While more easily called a bulb, the gladiola (Gladiolus spp.) grows from a corm, an underground modified storage stem. Rather than reproducing only by seed, the gladiola corm sprouts small stems called cormels or cormlets around its base during the growing season. These cormels slowly enlarge and replace the degrading mother corm. You can plant 1/2- to 1-inch-sized cormels.
Sizes of Corms
According to Ron Smith of North Dakota State University Extension Service, gladiola corms over 1 inch in diameter are most likely to produce leaves and flowers after planting. Smaller-sized cormels will yield leaves in order to produce food and enlarge the cormel. While any cormel, regardless of size, is capable of growing on its own, tiny cormels are difficult to handle, orient upright when planted and get to grow. Focus on dime-sized, 7/10 inch or larger cormels for planting.
Cormels begin to form once the main corm sprouts leaves in springtime. Slowly the mother corm degrades and the energy created by the leaves is diverted into making cormels. Do not dig up corms while the gladiola plant is actively growing. Wait until the plant leaves are killed by frost or naturally wither away in a dormant season. This ensures the cormels made that year are as large as possible. Keep the mass of corms together over the winter storage period and then separate cormels carefully to avoid crushing or bruising them in spring at planting time.
Depending on size, cormels are planted at a depth between 1 and 2 inches and 1 to 2 inches apart. Smaller cormels are planted more shallowly than larger ones. In lighter, faster-draining soils such as sand, plant them slightly deeper, whereas in heavier soils like clay, slightly more shallow. Soak the cormels in lukewarm water for 24 hours prior to planting. What makes planting cormels difficult is not immediately knowing which side of the cormel needs to be facing upward in the planting hole or furrow. That's why planting at least pea-sized cormels helps you determine which is the more pointed top and the flatter bottom where a root emerges.
When to Plant
Since cormels are so small, they are greatly affected by soil temperature and moisture. Wait until all danger of frost passes in spring before planting cormels. Ideally, the soil temperature should be in the 60 to 70 degree Fahrenheit range at planting time. Keep the soil evenly moist and do not compact the soil above the cormels with foot traffic.