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Right Plant Right Placeby Nicola Ferguson
Plants suitable for shallow soils over chalk
Gardeners with very alkaline or limy — as opposed to acid — soils have to contend with the unsuitability of their soils for growing rhododendrons, camellias, most heathers and a considerable number of other plants. Gardeners with shallow soils over chalk have to deal with the additional problem of excessively free drainage. With this second type of soil many of the plants so beguilingly described as 'revelling in lime' would, in fact, be inappropriate choices; these plants enjoy the fertility of an alkaline soil, but not all of them appreciate the dryness of one through which moisture passes extremely quickly.
The plants in the following list do well, rather than survive, in both the alkalinity and the rather dry conditions of shallow soils over chalk. (Occasionally, acid soil may overlie chalk but this combination is a relatively rare one.) For areas, within a chalky garden, which are particularly dry and hot, the list entitled 'Plants suitable for dry soils in bot, sunny sites' should be consulted. Almost all the plants which appear in that list do well in alkaline soils, but there are exceptions: hybrids and varieties of the common broom, Cytisus scoparius, and Berberis thunbergii and its varieties do best in a neutral soil (that is, one which is neither alkaline nor acid), as do Ulex europaeus 'Plenus', a form of gorse with double flowers, and Liriope muscari.
Many apparently obvious candidates for inclusion do not appear in this list. Some of these plants have been excluded because they become diseased on dry soils: for instance, without constant moisture Michaelmas daisies are, in general, prone to mildew (the exception to this rule, Aster x frikartii, has been included as a suitable plant). Another group of favourite plants for alkaline soils are the clematis but, particularly in the case of the large-flowered hybrids, they require moisture and a good depth of soil to succeed; even the vigorous Clematis montana and its varieties do best in a moist, well-drained soil. Some plants, like annual sweet peas, the larger antirrhinums or snapdragons, and the so-called florists' chrysanthemums, rather than the border varieties of the genus, are grown almost exclusively for large blooms; these will only be produced on soils which have plenty of moisture-retentive nourishment dug into them.
Occasionally, some members of a mostly unsuitable genus sound as if they might do well in chalk soils: in general, lilies, for example, prefer or need acid soils, but Lilium candidum, the madonna lily, does best in an alkaline soil. However, no lily of any sort must be allowed to dry out and, since this can be difficult to ensure on shallow soils over chalk, Lilium candidum is, in fact, as unsuitable as all other members of the genus tor inclusion in the present list.
There are also some plants which, most discouragingly, show their dislike of a shallow, chalk soil only some considerable time after they have been planted there. On dry, chalky soils, the various japonicas and flowering quinces often produce yellow leaves after having looked healthy when younger; and, after a few years on a shallow soil, the fast-growing Leyland cypress may become top heavy and Calocedrus decurrens begin to lose foliage from its upper branches.
Although the fertility of alkaline soil means that even gardeners contending with chalk have a large number of suitable plants to choose from, the range of plants with certain features is rather restricted. And, of course, there are the forbidden fruits of rhododendrons, camellias and most related plants. Indeed, the main problem for many gardeners who live in areas of shallow soil over chalk is not the high alkalinity or quick drainage of their soil, but their inability to overcome a longing tor, say, rhododendrons or heathers.
This longing can most easily be satisfied by growing otherwise prohibited plants in tubs, or similar containers, filled with some lime-free growing medium (sec list entitled 'Trees, shrubs and climbers suitable for growing in containers' lot a number of suggestions). Alternatively, certain small, manageable areas of the garden can have the alkalinity of the soil reduced and its moisture-retentiveness improved by the addition of peat and leaf-mould. In these areas the lime-tolerant, winter-flowering heaths or heathers and so on can be planted (for suitable plants see entries not marked with an asterisk in the 'Acid soils' list). The construction of anything other than the smallest of peat beds or walls is a more elaborate and ambitious project which only the enthusiast should attempt.
Finally, it is worth remembering that many plants associated with woodlands and acid, leaf-mouldy soils are not, in fact, adverse to alkaline conditions. The most important requirement of plants like hostas and astilbes, for example, is not acidity but moisture. By incorporating plenty of moisture-retaining substances such as garden compost, leaf-mould and peat, a range of plants even wider than the sizeable one given here can be grown in shallow soils over chalk.
Copyright © 1984 by Nicola Ferguson
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