How to Grow Gladioli in Your Flower Garden

Gladioli are perennial cormous plants with swordlike leaves and tall flower spikes that range in color from white to pink to brown. There are about 300 species within the genus Gladiolus, all members of the Iris family. Although some species are native to the Mediterranean, the vast majority of gladioli are from South Africa. Gladioli vary in size from miniature gladioli (such as the Glamini) that have flowers less than two-and-a-half inches in diameter, to giant gladioli that have flowers over five-and-a-half inches in diameter.

Gladioli grow from corms—underground stems that serve as food storage during the plant's growth. Although gladiolus corms look like allium bulbs, they aren't true bulbs. Instead of layers of scales, corms have a solid interior. Other plants grown from corms include crocus, elephant ear, ranunculus, and taro root.

Since gladiolus corms are similar to bulbs, gladioli are considered summer-blooming or tender bulbs, like dahlias. To plant them, make sure you choose healthy corms and select the right location:

1. 1. Choose corms. Look for smooth, firm corms with intact husks. Corms are graded by size, and the bigger the corm, the bigger the flower spikes. Look for jumbo corms or corms rated one through three for the best chance of flowering.
2. 2. Check your soil. Gladioli prefer medium-fertile, well-draining loam or sandy loam with a pH of 6.0 to 6.5. To increase nutrition and drainage, work some compost into the soil before planting. You can also grow gladioli in containers at least 16 inches deep and filled with potting soil.
3. 3. Plant in late winter or spring. Unlike winter-hardy spring bulbs (such as daffodils and tulips), tender bulbs cannot survive underground in cold climates. In warm climates (USDA hardiness zones seven and warmer), though, you can plant gladiolus corms up to a month before the last frost date. Choose a spot that receives full sun (partial shade is also acceptable). Dig a hole four to six inches deep and wide enough to fit the corm. Plant corms at least five inches apart and cover with soil.

There are a few things you can do after planting to keep your gladioli healthy.

1. 1. Consider succession planting. The bloom time of a gladiolus is about one week. If you want blooms all throughout the growing season, consider succession planting. Plant your first corms before the last frost, and continue planting every two weeks until late summer. Once they’re planted, gladioli take 70 to 90 days to bloom, so stop planting about three months before the expected first frost.
2. 2. Use mulch. Spreading mulch around the base of the plants will prevent weeds and help retain moisture.
3. 3. Water once a week. Give your gladioli the equivalent of one inch of rainwater once a week during the summer. A weekly thorough soak is better than daily sprinkles.
4. 4. Stake tall flower spikes. When gladiolus flower stalks that reach two to three feet in height, you may need to stake the tall flowers to prevent them from falling over. Place the stakes at planting time to avoid damaging the roots of the plants when they’re already established. Tie plants to the stakes with string or cloth at 10-inch intervals. You can also plant gladioli in clusters of five to 10 plants and surround them with a tomato cage for support.
5. 5. Propagate your gladiolus plants. Each spring, a new gladiolus corm forms on top of the old one, and the old corm dies. Small cormlets form on the side of the new corm, and you can use these propagate gladioli. This process can continue for about six years, at which point the plants may stop flowering. If you live in a frost-free climate (USDA zones seven and warmer), you can keep corms in the ground until you want to propagate; in cold climates, you'll need to dig up the corms before the first frost. Check corms for signs of rot or disease, and discard any that are damaged. Dry corms at least two weeks, then snap off the old corms and store the new corms in a dry, cool place (35 to 45 degrees Fahrenheit) in mesh bags or paper bags. To help smaller cormlets reach size, you'll need to plant them next spring, but don't expect them to flower until their third year in the ground.
6. 6. Prevent thrips. Thrips are gladioli's biggest pest problem. These tiny bugs burrow in flower bulbs and cause flower damage. At the first sign of thrip damage, spray plants with organic insecticidal soap. Thrips can overwinter on corms. You can kill thrips in corms by placing corms in boiling water for two minutes.

Since they have such a short bloom time, most people plant gladioli to use them as cut flowers. The best time to cut gladioli flower spikes is when the first floret starts to show color. In the morning or evening, slice the flower spike from the stem by placing a sharp knife between the second and fourth leaves and cutting up on the diagonal. Immediately insert the gladiolus flower spike in water and keep upright. Leave at least four leaves on the remaining plant, and allow the leaves to die naturally to ensure that a healthy new corm develops.

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