If roses could choose, they'd settle for clay (Filed: 19/10/2002)

Rock-hard in summer, waterlogged in winter: but don't despair of heavy soil, says Carol Klein in the second of our four-part series

Heavy soils are often assumed to be the most problematic of the lot. The mere mention of waterlogging or "clay" conjures up visions of double-digging and chiropractors' bills. But the benefits of heavy soils usually outweigh their problems. The inevitable hard work is compensated for by luxuriant growth and high fertility.

There are two big problems with clay soils. The very fine particles are packed closely together to produce a heavy, ill-drained substance that is easily waterlogged. The soil turns into mud during the winter/spring "rainy season", then bakes hard with deep fissures after only a couple of weeks of summer sunshine. In both cases it is impossible to work.

No matter what else you do to ameliorate clay soils, the first step is to attend to drainage in any areas that are constantly wet.

Bad drainage is often caused by a layer of compacted soil below the surface of the topsoil resulting from constant use of a rotavator, too much foot traffic or, most usually, by the cultivation of only the top layer over a long period.

Only exceptionally bad sites will need a drainage system with a soakaway or other means (ditches or storm drains in extreme cases) to take away excess water. Deep digging usually solves the problem. Incorporating plenty of gravel and organic matter into the bottom layer of soil will physically change it.

This is a counsel of perfection, hard enough when you are faced with a new, empty site and even more difficult when the land is already occupied by plants. None the less, it is worth doing - even in a piecemeal fashion - if drainage is dreadful.

Silty soils have much in common with clay and should be treated in a similar way. Although the particles are bigger and do not form clumps, they are close together and so drainage is poor. Adding bulky organic matter helps to avoid waterlogging. And silt is easier to work than heavy clay.

Those who are lucky enough to garden on loam will know that they have the best of all worlds. Loam retains water and nutrients just as well as clay soil while draining perfectly. But it must still be maintained, even though it is the envy of those who garden on mud or sand.

Whatever our soil, if we expect it to grow our plants we need to cosset and nurture it.

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Ground rules

1 Keep off clay soils as much as possible. If you need to work and dig when the soil is wet, use a plank to distribute your weight and prevent compaction.

2 If you are starting from scratch, make beds and borders narrow enough to reach in from either side. If your border is backed by a hedge, create a hidden path between the two to allow access for tidying and replanting.

3 Dig in autumn before the earth gets too wet and leave the surface of the soil rough so that frost and rain can break it up during the winter.

4 Although digging helps drainage initially, it is unnecessary once plants have become established. In fact, it can be counter-productive. Not only does digging cut through plant roots but it destroys organisms, fungi and bacteria whose action increases fertility and helps plants to feed.

5 Mulching clay soils with organic matter, rotted manure or home-made compost is the best way to suppress weeds, increase soil fertility and retain moisture, preventing cracking and improving texture. There is no need to dig it in: worms, insects and other organisms will make an efficient job of incorporating it gradually into the soil. Wood chips or chipped bark are also effective mulches but may deplete the soil of nitrogen, which is used in the "rotting-down" process.

6 One of the best ways to break up clay soil is by adding lime. This allows the soil to flocculate - a chemical process where the fine clay particles combine to make much larger crumbs. Water and air can then circulate much more easily and consequently the soil becomes more friable. The best time to apply lime is in early spring, never with manure as the two can react to produce ammonia gas and release nitrogen into the air. There are various forms of lime. The longest-lasting and most expensive is calcified seaweed, which has the added benefit of containing some minerals and trace elements.

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Plants for heavy soils

Clay soils can be acid, alkaline or neutral. If you are not sure what yours is, it is worth buying a soil-testing kit. The plants growing in nearby gardens or in any wild areas - even waste ground - will indicate the acidity/alkalinity of your soil and give you a feel for what thrives.

Happily, a wide range of shrubs, trees, perennials, bulbs and vegetables will grow well in clay. And those that occur naturally in wet or reasonably fertile ground love it.

The better the soil is cultivated and maintained, the better the results.

Narcissi and snowdrops perform and increase well but tulips are best treated as annuals on clay.

The great majority of herbaceous plants grow well. Hemerocallis and fibrous-rooted irises such as I. ensata, sibirica and the delicious black Iris chrysographes will feel at home. Large-leaved perennials such as hosta, ligularia and rodgersia grow magnificently in clay soils and many of the sedges and ornamental grasses - carex, molinia and miscanthus, relish heavy conditions.

In shady clay, pulmonaria, hellebore and primula will grow apace; out in the sun, cranesbills, schizostylis and the North American daisy tribe, from asters to rudbeckia, will grow strongly alongside phlox and aconite. Aconite belongs to the ranunculaceae family, which thrives on clay almost without exception.

Most viburnums grow well on clay but probably prefer alkaline conditions. The same is true of mahonia and berberis. The whole gamut of widely cultivated shrubs from weigela to skimmia should do well, and if roses could choose where to grow they would probably opt for clay.

Ivy, honeysuckle, clematis and wisteria are all gross feeders and should race away in heavy soil.

Oak, ash and alder, apples and pears enjoy clay and so too do many ornamental exotics. Cercidiphyllum, magnolia and cornus all do well in my heavy clay.

In the kitchen garden, onions, potatoes and leaf crops all enjoy the high fertility that substantial soils usually ensure. It is here that the argument for growing vegetables in deep beds is most pertinent so that, after initial preparation, there is no need to dig or walk on the soil.

-- Anonymous, October 21, 2002

Answers

"leave the surface of the soil rough so that frost and rain can break it up during the winter." Didn't know that one...

Every year I ask Santa for a solar-powered jack hammer to break up these soils. My letters keep getting lost.

-- Anonymous, October 22, 2002

BrookS, your letters aren't getting lost. We read them all, and pass around the funniest ones. Then, we throw them away.

See? They aren't lost.

-- Anonymous, October 22, 2002