soil a fair bit, then pull them out of their dark cooling place and set them in a sunny window to grow the rest of the way. Once they are out of cooling, they will need light, and water when the soil is dry.
When your forced bulbs are done blooming, if they are hardy in your gardening zone, you can plant them outdoors as soon as the ground is workable (in the warmer portions of the North State, this is all winter). Due to being depleted by forcing, your bulbs may not bloom for the first one or two seasons in the ground. For the best performance, plant with bulb specific fertilizer and water in well – especially during dry conditions.
Writing recently in the Chico Enterprise-Record, Butte County Master Gardener and Paradise Garden Club member, Carolyn Melf noted:”…a major problem with paperwhites is their tendency to develop long, lanky stems and leaves that require support. But there is a solution to this problem.
Researchers at Cornell University have discovered how to reduce the growth of the leaves and stems by one-third, while still producing normal-sized flowers, simply by watering with isospropyl (rubbing) alcohol. Steps to stunt paperwhites with rubbing alcohol
1. Place the bulbs, pointed side up, in about 3 inches of pebbles. Push the bulbs down, leaving half of each bulb exposed. Add water to barely touch the bottom of the bulbs, and wait one week.
2. When shoots reach 1″-2″ above the top of the bulb, pour off the water.
3. Replace the water with a 5 percent solution (one part rubbing alcohol to 10 parts of water).
4. Continue to use the alcohol solution for all future watering. You will see results in just a few days.”
Cooling Period guide for forcing selected bulbs, from The Brooklyn Botanic Garden Gardener’s Desk Reference, Henry Holt and Company, Inc. (1998):
Crocus: 15-week cold treatment; cover completely in potting soil.
Daffodil (which are Narcissus other than paperwhites): 16-17-week cold period; plant bulbs with top 1/3 out of the soil.
Tulip: 15-16-week cold period; plant bulbs with top 1/3 out of the soil.
Grape Hyacinth (Muscari): 15-week cold period; cover completely in potting soil.
Fritillaria: 13-15-week cold period; plant bulbs with top 1/3 out of the soil.
Iris: 15-week cold period; plant bulbs with top 1/3 out of the soil.
Snowdrop: 15-week cold treatment; cover completely in potting soil.
Good Resources for Bulbs:
Most independent nurseries sell a wide variety of spring bulbs, those with some still in stock may have them on good discounts beginning right after New Years. If their gift shop is unheated, even the unusual bulbs may well be pre-chilled for you.
Also try the following mail-order sources – often if you place your fall order in early summer- before July 1, these sources offer early-bird discounts:
Old House Gardens: both the catalogue and staff are so very helpful with all things bulb. They even sell a reproduction hyacinth glass, hand-blown by a glass works on Cape Cod and based on a New England design from the Victorian era.
Brent and Becky’s Bulbs – http://www.brentandbeckysbulbs.com/
White Flower Farms – http://www.whiteflowerfarm.com/
And finally, while I have not yet tried forcing any native bulb varieties, Telos Bulbs has an excellent California native bulb selection.
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In a North State Garden is a weekly Northstate Public Radio and web-based program celebrating the art, craft and science of home gardening in Northern California. It is made possible in part by the Gateway Science Museum – Exploring the Natural History of the North State and on the campus of CSU, Chico. In a North State Garden is conceived, written, photographed and hosted by Jennifer Jewell – all rights reserved jewellgarden.com. In a North State Garden airs on Northstate Public Radio Saturday mornings at 7:34 AM Pacific time and Sunday morning at 8:34 AM Pacific time. Podcasts of past shows are available here.