When to Plant Gladiolus

The rule: start around the last frost, then keep planting

Gladiolus are tender summer bulbs. Illinois Extension notes that cold can kill them if they’re left out over winter or planted too early in spring. Most extension services start planting once frost danger has passed; Iowa State, Mississippi State and Colorado State all use that rule. Some start a little earlier. WSU allows planting as early as a month before the average last frost, and Illinois Extension starts about two weeks before it.

The dates for your zone above put the first planting from about four weeks before the last frost to a week after it. If your spring tends to be cold and wet, use the later end.

Why plant every two weeks

Each corm sends up one main spike, and a planting all blooms within a short span. To have flowers from midsummer into early fall, plant a few corms at a time every two weeks. Iowa State and Illinois Extension use two-week intervals, WSU 10 to 14 days, and Mississippi State cut-flower growers 7 to 14 days.

Stop in time for the last batch to flower before fall frost. Iowa State makes the final planting in early July. To figure your own cutoff, count back from your average first fall frost: Iowa State finds corms bloom 8 to 10 weeks after planting, and WSU gives 65 to 100 days depending on variety.

Timing by region

Upper Midwest and North. In Iowa, it’s generally safe to start in early to mid-May, with the last planting in early July (Iowa State). In central Illinois, where the average last frost is around May 14, Illinois Extension starts about two weeks earlier and repeats every two weeks until the beginning of July.

Inland Northwest. WSU’s Spokane County Master Gardeners, in zone 6A, plant from about a month before the last frost and continue every 10 to 14 days.

Rocky Mountain West. Colorado State plants once the danger of frost has passed and finds gladiolus perform well there.

The South. Mississippi State plants after the last frost, from mid-March to mid-April depending on location in the state, with successions 7 to 14 days apart. Large-flowered types are perennial in zones 8 to 10 there, so corms can stay in the ground.

Florida. Timing flips. The UF/IFAS North and Central Florida gardening calendars list gladiolus for planting in July, and the South Florida calendar plants in midsummer and then every two weeks in September to stagger bloom.

What happens if you plant too early or too late

Too early. Gladiolus are tender, and Illinois Extension warns that planting too early in spring is one of the ways cold kills them. Stay within the window for your zone rather than pushing it.

Too late. Plantings made after early July in the North may not bloom before the first fall frost. Remember the corms also need time in the ground after flowering: WSU digs them six to eight weeks after bloom, when the leaves begin to die back.

Digging in fall

Where winters freeze, the planting season ends with digging. Lift corms as the leaves die back, cure them at 60 to 70°F for two to three weeks, and store them dry at about 35 to 45°F until next spring’s first planting. For depth, spacing and care, see how to grow gladiolus.