Hardiness of Gladioli
Gladioli are considered "tender" bulbs. They can be grown year round only as far north as Tennessee and into Virginia (Zone 7), where average annual minimum temperatures dip no lower than 5° to 10°F. In colder zones, farther north, they will only grow between frosts and must be dug up and stored over the winter.
Locating And Planting Gladioli
In the South, plant glad corms anytime. In the North plant them anytime from early spring to midsummer, always allowing each successive planting 3 months to mature, bloom and store nutrients before the first killing frost arrives in the fall. If you want to push the envelope, try [[Plant Summer Bulbs Early]]. Otherwise plant the corms in a sunny location. Good drainage is very important--gladioli do poorly in wet, clayey soils. If necessary, build gladiolus beds higher than the surrounding soil surface, and add lots of organic matter (such as sand, chopped leaves, peat moss) to the soil to improve drainage. If possible, loosen the soil to a depth of 1 foot when preparing the bed for planting. Although glads tolerate many soil types, they are happiest in soil that is rich, sandy, and slightly acidic (pH 6.5 to 7.0).
The planting depth of gladiolus varies with corm size. Plant small corms (1/2-inch diameter or less) 3 inches deep; medium corms, (up to 1 inch) 4 to 5 inches deep; and large corms (1-1/4 inches and up) 6 to 8 inches deep. Measure the depth from the bottom of the hole. Plant corms in groups of 5 or more, spaced 6 to 8 inches apart, or, for cutting rows, 6 inches apart in furrows dug 5 to 6 inches deep. Be sure their pointed growing tips point upwards and press their bottoms into the soil, using a slight twisting motion to assure good soil contact for the root plate. Fill the hole with soil, spread a small handful of slow release nitrogen fertilizer to the surface soil over each clump of corms and cover each bed with mulch. To arrange a succession of blooms over many weeks, some northern gardeners plant corms every 3 weeks until July. Flowers mature 8 to 10 weeks after corms are planted.