Garden: Under the greenwood tree

Choose shade-giving trees with care: both plants and people will benefit

THERE IS a period in the year when the word "shade" is no longer such a dirty word in the gardener's vocabulary. After the prolonged hot, dry weather we have had lately, those of us with trees in our gardens are profoundly grateful for them.

This gratitude is not shared, however, by plants languishing underneath, because what gardeners call "dry shade" - that area of soil under a deciduous or evergreen tree or large shrub - presents some hefty obstacles for plants determined to grow and flourish. We may like to sit under a tree in the afternoon heat, but the droughty soil that is so often associated with such a place puts stress on the vast majority of plants.

In the wild, herbaceous perennials that grow in woodland almost always begin to grow early in the year, and get their flowering over and done with before the tree cover becomes fully established. Think, for example, of dog's mercury and bluebells in our native woodlands, spring-flowering and quick to die down to nothing above ground thereafter. Our native trees, especially oaks, are slow to come into leaf, so that it is usually mid- May before their combined shade begins to make any impact on what lies below.

If we want to grow a deciduous tree or large shrub in our gardens, it makes sense to look out for those that are comparatively slow to come into leaf, for underneath them, we can then plant our snowdrops, aconites, pulmonarias, hellebores, herbaceous geraniums and other plants that flower early. Oaks, ashes and walnuts spring immediately to mind, together with the Judas tree, Cercis siliquastrum, Gleditsia triacanthos, and the tree of heaven, Ailanthus altissima. An eminently suitable plant, because it can be both a magnificent tree and a manageable shrub, is Catalpa bignonioides, the so-called Indian bean tree.

C bignonioides enjoyed a vogue in suburban gardens between the wars, but it is rare that you now see a young plant growing, except in the less vigorous, more shrubby, yellow-leaved form called `Aurea'. This seems to me a pity, because the type tree has a definite majesty when fully grown. It has a spreading crown of branches and enormous, heart-shaped leaves, measuring up to 12in long and 8in across. This tree is an obvious candidate to place at the end of a vista, to act as a focal point, to serve as a specimen in the middle of a lawn or give verticality to a large mixed border. The leaves do not unfurl until well into May but, in any event, the configuration of branches is not unattractive.

You may have come across one of these trees in a public garden in the last few weeks and been impressed by the large, white, two-lipped flowers, spattered inside with yellow and mauve markings. Even more remarkable are the dead-straight, brownish-black beans, as much as a foot long, hanging conspicuously down among the leaves from early autumn and persisting even when the leaves have fallen.

These days, the golden-leaved form is more widely available, presumably because it is less vigorous. In spring, the young leaves have striking, bronze-purple tints. This colour soon disappears, leaving the foliage a soft yellow.

Although eminently garden-worthy, this form has the infuriating habit of most yellow foliage of burning up in too hot sunshine; the colour also loses definition in late summer, turning a sickly yellow-green. All catalpas require a sheltered place in reasonable soil but, considering the large surface area of each leaf, they are not particularly thirsty plants. They are certainly hardy, although the buds and young growth can be caught by very late frosts in spring.

You can grow them as specimens, with a single trunk, or "multi-stemmed", so that they will make more of a shrub. Even better, however, is to cut them down hard every spring, to three buds from the ground, so that they grow long new shoots with leaves that are even bigger than usual. This is the best solution if you want to grow them as features in a shrub border, where they will make a bold and adventurous statement. What is more, in such a situation they act as good summer ground cover, so that there is no room underneath for anything to languish in dry shade.