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Bulbs need a cooling-off period to bloom

This week a friend gave me a lovely gift, a simple hyacinth vase with a single bulb. If you love plants, you already understand. A bulb is a perfect gift: a complete package, containing its own nutrients. If you simply plant it, or in some cases just provide water, it will grow roots, leaves and then flower. A bulb is a gift of future pleasure.

During the winter, gardening discussions may address the many nuances of "forcing bulbs." In horticulture, forcing means manipulating growing conditions so a plant performs differently from its norm -- typically blooming earlier than usual.

Spring flower shows are full of forced bulbs as well as forced herbaceous and woody plants, all flowering weeks or months early. (Annuals and perennials without woody stems are called "herbaceous." Shrubs and trees are "woody.")

When you have a bulb in hand, though, how can you tell if it's ready to be planted or needs this "forcing" treatment?

Two types of bulbs need no cold period, no special treatment or forcing. Amaryllis and paperwhites (a tender narcissus species), usually sold before Christmas, are perfectly primed to respond the minute you provide them with a growing medium.

Paperwhites just need to be placed with their bottoms touching water. Put the large amaryllis bulbs just part way into planting mix (the top two-thirds of the bulb above soil level), with a little moisture and light, and soon the roots and shoots will grow.

If you find any of these bulbs left over from holiday sales -- if they still feel firm -- grab them and enjoy. Once you learn the steps, you can regrow an amaryllis for years. (Paperwhites are generally spent after forcing; toss them.)

As for my hyacinth gift, how would I know if it needs forcing? Since the gift came in a vase from a real gardener, I assume this one's ready to grow. Hyacinth vases are intended for growing a single bulb in water, just like paperwhites. I'll just add the water.

But if I didn't know the source, the intelligent question would be whether this bulb was forced, or prechilled.

Tulips, narcissus (daffodils), hyacinths, crocuses and other hardy, spring-blooming bulbs require a certain period of cold or freezing weather to prepare them to flower. We plant them in fall, they start to grow roots, and then they freeze in the ground. Perfect.

But friends in Florida cannot plant the same bulbs in the fall for flowers in the spring. (Preforced bulbs are a nice gift for them.)

No cold equals no flowers. Each species has a different requirement for how cold the temperature must be, for how long a period, so different folks enjoy different spring bulbs from the South to Alaska.

So if you didn't get your tulips and daffodils planted, you now must provide some version of winter if you want them to flower ever again. Left inside in a bag, they will simply shrivel or rot.

>Survival outside

It is too late to put most bulbs into the ground now that winter has arrived. If you can still dig holes or trenches, you could bury the bulbs and you might see leaves in the spring and perhaps flowers in the second year -- if they live.

The question is whether those bulbs have a chance to grow some roots before freezing hard. Bulbs with roots will generally not freeze and die (although every plant has a freeze tolerance level); bulbs without roots can die at just a little below freezing. That's because root growth is accompanied by changes in the bulb's fluids -- an increase in concentrated sugars and carbohydrates -- so the bulb's cells have a lower freezing point.

In effect, bulbs with roots have a kind of antifreeze. In parts of Alaska or Canada, some grape hyacinths (Muscari) live for decades, even through some winters when the snow comes late but the soil freezes to minus 30 degrees Celsius (minus 22 degrees Fahrenheit). And what if you left some bulbs planted in pots outside? If they grew roots, they may do fine, but those bulbs' roots are inches from the air, so they could experience colder temperatures than they can tolerate, or they may just dry out.

They may also rot. Their survival is more likely if you sank the pots into the soil, or clustered them where temperatures stay close to freezing.

>Forcing for inside

Professional growers keep bulbs, planted in pots, in coolers at various temperatures just above freezing for many weeks, and they have to honor the same requirements that home gardeners do: Hyacinths need a cold period of 11 to 14 weeks, grape hyacinths 10 or 11 weeks. Most of the others -- crocus, iris, Chionodoxa (Glory-of-the-snow) and narcissus -- need about 15 weeks of cold. Tulips need the longest, from 15 to 17 weeks.

Growers face critical timing issues -- getting the flowers to open exactly at the right time for a flower show or Valentine's Day or Easter -- so they manipulate the temperatures very carefully: two weeks at 45 degrees, three weeks at 35 degrees, etc.

Then they hold the bulb pots at 50 degrees for the final weeks. If the plants get too cold, they produce fewer flowers; too warm makes them leggy, or the buds blast.

For indoor forcing, some home growers plant the bulbs in pots and chill them in a cold cellar, unheated basement or above a freezing garage. Even easier, store them unplanted, in a pot covered in peat moss, in a refrigerator at about 35 to 40 degrees for the required weeks. Then plant them in pots, put them in a cool place, and slowly introduce them to light and water.

For home growers, the exact chilling temperatures aren't as crucial as for growers, but it is critical that you introduce the plants gradually to indoor temperatures. Try to move them from 35 to 45 to 55 degrees slowly, over many days. Only after you see growth begin should you move them into some direct sunlight and let the warmth "force" them into flower.

Even then, your forced bulbs will last longer if you keep them in a cool location at night.

Thanks go to bulb givers, and most of all to the bulbs themselves, as they are the gift of spring.

Sally Cunningham is a garden writer, lecturer and consultant.