12 Essential Things to Know about Corms

Corms are swollen underground stems that look like small stones. They're solid with tough skin and don't have any leaves or scales. Corms sprout from the top and may have buttons or eyes from where the new plant grows. A corm is the base of the plant's stem and shrivels up as the plant uses the nutrients.

Freesia, crocus, gladiolus, and taro have corms.

How you plant corms depends on the specific plant. Some need to be planted in spring, while others should be planted in autumn. Generally, you'll place corms between two and five inches below the soil, but you might not need to worry too much about the depth; many corms produce roots from the top and bottom.

The bottom roots are normal fibrous roots, but the ones that form on the top are contractile roots. They pull the corm deeper into the soil as they grow in response to light and temperature changes. Once the corm reaches a place where the temperature is suitable, and there is no light, the contractile roots stop growing.

After the first frost but before the ground is frozen, dig up your corms to keep them safe over winter. Leave the husks, but cut off any stems. Let them dry for a few weeks, then store them in a dry, dark place for the winter. If you live in a warm climate, you should still dig up your corms every few years to divide them so they don't get too crowded.

When you dig up your corms, you'll probably see new, smaller corms forming around the top. You can plant these ones, but they may take a year or two to get large enough to produce flowers—the larger the corm, the better the bloom.

Bulbs are divided into layers, like onions, and this is the main difference between a bulb and a corm. Corms are solid; bulbs are not. The plant's stems and leaves are in the bulb, with the bottom of the bulb being the stem where the roots grow.

Deep inside the bulb, surrounded by layers of nutrients, is a bud that will eventually grow into the flower. Most bulbs survive year after year.

Hyacinths, daffodils, and tulips all grow from bulbs.

When you should plant bulbs depends on the plant and where you live. You can plant spring-blooming bulbs as soon as evening temperatures dip below 50 degrees F and at least six weeks before the ground freezes. If you live in a warmer climate, you may need to keep your sping-blooming bulbs in the fridge until the ground cools. Summer-blooming bulbs don't take as much planning. You can put them in the ground in early to mid-spring.

As a rule of thumb, plant each bulb two to three times as deep as it is tall, with the pointed end facing up. Keep them in clusters for the best effect, and ensure they have good sunlight and drainage.

Spring-blooming bulbs can survive the winter underground and then return in the spring. Bulbs that bloom in the summer, though, are not winter hardy. You can treat them as annuals and let them bloom for one year, dig them up, store them in a cool, dry place over the winter, and plant them again in the spring.

You should check any bulbs you leave in the ground every few years to see if they need to be divided. In time, bulbs will form smaller bulbs at the base, and the area will get too crowded.

Unlike bulbs and corms, tubers do not have a rounded shape. They are longer, thinner, and have a fleshy texture with tough skin for protection.

There are stem and root tubers, the difference being where the plant stores its energy. Potatoes are stem tubers, while sweet potatoes are root tubers. Most tubers only last one growing season because the plant completely uses the nutrients, causing the tuber to shrink.

Other examples of tubers are dahlias, peonies, ranunculus, caladium, and some begonias.

When planting tubers, place them in the ground sideways, with the buds facing up. The buds are easy to identify: just think of the eyes on a potato! The specifics for when and how deep to plant depends on the tuber.

For example, it's best to plant peonies in early fall, about six to eight weeks before the first hard freeze, and never more than two inches underground. On the other hand, you should plant dahlias in the spring, right after the last frost, and only about an inch below the surface.

Many tubers don't last more than one season. The plant uses all the stored energy, and the tubers shrivel up and don't return. Some plants form new tubers, though, which is how potatoes grow.

Shoots grow from multiple places on a tuber, and you can cut them up and plant each shoot as a new plant, which is something you can't do with bulbs and corms.

Rhizomes are very similar to tubers, the main difference being that tubers grow underground while rhizomes grow along the surface. The buds on a rhizome are the flowers and leaves for the following year's growth. They spread fast and are often really hard to get under control.

Rhizomes can be dense, like ginger, with compact nodes that don't spread; running rhizomes grow horizontally along the surface, like bamboo or poison oak.

Other plants with rhizomes are turmeric and lily of the valley.

Generally, you should plant rhizomes close to the surface. How and when depends on the specific species. Many rhizome plants, including ginger, should be in the ground right after the last frost. Others, like turmeric, take a long time to grow and should be started indoors in the winter.

Rhizomes are pretty hardy, so you don't necessarily have to dig them up to store them over the winter, but you can, especially if you want to divide them. Do this right after the first hard frost but before the ground is frozen.

You can break apart rhizomes with your hands or use a knife to cut them; just make sure each section has a few buds because that's where the shoots will grow.