How to Force Bulbs Indoors for Winter Flowers

Bulbs that need cold, and bulbs that do not

Clemson Cooperative Extension says the easiest and most common hardy bulbs to force are crocus, daffodils, hyacinths and tulips, and that Dutch iris, snowdrops and grape hyacinths are easy too. All of them need a cold period first. University of Wisconsin–Madison Extension says paperwhite narcissus and amaryllis are the easiest bulbs to force because they need no cold treatment: paperwhites bloom in 3 to 5 weeks and amaryllis in 6 to 8.

Those two are covered on their own pages: paperwhites for growing them on pebbles and keeping the stems short, and amaryllis for the bulb you can bring back every year. The rest of this guide is the hardy bulbs.

Choose varieties sold as good for forcing, and the largest bulbs you can find (Wisconsin, Missouri). Wisconsin says shorter varieties make a better indoor display and do not flop as easily, names ‘Tete-a-Tete’ as a very good small daffodil for forcing, and says late-blooming varieties generally need longer cold than early ones.

Potting

1. A pot with drainage holes. Iowa State uses one twice as deep as the bulbs; Missouri prefers short azalea or bulb pots 4 to 8 inches across; Clemson leaves at least 1 to 2 inches of soil under the bulbs. Clemson soaks clay pots overnight so they do not pull water out of the mix.
2. A well-drained mix, no fertilizer. Commercial potting soil works; Clemson prefers equal parts potting soil, peat moss and perlite. The bulb already holds the food for its flowers (Clemson, Missouri).
3. Crowd them. Missouri fits six tulips, three hyacinths, five daffodils or 15 crocus in a 6-inch pot; Iowa State 5 to 7 tulips, 3 to 5 daffodils, 3 hyacinths or 10 to 12 crocus. Do not press the bulbs into the mix (Clemson, Iowa State).
4. Tips at the surface. Missouri leaves only the tip of a tulip or hyacinth showing, about half of a daffodil, and sets small bulbs about an inch under the surface. Set tulips with the flat side toward the rim, so the first leaves arch over the edge (Clemson, Iowa State, Missouri).
5. Water and label. Water until it drains through, and label each pot with the variety and the date it went into the cold (Missouri, Iowa State, Wisconsin).

The cold period

This is where roots form and the flower stem grows inside the bulb. Iowa State says at least 12 to 15 weeks, Clemson 12 to 16, and Missouri 13 or 14 for most varieties; Clemson says stems come up short, and flowers may abort, if the bulbs get less than 14 weeks. Wisconsin gives each bulb its own cold period in its table and says late-blooming varieties need longer.

• Where: a refrigerator, an unheated basement or crawl space, a cool cellar, a cold frame or an outdoor trench (Iowa State, Clemson, Missouri). Wisconsin says the steady cold of a refrigerator suits bulbs better than a place that freezes and thaws, which rots them.
• Dark, and not frozen. Iowa State keeps the bulbs in complete darkness and does not let them freeze. Wisconsin shields them from light and covers pots in the refrigerator with an open plastic bag so they need watering less often.
• No fruit beside them. Iowa State, Wisconsin and Clemson all say ripening fruit, apples above all, gives off ethylene gas that harms flower development. Clemson says the bulbs are poisonous, so it does not recommend refrigerator storage in homes with young children.
• Outdoors: Iowa State digs a trench at least a foot deep in late October or November, sets the pots in, covers them with straw and a tarp. Missouri puts a cold frame in shade or on the north side of a building, covers the pots with leaves, straw or similar, and says a few pots can sit against a building under mulch with a box turned over them. Clemson closes the cold frame early in December and opens it on mild days.
• Keep them moist. Check pots now and then and water if the mix is dry, but too much water rots bulbs (Wisconsin, Missouri).

Bringing them into bloom

The bulbs are ready when roots show through the drainage holes and pale shoots have come up: Iowa State looks for both, and Wisconsin for shoots one to two inches tall. Move the pots to a cool spot in low or indirect light until the shoots turn green, about 4 to 5 days in Iowa State’s method, then to a bright window. Turn the pots so the stems grow straight (Clemson, Iowa State).

Most bulbs flower 2 to 4 weeks after leaving the cold (Iowa State; Wisconsin says three to four; Clemson three to four for buds). The flowers last longer in a cool room at night (Clemson). For a run of flowers through winter, take a few pots out of the cold every week or two (Iowa State, Missouri).

Hyacinths in a glass

Hyacinths can be forced in water alone. Iowa State uses an hourglass-shaped forcing vase, or a container part-filled with pebbles, with pre-chilled bulbs; if you cannot buy them pre-chilled, it chills them in a paper bag in the refrigerator for 12 to 15 weeks. Wisconsin says unplanted hyacinths need less room and chills them 4 to 6 weeks in a paper bag. Either way the water should reach the roots but never the base of the bulb, which rots if it sits in water (Iowa State, Wisconsin). Iowa State changes the water once or twice a week or when it clouds, and says flowers open in 2 to 4 weeks. NC State’s plant toolbox says to wear gloves, since the bulbs can irritate skin.

After the flowers

Iowa State and Clemson say tulips, hyacinths and most other forced bulbs are usually discarded, and hyacinths forced in glasses always. Daffodils are the exception: keep them in a sunny window and watered until the leaves yellow, then let them go dry, and plant them in the garden in fall at the normal depth (Iowa State). Clemson says a forced daffodil will not flower the next year but may the year after. Wisconsin says none of these bulbs can be forced two years running, and that bulbs grown without potting mix seldom bloom again.

For when and how deep to plant them outdoors, see When to Plant Fall Bulbs by Zone: Tulips, Daffodils, More.

Common mistakes

• Too short a cold period. Clemson says under 14 weeks gives short stems, and flowers may abort.
• Chilling bulbs beside apples. Iowa State, Wisconsin and Clemson all warn against it.
• Letting the pots freeze and thaw. Wisconsin says it rots the bulbs.
• Bulb sitting in the water of a forcing glass. Iowa State and Wisconsin say it rots.
• Feeding forced bulbs. Clemson and Missouri say the bulb already holds the food it needs.

Where there is no source here

We found no Texas, Florida or Louisiana source on forcing hardy bulbs that we could read. Clemson writes for South Carolina; the other sources are Midwestern. Penn State’s and Minnesota’s forcing pages did not load at the addresses we tried.