What You Need to Know to Force Bulbs
Brighten dull winter days with pots of spring bulbs you plant this fall.
Make plans now to chase away the winter blues with pots of fabulous spring bulbs. Tulip, hyacinth, crocus—you can trick these (and other) spring bulbs to unleash their flower show a few months early. The secret? Giving bulbs a cold treatment that mimics winter’s chill. Coaxing flowers to bloom ahead of their natural time is called forcing.
It’s not a difficult technique to pull off, and you probably already have most of what you need: potting soil and pots. Bulbs are the one thing you’ll need to get. Certain bulbs perform better than others when it comes to forcing, but good candidates include crocus, grape hyacinth, Dutch hyacinth, Iris reticulata and tulips.
Shorter tulips like the Greigii group—including its well-known member, ‘Red Riding Hood’ tulip (above)—adapt readily to forcing. Greigii tulips offer a really eye-catching pot with their variegated leaves and shorter-stemmed blooms.
Choose Your Containers
After forcing bulbs, you can add them to planting beds the following growing season if you grow them in soil. (Bulbs forced in water, on the other hand, don’t successfully transition to planting beds.) With forcing, aim to plant bulbs shoulder to shoulder, and select pots based on that method. For instance, choose a 6-inch-wide pot for six tulips. Six hyacinths need a slightly larger pot because the bulbs are bigger. Special pots known as bulb pans are shallow and provide a just-right depth for planting. Otherwise, basic plastic pots (like the kind you buy annuals and perennials in) work fine.
Plant the Bulbs
Fill the base of pots with soil, then add bulbs and more soil. Bury bulbs so the tip is just visible above the soil. Keep soil 1/2 to 1 inch below the pot rim, because as bulbs grow, they’ll push up soil. Water pots after planting until water exits the drainage holes.
Bring on the Chill
Stash pots of bulbs in a mostly dark spot where temps hover in the 35 to 45 degree range. Don’t allow pots to freeze. Good places to store pots include an extra fridge, an unheated shed or garage, cold frame or greenhouse. If you keep bulbs in areas where rodents might forage, cover pots to protect bulbs. In mild winter regions, sitting pots outside provides enough chill to trick bulbs into blooming. In cold winter regions, protect bulbs from freezing. Some gardeners store pots of bulbs beside a compost pile, covering them with leaves.
How Long to Chill
During chilling, bulbs grow roots, which help support and nourish the plant while it’s growing. Different bulbs need different amounts of cold to trigger flowering.
- Crocus, Dutch hyacinth: 8-10 weeks
- Grape hyacinth: 10 weeks
- Iris reticulata: 13-14 weeks
- Tulips: 12-16 weeks
During chilling, check pots regularly. Gently lift them and inspect the bottom—eventually you’ll see roots poking out. Water pots when soil dries out. Remove pots from your chilling area when you see the first green shoots poking through. Place pots near a bright window in a cool room as growth unfolds.
Create a Display
Slip pots of bulbs into decorative containers for an eye-catching look. Keep soil moist as bulbs continue to grow and flower. Bulb flowers last longest in a cool room (65 F maximum—less is better). Keeping bulbs near a bright window where air is naturally cooler helps extend the flower show.
Snip flower stems after blossoms fade. Shift pots to a less conspicuous—but still sunny—spot and continue to water and care for leaves until they fade. After leaves die back, store bulbs (in pots or lifted from soil) until you’re ready to plant them outdoors. Don't forget to label bulbs. Expect bulbs to put on a full flower display the second year after planting.