Corms

One way plants store food and reproduce is with a structure called a corm.

​While most corms produce inedible flowers, some very popular edibles also get their nutritional jumpstart from corms. Saffron crocus, taro, and banana plants use corms to carry them through the off-season.

​​A corm is a type of perennating organ. Perennation is the ability of some organisms to live through Hard Times, such as drought or extreme cold, emerging healthy enough to grow again when conditions are more favorable. The food and water storage organs typical of many plants are bulbs, rhizomes, tubers, and corms.

​Different types of geophytes

Corms are similar to bulbs, tubers, and rhizomes, in that they are all geophytes. Geophytes are structures that store food and water. If you cut these structures in half, however, they look very different:

Crocus growing from a corm (Fred Souret) CC BY-SA 4.0

• bulbs - layered flowers and leaves, papery coating, flat bottom and pointed top, produce new bulbs called offsets from the basal plate (onions, garlic, tulips, lilies)
• corms - solid, enlarged stem bases, papery coating, flat bottom and pointed top, produces new corms called cormels (crocus, gladiolus)
• rhizomes - swollen, underground stems, adventitious roots at intervals (ginger, turmeric, iris)
• tubers - everything else; no papery covering, several growing points called eyes, no offsets or cormels (potatoes, dahlias, gloxinia)

Gladiolus corm with cormels (Hardyplants) Public Domain

Corm structure

Each corm is a segment of underground stem that includes at least one growing bud. Corms are often covered with a papery coating called a tunic. The tunic is made out of dead petiole sheaths that were produced the previous year. The tunic protects the corm against insect and animal feeding, flooding, and drought. The inside of a corm is tissue filled with carbohydrates. The flat bottom is where roots emerge and cormels are produced. In some cases, the parent corm is used up completely and is replaced by the cormels it produced. In other cases, the corm simply becomes larger. In yet other cases, a new corm grows from the top of an old one that withers and flattens. Over time, this type of corm ends up becoming a stack of used up corms.

Coppertip corm stack (Velela) CC BY 3.0

Many corms produce two different types of roots. Familiar fibrous roots anchor the plant in the soil and absorb water and nutrients. Contractile roots pull corms deeper into the soil, away from animals and extreme temperatures. Contractile roots stop pulling the plant downward as conditions above improve.

Caring for corms

When a plant grows from a corm, it draws moisture and carbohydrates from that underground structure. By the time the plant reaches its mature size, much of those resources have been used up and must be replenished. Use these tips to help your corms stay healthy:

• Do not remove leaves until they are fully brown; green leaves are producing food energy to be stored in the corm
• Fertilize/feed plants lightly right after flowering and just as they start putting out leaves
• In summer, allow spring-blooming corms to dry out, but water fall-blooming corms
• Dig up and divide corms every 3 to 4 years, to reduce crowding

Do you any corms in your garden?