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Here is a rule of thumb quick reference for planting spring flowering bulbs:
These guidelines are usually pretty safe to plan on planting spring flowering bulbs, but the arrival of winter in the northern and midwest zones is unpredictable. So be mindful of these details to adjust your planting plans:
Spring blooming bulbs must be established before beginning their required cold, dormancy. Roots will quickly develop from the bottom, and a little later stems will sprout from the tops. (Spring blooming bulbs for zones 9 & 10 do not require a cold period.)
Avoid, if possible, planting when there is still good possibility of a long enough warm spell for the stems to sprout from the soil. If air and soil temperatures are cold enough, the stem sprout will grow only until just below the surface and stop when the soil temperature is further reduced by freezing temperatures.
Spring bulbs can be planted once night temperatures are consistently between 40 and 50 degrees F, but before the first hard frost. The soil will be cool enough for optimal rooting.
Bulbs should be planted about six weeks before the ground freezes. Bulbs that have had enough time to root will also have gone through a natural process that prevents them from freezing.
After the ground freezes apply 2 - 4 inches of mulch to maintain a more consistent soil temperature. The mulch will also prevent rapid thaw/freeze cycles in early spring, which can heave your bulbs up out of the ground. If your region experiences climate extremes or rapid changes, such as around the Great Lakes or the Northern zones 1 - 3, you may want to choose later blooming bulbs that may not be affected by late freezes.
Planting Summer Blooming Bulbs
Summer blooming bulbs, which include rhizomes, corms, tubers and tuberous root plants, can be planted in spring once the soil begins to warm. The hardier summer blooming bulbs such as iris and daylily can be planted fairly early and not be bothered by cool soil. Tender bulbs, like begonias, dahlias, gladiolus, the tropical cannas and caladiums, will prefer to wait until later spring when the soil is quite warm.
Planting Fall Blooming Bulbs
Fall blooming bulbs can be planted in late summer to early autumn. Many will bloom the first fall and overwinter in the ground. Some will produce their foliage in the spring and blooms in the fall. Autumn crocus and some varieties of Colchicum are hardy to zone 5.
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