Difference between bulbs and corms
Bulbs and corms both occur on plants which have evolved reservoirs in which to store energy during dormant periods. But the plants swell in different parts of their anatomy.
A bulb is a swelling of fleshy scales at the base of the leaves. The scales enclose next year’s stem and flower bud; the onion is perhaps the best-known example.
A corm on the other hand, is the swollen base of a stem. The new corm grows on the back of the old one. which starts to wither once the plant has finished flowering. Crocuses and gladioli are familiar examples.
For practical gardening purposes the differences have little importance. The key thing is to understand the likes and dislikes of the individual plants and varieties.
Dividing corms in spring
Some corms grow in clumps which can become very congested. The young, healthy cormlets are found at the edge of the clumps, while those in the centre are often dried up or decaying. If you are lifting a clump for winter protection in autumn, you can always do your dividing at that stage. But the clumps of hardy species tend to get neglected – which is when a spring division becomes necessary.
Crocosmia provides a good example. This is a genus of yellow and orange plants which came originally from southern Africa. Despite their tropical origin they are usually very hardy so you don’t need to lift them for winter. However, the groups can become overcrowded, so it is wise to lift and divide them in spring. Do it Just as the young shoots are breaking through to the surface. Replant them immediately in soil enriched with plenty of rotted manure or compost.
You would often see corms being sold off in shops at the end of the season at a bargain price. They look healthy enough, except that their skin is often missing. The outer skin, or ‘tunic’, is a corm’s protection against disease and drying out. Corms that have been lying around in a warm shop unprotected are likely to have become very dry and the loss of moisture will affect their performance. Worse still, disease easily penetrates unprotected tissue. The best advice, therefore, is to avoid end-of-season corms – and bulbs and tubers too, for that matter.
Protecting corms from frost
There are certain hardy corms, such as crocuses, which need no protection. But most gladioli and other corms are damaged by frost. Unless you lift and store them in a cool, dry place for the winter months you are liable to lose them.
If you find lifting a chore, go for the hardiest corms. A few of the less showy gladioli will survive a mild winter in the ground. The hardiest of all is the wine-red Gladiolus byzantinus. Otherwise, the only possible alternative is to try giving the soil a top-dressing of organic material to act as a protective winter blanket. The method is by no means guaranteed to work, but scientific research suggests that a layer of straw or dried bracken acts as a good insulator. Remove it in spring. Aim to make the layer at least 3-4in thick. There is no point in using peat or rotted manure, since both tend to become waterlogged and freeze.
Disease can very easily enter a corm at the top, where the leaves once sprouted. For this reason you should never lift green plants and cut off the foliage; it will leave a gaping wound. Instead, leave the plants in the ground until the first frost has blackened the foliage. This seals the tissues naturally.
Having lifted the corms, discard any which appear to be damaged already, or which have unusually wide, open necks. These will be prone to disease and may infect healthier specimens. Having made your selection of sound, narrow-necked corms, spread them out upside down in a shed out of the sun and allow them to dry. It is best to allow them to dry out naturally, since heating tends to shrivel them. When they are dry. after about 10-14 days, rub off any excess soil, taking care not to damage the fibrous skin, or ‘tunic’. Do not wash the soil off. since this will encourage the development of storage moulds later.
When the corms are clean, spread them out and dust them with flowers of sulphur. This helps to keep mould at bay. For storage, place the corms in trays or nets and keep them in any cool, dry, frost-free building. Inspect them from time to time and remove any suspect corms before they infect the others. Small corms are particularly vulnerable to drying out and shrivelling, so store these in dry sand or peat in a sealed polythene bag.
Do make sure, though, that the storage site is properly cool and dry. If the sand or peat moistens, the corms may start to root prematurely, before you get them back into the ground next spring.
How deep to plant corms
Some bulbs, a number of daffodils and tulips among them, definitely benefit from deep planting. But corms do not. New corms are produced each season on top of the dying originals, and they alter their position in the soil, automatically adjusting themselves over a period of years to the depth that suits them best. Planting too deep seems merely to stunt their natural development, so for corms stick to the basic rule: place each one in a hole twice its depth so that the top of the corm is covered by a layer of soil as thick as the corm is high.
Corms Problems and Cure
Stems, leaves or flower buds covered with small green or black insects, and often stunted or malformed
Aphids (greenflies or blackflies)
Spray with derris. malathion or a systemic insecticide such as dimethoate
Flowers and foliage of gladioli have silvery streaks and patches which ultimately turn brown
Dust corms with HCH powder before storing; spray affected plants outdoors with malathion
Leaves of gladioli turn yellow and topple over, usually before the flowers appear. Corms show black spots or lesions and later the whole corm shrivels
Remove and burn infected plants; soak the remainder in benomyl or thiophanate-methyl before storing. Replant in a fresh site
Soft rot at the base of leaves, often causing plants to topple and die. Corms have round, shrunken craters with distinct shiny margins
Remove and destroy infected plants, and treat healthy stock with a fungicide as for dry rot
Foliage on anemones has patches of waxy whitish powder: sometimes leaves are also distorted
Spray with mancozeb
Leaves, particularly on narcissi, show yellow or grey streaking or mottling; vigour gradually diminishes
No cure available. All infected bulbs must be destroyed