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The Basics: Forcing Bulbs for Indoor Bloom

By Kathy LaLiberte

The reason people force bulbs is to make them grow and bloom earlier than they would have naturally. But forcing bulbs sounds like a bit of an oxymoron, doesn't it? The key is to set the proper stage. That starts with a comfortable setting, refreshments, cool temperatures and low light. When show time arrives, the lights and the temperature come up and the urge to emerge is irresistible.

Many people don't realize that there are really only two types of bulbs for indoor growing: those you need to pre-chill and those you don't. Here's how to tell the difference.

Bulbs That Do Not Need Pre-chilling

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Amaryllis and paperwhites are the most commonly forced bulbs.

In this category are amaryllis, freesia, and the tropical narcissus varieties, such as paperwhites. These zone 8 to 9 bulbs have never known winter, so don't expect a cooling period.

You can plant these bulbs in a pot filled with soil, or just grow them in a shallow bowl and use pebbles to hold the bulbs in place. They'll usually bloom just four weeks after "planting". To help keep stems short and sturdy, provide indirect light and temperatures of about 50 degrees F. for the first two weeks, then warmer, brighter conditions after that. If you're growing your bulbs in water, it should cover no more than the bottom quarter to third of the bulb.

Amaryllis are available in many interesting colors and forms: There are bright reds, but you can also have white, lilac or peach, with single or double blossoms.

Paperwhites offer beauty and a strong scent. Buy a couple dozen of these no-chill bulbs and store them in a cool, dry place. Start some every few weeks for blooms right through January.

Good Choices for Forcing

Bulbs That Don't Require Chilling: The following bulbs are the easiest to force into bloom because they don't require a chilling period:

  • Amaryllis: Blooms appear six to eight weeks after planting.
  • Paperwhites: Blooms appear three to five weeks after planting.

Bulbs That Require Chilling: The following bulbs can be forced for indoor bloom, but they need to be chilled, a process that replicates what happens in the bulb's outdoor growing environment. The list includes the number of weeks of chilling, followed by the number of weeks until it blooms (in parentheses).

  • Glory-of-the-Snow (Chionodoxa), 15 (2-3)
  • Crocus, 15 (2)
  • Hyacinth, 11-14 (2-3)
  • Grape Hyacinth (Muscari), 14-15 (2-3)
  • Daffodil, 15-17 (2-3)
  • Star of Holland (Scilla), 12-15 (2-3)
  • Tulip, 14-20 (2-3)

Bulbs That Need Pre-chilling

All other spring bulbs require a chilling period before they'll bloom. This includes tulips, daffodils, hyacinths, crocus, Dutch iris and scilla. Cool temperatures stimulate a biochemical response inside the bulb that "turns on" the embryonic flower so it starts developing. Most bulbs require 16 to 18 weeks of cold before the flower is fully formed. At that point they're ready for light and warmth. If you cut the time short, the flowers will emerge but they will not be fully formed.

For indoor blooms, the easiest bulbs are crocus, hyacinth, muscari, and mini-daffodils. Tulips and standard daffodils are a little trickier.

Bulbs seem to look best in a shallow, wide pot. There should be about 2" below the bulbs for root growth. The very top of the bulbs can be even with the pot rim. Use a standard potting mix (2/3 soilless mix, 1/3 compost/soil). Potted bulbs also look best when they're crowded, so snuggle the bulbs together about 1/2-inch apart. It's also good to stick with one variety of bulb per pot, because cooling and bloom times vary. Label each of your pots with variety name and planting date. Otherwise, you won't remember what's what when February comes.

Finding the right place to chill your bulbs is the real challenge. The bulbs need to be kept at 35 to 45 degrees F. for the entire cooling period of 16 to 18 weeks. If you live where outdoor winter temperatures rarely get below 25 degrees F, just keep the pots of bulbs moist and store them right in the garden beneath a layer of straw. If your winter temperatures are colder than that, the potted bulbs can be stored in an unheated basement, a ventilated crawlspace, or a cold frame. If you have space in your refrigerator, you can chill the bulbs there. Be careful not to store the bulbs with any kind of fruit, which emits a type of gas that can harm the bulbs.

Check the sidebar for specifics about how long to chill your bulbs. Generally, for flowers in January, you should plant in September or early October. For February flowers, plant mid-October. For March blooms, plant in late October or early November. Keep the soil moist, but not wet for the entire duration. And keep the bulbs in the dark or they may start growing before they're fully chilled.

Time for Spring!

Once you remove your bulbs from cold storage, allow three or four weeks to bloom time. Wake the bulbs gradually, starting with about two weeks of indirect sunlight and 60 degree temperatures. When shoots are three to five inches high, move the pots to a 68-degree environment and a bright, sunny window. Once buds color, move the pot to indirect light again to prolong bloom.

After blooming, most people discard potted bulbs. If you find that impossible, keep watering the pots and start adding some fertilizer. When the foliage yellows (usually after a month or two), you can remove the bulbs and plant them outdoors. Just remember, it may be several years before the bulbs bloom again. Paperwhites and other tropical narcissus will not rebloom.