How to Force Cold-Hardy Bulbs
"Forcing" is a misnomer, because it sounds too much like
work. We're just tricking the bulbs into thinking winter is over
quite a bit sooner than it is. Forcing is an easy sleight of hand
that offers the soul-restoring scents and colors of spring at a
time of year when spirits sorely need reviving. But you need to
plant now (October or November) to enjoy the results when the snow flies! Although
we usually think of Daffodils, Hyacinths, and Tulips, many of the
smaller bulbs are also extremely easy and gratifying to force: Crocus,
Muscari (Grape Hyacinth), Scilla, Dwarf Irises, and Anemones will
also give great results.
Forced bulbs can be divided into two groups: those that require
a chilling period and those that don't. When bulbs do need
chilling, what they actually require is many weeks less than typical
northern winters. See the list at the end of this essay for details. See the growing guides
for Paperwhite Narcissus and Amaryllis
for directions for these bulbs that do not require a chilling period.
In a nutshell, here's what you do to force bulbs that do need
chilling: pot the bulbs in any well-draining potting mix, water
them, set aside in a cool but not freezing dark spot for the required
minimum time, then bring them into warmth and light in the house.
The bulbs think spring has arrived and quickly sprout and flower.
It's that easythe bulbs do most of the work.
This is a great project to do with young children, too, if you
want to invite the kids or grandkids to participate. The actual
planting is a little messy, so it's a good idea to spread some
newspapers to catch any spilled soil, gather all your pots in one spot,
and do all the planting at one time.
Containers and potting mix: You can use any pot you like
to hold bulbs you want to force, as long as it allows room for root
growthabout 3-4in. of space below the bulbs. This is a great
opportunity to showcase flea market finds and tag sale treasures,
or simple terra cotta pots. If you choose a pot without a drainage
hole in the bottom, you'll have to water your bulbs very
carefully, because bulbs that sit in soggy potting mix will soon
rot. Consider using a ceramic or terra cotta pot if you're
forcing tall Daffodils or Tulips. They can be top-heavy when in
full bloom and may topple if grown in light-weight plastic pots.
We recommend that you plant bulbs in a soilless potting mix (available at garden centers and hardward stores). A soilless mix holds moisture but allows excess water to drain away readily.
Potting the bulbs: To pot the bulbs, begin by placing potting
mix in a plastic tub or bucket. Slowly add water and stir until
the mix is moist but not soggy. This is an ideal job for
a very young assistant, if you'd like to invite a child or
grandchild to join the fun. Add the moistened mix to the container
until the pot is about three-quarters full. Set the bulbs root-side
down on top of the mix (or on their sides if you can't tell
which end is up, as with Anemone blanda). Space the bulbs
much more closely that you would in the garden--they should
almost touch. Then add more mix. Cover small bulbs completely with
a 1/2in layer of mix; cover larger bulbs up to their necks, leaving
the tips of the bulbs exposed. Water thoroughly after potting.
Chilling the bulbs: To force cold-hardy bulbs into bloom,
you must first encourage them to produce new roots by keeping them
cool and moist for a period of time that varies by type of bulb(see listing below).
The ideal rooting temperature also varies, but most bulbs flower
best if stored at 40-60°F for the first 3-4 weeks after potting,
then at 32-40° for the balance of the cooling period-- a shift
that mimics the drop in soil temperature outdoors as fall turns
The easiest way to chill bulbs is to put them outdoors and let
nature do the rest. To insulate the bulbs from rapid changes in
air temperature and from freezing cold, bury the pots in a pile
of dry leaves held in place by a plastic tarp or in a pile of mulch,
such as bark or wood chip, and cover the
pile to prevent formation of a frozen crust. You can also chill
bulbs in a cold frame if you are lucky enough to have one; a cold basement, an unheated garage (provided the temperature
doesn't fall below freezing), or a refrigerator. In such locations,
it may be difficult to arrange for the ideal shift in temperature
described above. Fortunately, most bulbs haven't read the manuals,
and they will root beautifully if the temperature does not stray
too far above or below 40°F during the rooting time. Professional
growers fill huge walk-in coolers with potted bulbs and control
the temperatures precisely. Using an old refrigerator
in a basement can get great results without ever touching the
The possible downside to outside storage has four little legs.
If mice or other rodents have access to your bulbs, they will devour
all but the varieties that are poisonous or distasteful to them (such as Narcissus).
Protect potted bulbs with steel mesh, such as hardware cloth.
Please note that moisture is as important as temperature
in the successful chilling of bulbs. Check the potting mix in the
pots every few weeks and water thoroughly when the surface of the
mix is dry to the touch.
Toward the end of the recommended rooting time, begin checking the
pots for signs that the bulbs have rooted. If you see fleshy white
roots poking through the drainage holes in the bottom of the pots,
the bulbs are usually ready to bloom. If you don't see roots,
give the bulbs more time in cold storage. Don't judge readiness
by the appearance of shoots from the tops of the bulbs; without
roots, the bulbs won't flower properly.
Once the bulbs have rooted, you don't have
to bring them out of the cold immediately. Most will tolerate extra chilling
time, allowing you to orchestrate a succession of winter bloom.
Bringing the bulbs into bloom: When the bulbs have rooted,
bring the pots out of cold storage and set them in a bright window
in a cool room (one where the temperature stays below 65°F).
Bright light will help keep the leaves and flower stems compact;
in weak light, they tend to flop. You are likely to find that the
bulbs have produced white shoots during cold storage. Sunlight quickly
turns them green.
Keep a close eye on the moisture needs of the bulbs as they send
up leaves and flower stems. Initially, the bulbs probably won't
need to be watered more frequently than once a week (if that much),
but by the time they bloom, you may need to water them every day
Most bulbs will bloom 2-5 weeks after they come out of the cold,
heralding spring with their bright colors and sweet fragrances.
Duration of bloom varies with the type of bulb and the variety but
is generally shorter than you'd expect of bulbs in the garden.
Warm temperatures and low humidity indoors speed the decline of
the flowers. Keeping the pots out of direct sunlight and moving
them to a cool room at night helps prolong bloom.
When the blooms fade, we usually recommend that you toss the bulbs
on the compost pile. If you keep them in a sunny window and continue
to water them, forced bulbs can be planted in the garden after the
threat of hard frost has passed, but they won't bloom well
again for at least two years. Tulips rarely bloom again, but Daffodils,
Crocus, and Grape Hyacinth are more likely to be worth the effort
Forcing Hyacinths without soil: Hyacinths can be forced
in pebbles and water, or in glass jars. They still require a cool
rooting period if forced this way. Special forcing glasses, in use
since Victorian days, are shaped like an hourglass and keep the
bottom of the bulb dryonly the bulb's roots reach down
into the water. If you are using pebbles in another type of container,
place a 2-3in layer of pebbles, such as pea stone, marble chips,
or river rocks, in the bottom of the bowl or pot. Set the bulbs
on top of the pebbles, then fill with more pebbles, leaving the
top 1/3 of the bulbs exposed. Add enough water to create a reservoir
for the roots, but be sure the bases of the bulbs stay above water
level. If they sit in water, the bulbs will rot. Then place the
container in a dark, cool area (40-50°F) for 4-8 weeks. Check
the water level occasionally and add more water as necessary, keeping
the water level below the bottom of the bulb. When roots have developed
and leaves begin to grow, it is time to move the bulb into a bright
window in a cool room (one where the temperatures stay below 65°F).
Bulbs forced in water can be planted in the garden after the threat
of hard frost has passed, but they won't bloom well again for
at least two years--if ever.
Recommended cooling period: Professionals often recommend very
lengthy cold periods, but we've had good results at home using
the minimums listed here. Remember that bulbs can remain chilling
for longer than the minimum. Please note that Tulips do require
the longest period to flower successfully.
- Anemone blanda, Crocus, Leucojum aestivum, Muscari:
- Chionodoxa, Galanthus, Dwarf Iris, Miniature Narcissus, Scilla:
- Daffodils, Hyacinthus: 12-14 weeks
- Tulipa: 14-16 weeks.
Revised: November 2007