How to “Force” Bulbs

October 30th, 2018

   One antidote to the months of cold weather coming our way soon is blooming flowers.

Grape hyacinths forced into bloom. (Credit: Netherlands Flower Bulb Information Center)

   You could just go out and buy some blooms whenever you’re fed up with snow and dark. Or you could get some ready yourself via the botanical trickery known as “bulb forcing.”

   Forcing involves planting and chilling bulbs in pots now, then taking them inside in winter for a floral show that’s weeks ahead of schedule.

   I do it some years, but no one does it better and knows more about bulb-forcing than New Jersey writer, speaker and photographer Art Wolk.

   Wolk has won hundreds of forced-bulb blue ribbons at the Philadelphia Flower Show over the years and wrote a 2012 book called “Bulb Forcing for the Beginner and Seriously Smitten” (AAB Book Publishing, $32.95 hardcover).

    I’ll give you his how-to highlights here, but if you end up “seriously smitten,” check out his book, which is a very fun read, by the way.

   First off, Wolk prefers to call the technique “bulb enticing.”

   “Bulb forcing is such a terrible term,” he says. “It sounds like an outright assault – as if you’re jamming botanical specimens into a pipe with a too-narrow opening.”

   Whatever you call it, Wolk says that “having a pot of tulips on your side of the window when icicles are on the other is like telling winter you don’t care an icy iota what it does.”

   Wolk recommends clay pots over plastic ones and a potting mix that’s light, airy and high in foam-like perlite (ideally 10 to 30 percent of the total). He uses Fafard 1-PV or Sunshine Mix No. 4.

Pack bulbs tightly into the potting mix.

   Fill the pot two-thirds to three-quarters full with moistened soil mix and press bulbs into the mix so tightly together that they’re touching. Then fill to the top and tamp down.

   The key to getting bulbs to flower early is fooling them into thinking they’ve gone through winter. That means storing them at 40 to 50 degrees (48 degrees is ideal) for 8 to 13 weeks, depending on the bulb variety. (See below for details on that.)

   The best time to begin storage: now through early November.

   One option is burying potted bulbs in an 18-inch-deep pit outside – covered with 2 feet of leaves to keep the ground from freezing. (Use a tall stick to mark your site.)

   Other options: 1.) storing the pots – surrounded by leaves or pine needles – in a cardboard box in an unheated garage or window well or 2.) placing them in a refrigerator.

   Wolk’s method is storing pots in a plastic-covered cold frame equipped with a heat cable and thermostat to keep the temperature a consistent 48 degrees. He lowers the temperature to 41 and ultimately to 35 degrees toward the end of the chill time.

The out-of-use vegetable garden is a good place to bury pots of forced bulbs.

   When the prescribed chill time passes in mid-winter, retrieve the pots, hose them off and move them inside. Leaf shoots already should be showing.

   Place the pots in a cool, bright location (55 to 68 degrees is good, 70 is too warm); water when the soil starts to go dry, and turn the pots a quarter of a turn each day so the flowers grow uniformly.

   Depending on the bulb, flowers will emerge and open in one to five weeks.

   Supplemental lights (fluorescent, high-pressure sodium or metal halide but not too-hot incandescent) can be used if you don’t have a bright room. Turn them on no more than 18 hours a day.

   A few things that can go wrong:

   * Inadequate chill time or too-warm soil can cause a failure to bloom.

   * Inside temperatures of 70 degrees and above can abort the flower buds and lead to overly long and floppy stems.

   * Voles could eat the buried bulbs. If you have these rodents around, cover the pots with hardware cloth.

   * Disease can rot the bulbs. Use only healthy bulbs, fresh potting mix, and pots soaked 15 minutes in a 10-percent bleach solution, then rinsed.

   * Fruits and vegetables give off ethylene gas that can rot bulbs, so don’t store bulbs near them.

   As for what to force and when:

   * The easiest to force: daffodils and early bloomers such as crocus, scilla, Iris reticulata, grape hyacinths, Dutch hyacinths and species tulips.

   * Give 9 weeks of chill time: Arisaema, crocus, ipheion, Iris reticulata, scilla.

   * Give 10 weeks of chill time: Allium, Grecian windflower, lilies, grape hyacinths, early tulips.

   * Give 11 weeks of chill time: Dutch hyacinths, early daffodils.

   * Give 12 weeks of chill time: Mid-season daffodils, mid-season tulips.

   * Give 13-14 weeks of chill time: Late-season daffodils, late-season tulips.  

   Good luck, and may the force be with you.