How to Grow Gladiolus

Gladiolus are grown mainly as cut flowers: tall, straight spikes of florets that open from the bottom up. They grow from corms, swollen underground stems, and belong to the iris family. The name comes from the Latin gladius, “sword,” for the blade-shaped leaves; older books call them sword lilies.

Types to choose

WSU Extension describes three main groups:

• Large-flowered (grandiflora) hybrids. Spikes of 4- to 6-inch flowers on plants 3 to 4 feet tall. Hardy in zones 7 to 10.
• Miniatures. The same ruffled, frilled flowers at 2 to 3 inches across, on stems 2 to 3 feet tall. These are the best choice for containers.
• Hardy Gladiolus nanus hybrids. Shorter, 18 to 24 inches, with fewer florets per spike, and reported to survive to zone 5 with winter mulch.

Mississippi State notes that corm size tracks flower size, so buy the largest corms you can. Iowa State finds corms 1½ to 2 inches across bloom best; smaller ones bloom less well.

Where to plant

Give gladiolus full sun, at least six hours, and a spot sheltered from strong wind but with good air circulation. The soil should be moist but well drained; gladiolus need steady water but won’t tolerate wet feet. WSU prefers medium fertility and a pH of 6.0 to 6.5, Mississippi State 6.5 to 7. Amend very sandy or heavy clay soil with compost.

WSU recommends moving gladiolus to a fresh bed after a few seasons, since they deplete the soil. It also says to keep them away from beans and peas, which it reports they can inhibit. Treat that as garden lore rather than tested fact.

How to plant gladiolus corms

Start planting around the last frost; the dates for your zone above show the window, and when to plant gladiolus explains how far ahead you can push it.

1. Depth. Large corms go 4 to 6 inches deep (Iowa State); small ones about 3 inches. WSU’s rule is three to four times the corm’s diameter; Mississippi State plants 3 to 5 inches deep, and Colorado State 3 to 6.
2. Spacing. Four to 6 inches apart, pointed end up. Illinois Extension crowds them to 2 to 3 inches for a cutting row but uses 6 inches in landscape beds. Space rows 12 to 36 inches apart (Mississippi State), or set corms in groups of five or more.
3. Stakes. Put stakes in at planting for tall types, so you don’t spear corms later.
4. Water thoroughly after planting.
5. Repeat every two weeks until early July for continuous bloom.

Once shoots appear, mulch 2 to 4 inches deep to hold moisture and smother weeds.

Water, feeding and staking

Give about 1 inch of water a week during dry weather, and, as WSU advises, keep watering after bloom for as long as the leaves stay green.

WSU side-dresses with a complete fertilizer such as 5-10-5 or 10-10-10 twice: when plants are 6 to 10 inches tall, and again when flower spikes emerge from the leaves. Keep fertilizer 4 to 6 inches away from the stems.

Tie tall spikes loosely to their stakes every several inches as they grow.

Cutting gladiolus

For use at home, cut when the bottom two or three florets show color or the lowest one has opened. For flowers you need to hold for a few days, cut a little earlier, when the lowest buds are just coloring. Mississippi State makes a slanting cut above the second to fourth leaf, leaving the lower leaves on the plant. Stand cut spikes upright, since they bend toward vertical if laid flat, and keep them away from ripening fruit, as gladiolus are sensitive to ethylene.

Digging and storing corms

Where winters freeze, dig the corms in fall. In zones 8 to 10 large-flowered types can stay in the ground.

1. Dig when the leaves begin to die back, usually six to eight weeks after flowering. Use a garden fork to avoid nicking the corms.
2. Cut the stalks 1 to 2 inches above the corm. Throw away diseased or damaged corms.
3. Cure in a single layer in a warm, dry place at 60 to 70°F for two to three weeks.
4. Twist off and discard the shriveled old mother corm at the base. Leave the husks on.
5. Store in mesh bags in a dry, airy, frost-free place at about 35 to 45°F, and check them over winter.

Save the tiny cormels too if you like. Planted 1 to 2 inches deep the next spring, they reach flowering size in two to three years.

Problems

• Gladiolus thrips. The most common pest. Look for silvery, streaked leaves, deformed buds or spikes that never open. Inspect leaf sheaths and buds with a magnifier.
• Corm and stem rots, including Fusarium yellows. Discard damaged corms at digging and plant in well-drained soil.
• Botrytis blight (gray mold) and bacterial scab. Common diseases noted by WSU; start with clean corms and good air movement.
• Squirrels and voles. Where they dig, WSU plants corms in hardware cloth cages.