While it may seem strange, spring-flowering bulbs need to be planted in fall in order to bloom come show time. The bulbs need a certain amount of time to get established before winter's freezing weather sets in, and they need enough time exposed to cool soil temperatures to be properly chilled. But fall doesn't occur at the same time on the calendar in San Antonio, Texas, as it does in Minneapolis, Minnesota. So how do you know whether to plant in September or November?
USDA hardiness zone map. The simplest solution is to use the USDA Climate Hardiness Zone Map as your planting guide. This map breaks the country into 11 growing zones based on average annual winter minimum temperatures. First use the map to find which hardiness zone you live in. Then follow the table below to know when to plant spring-flowering bulbs in your zone.
Hardiness Zone Average Annual Winter Minimum Temperature When to Plant
Zone 1 below -50oF Early September
Zone 2 -50 to -40oF Early September
Zone 3 -40 to -30oF September
Zone 4 -30 to -20oF Late September to early October
Zone 5 -20 to -10oF Late September to early October
Zone 6 -10 to 0oF Mid-October
Zone 7 0 to 10oF Early November
Zone 8 10 to 20oF Early November
Zone 9* 20 to 30oF Early December
Zone 10* 30 to 40oF Mid-December
Zone 11* Above 40oF Late December
* Additional chilling may be needed to grow spring-flowering bulbs in these regions.
Special planting considerations. In coldest areas (USDA Climate Hardiness Zones 1 through 4), bulbs grow and perform best if planted early enough (September) to get established before the ground freezes. Mulch the bed a month after planting with a 3- to 4-inch layer of hay, straw, or shredded leaves. This will allow the soil to stay warm enough for the bulb roots to get established and will protect tender bulbs from freezing injury during winter, especially if the snow cover is sparse.
Bulbs in warm areas. In warmest-winter areas (zones 7 through 11), select bulb varieties that are best adapted to warm winters, such as wild tulips that are native to southern Europe. Most large-flowered tulips, hyacinths, and crocus will need supplemental chilling. To chill the bulbs before planting, place them in the refrigerator crisper for 8 to 10 weeks (but keep bulbs away from fruits or vegetables; they give off ethylene gas, which can cause the bud inside a bulb to abort), then plant. Since the ground rarely freezes deeply, if at all, in these areas, bulbs can be planted into December or even early January.
Even within a given climate zone, fall temperatures can vary widely from year to year. A good rule of thumb is to plant spring-flowering bulbs when the soil temperature 6 inches below the surface is below 60° F.
Bulbs that were not planted in fall at the proper time can be forced indoors this winter (see how-to project on forcing paper whites) or, if the ground still isn't frozen, planted in the garden. Depending on the severity of the winter, bulbs planted out late in the season may not flower the following spring.
Article published on June 23, 2008.