Hardy, spring-flowering and summer-blooming bulbs can be grown throughout Wisconsin and left in the ground year-round. Divide hardy bulbs immediately after flowering if they need to be divided Tender summer-flowering bulbs should be dug up and stored over winter in Wisconsin and re-planted in spring.
Bulbs that bloom in spring, such as tulips, daffodils, crocus and hyacinth, must be planted the previous fall. In Wisconsin, plant these bulbs from mid-September to late October, or at least three to four weeks before the ground freezes for the winter.
Plant hardy, summer-blooming bulbs like lilies from early spring through midsummer for blooms the same year. Although iris are treated similar to lilies, they are technically a rhizome — a swollen bulb-like root. Plant iris from early spring through midsummer. The later you plant, the less likely they will bloom in the current year.
Plant tender, summer-flowering bulbs such as gladiolus, canna, dahlia and tuberous begonia in mid- to late spring, after all danger of frost has past. In southern Wisconsin, plant in late April to early May and in northern Wisconsin, plant in late May to early June, depending on the average date of your last spring frost.