Bulbs, corms, tubers and rhizomes can all be described in general terms as being ‘storage depots’ for the foods that sustain the plant during its resting period, enabling them to grow and flower later. The most important characteristic they share is that, by and large, they exist underground. The rhizome is an occasional exception to this generalization as it sometimes grows horizontally on the surface. The Bearded Iris is a case in point, the rhizome being partly exposed above the ground as the plant grows.
It is in the decorative field that bulbs, corms, rhizomes and tubers excel, and few other types of plants can rival them in their brilliant colours. They are all the more valuable in that they generally last over several weeks. Thus, by careful selection it is possible to have flowers from this type of plant blooming in the garden practically the whole year round.
Among them, there is a wide range of shapes and sizes, from some Snowdrops that stand only 7.5 cm (3 in) tall to the stately Golden-rayed Lily, which towers above its neighbours, growing to a height of 1.5 m (5 ft). Almost every conceivable colour and shade of flower are found among bulbs, corms, tubers and rhizomes, so that really every taste can be accommodated.
In the gardens, they are invaluable for infilling the spaces between shrubs, herbaceous perennials and other plants, so helping to give brilliant colour to the beds. Better still, they bring this beauty to a border or bed at a time when the other occupants are either lying dormant during winter, or have already passed their peak
An advantage of the early spring-blooming bulbs is that when their flowers have died and their foliage withered, they can be dug up to make room for the summer flowers – the tuberous Begonias for example, or Dahlias and Gladioli which are grown from corms. A further value of some of these plants (bulbs, corms and tubers in particular, as these are more usually lifted after flowering), is that, like annuals and biennials, they provide opportunities to ring the changes and to introduce new ideas without waiting the relatively long time that it takes for shrubs or herbaceous plants to reach maturity.
From a practical gardening point of view, there is no difference between bulbs and corms and they are handled in a similar manner.
This is composed of fleshy, modified leaves or leaf bases. These are either tightly wrapped round each other and enclosed in an outer skin or scale, as with Tulips and Narcissi, for example, or the scale leaves are all separated from one another but closely grouped, as with Lilies. In both cases, the bulb so formed contains the young plant.
This is a swollen, underground stem, which is often enclosed in a paper skin. It resembles a bulb in appearance, but in fact it is solid and not composed of overlapping, fleshy leaf bases, nor does it contain the young plant. At the top of the corm there is a bud, from which both shoots and new roots appear. Typical examples are Crocuses and Gladioli.
This is most frequently an underground stem, that generally lies horizontally in the soil. It looks similar to a root and carries both roots and leaf buds. Typical rhizomatous plants are Bearded Irises and Lilies of the Valley.
This is either a thickened fleshy root or an underground stem. The Dahlia and the Potato respectively are well-known examples of these two types.