Adam's Garden

At this time of year bulbous plants are much in evidence, producing welcome colour to the sometimes drab winter scenery. First come the snowdrops, then daffodils and grape hyacinths, and then on to tulips and many others. There is some confusion between bulbs, corms, rhizomes and tubers so I shall try to provide some kind of rough explanation.

These underground storage systems, as I shall collectively call all of the above, have evolved primarily in order for the plant to survive adverse weather conditions during that part of the year they are not growing. In the case of daffodils this means the bulbs sit out the hot summer conditions, starting their life cycle in the winter, and in the case of autumn flowering plants like Schizostylis it means sitting out the winter before a more conventional life cycle begins in spring and summer. The benefit to the gardener of this off-season dormancy is they can be lifted for storage, out of harms way - vermin and disease - and put back in position at the appropriate time of year.

The structures of these systems vary markedly.

A bulb can be thought of as an embryonic plant. It has a tough basal plate and above this a shoot protected by modified leaf bases called the scales. Food is contained within the scales - starch, sugar and proteins - and roots burst out from the basal plate once dormancy is ended. Bulbs are often protected by a thin skin known as the tunic and they are mostly perennials, renewing the food stored in the scales each year. An onion is a good example of a bulb.

A tuber, such as the common potato, is a swollen underground stem which has been modified to store food reserves. It consists almost entirely of food reserves, and the surface is covered in buds which provide shoots when the conditions are right. This can be seen in potatoes that sprout when stored in the larder!

A corm is like a tuber but with a larger proportion of stem tissue. The food reserves are contained in the fleshy stem and the roots come from the base of this stem. The upper part has one or two buds, protected by modified leaves, and these buds produce the new plant. The key difference between a bulb and a corm is the latter exhausts its food reserves, leaving a dry husk that is replaced with a brand new corm large enough to flower and a number of miniature corms called cormels. Gladioli grow from corms and because they are not frost hardy need lifting for winter protection.

A rhizome is an adapted underground stem, growing horizontally, and is often thickened. Leaves and flower stalks grow from the top and the roots emanate from the underside. Irises are probably the most instantly recognised of plants in this category.

Root tubers are formed by roots that have transformed into storage organs and in the process have lost the ability to absorb nutrients in the usual manner. Other roots, normal in appearance, carry out this process for the plant. The buds are generally found on the collar at the base of the plant's main stem. Dahlias have root tubers.

When dormancy is over for any of these the roots are first to appear, anchoring the plant and absorbing water and mineral salts. Only then will the aerial parts appear. These ensure food supplies, including those for the following year which means it is important not to disturb the plant whilst it is growing. If conditions are unfavourable bulbs will continue to develop underground, using energy reserves to create new scales or secondary bulbs. In this way some lilies will remain apparently dormant for a whole season.

Although normal sexual reproduction occurs to produce seed, vegetative reproduction also occurs, producing plants identical to the parent. Bulbs and corms produce offsets, kind of mini bulbs, that can be detached and grown on. Some lilies can even produce bulbils, miniature bulbs, in their leaf axils. After flowering these drop off and will eventually grow into bulbs of flowering size. Stolons are produced by some kinds of tulip. These are runners up to 2 feet long or more thrown out from the base of the bulb and each produces new bulbs from the tip. Rhizomes advance by growing outwards, forming new shoots at the nodes.

It would be fun to try out a small bed using only plants in the category formed by our theme. Gaps could be minimised by growing on potted plants inside and sinking them when quite well advanced and once exhausted daffodils and tulips could be lifted and stored to prevent damage. Good luck to you if you try out this idea. Happy gardening.

You can find more articles in the archive under Soiled Hands.

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