Monday, October 15, 2007

Beautiful gardens in wet, sticky soil

In soil that stays wet, the roots cannot take in air because the air spaces within the soil are filled with water. The roots then 'drown' and rot, causing plants to die. Some plants, however, are specially adapted to life in wet soil, particularly bog-garden plants and those that grow in shallow water along pond margins. Some border plants are also happy in permanently moist soil that is not waterlogged, such as astilbe, canna, hosta, lobelia, lomandra, monarda, patersonia and schizostylis.

In a garden with a problem wet area, a common solution is to use the site to build a pond. However, the wet spot will often dry up in hot weather. Adding a liner will not help since in very wet weather ground water will rise up under the liner to make it balloon up, pushing out planting baskets. The best solution for a naturally damp spot is to use it for moisture-loving perennials, digging in lots of well-rotted organic matter to help it stay moist even in dry spells.

Plants for clay soil
Once established, many trees and shrubs like clay soil, but they can suffer from waterlogging in winter, so plant them on a slight mound. The best trees for clay soil include Callistemon 'Dawson River', coachwood, Monterey cypress, red-flowering gum and silky oak. Clay-loving shrubs include aucuba, chaenomeles, dogwood (Cornus), elaeagnus,
eriostemon, hydrangea, Indian hawthorn, oleander, philadelphus, shrub roses, salix and viburnum. On improved clay, many perennials will also do well, including agapanthus, astilbe, euphorbia, hellebores, hippeastrums, hosta, rock lilies and Swan River daisies. For planting under deciduous trees, daffodils, snowdrops and primula are good companions. Annuals and tender perennials are slow to get away on cold, clay soils, so, in frosty areas, they need to be confined to con­tainers until later in spring.

Improving clay soil
Clay can make a good garden soil if it is improved, which need not be hard work. Don't dig deeply because this can bring to the surface infertile, yellow or blue subsoil. Improve the soil by digging in a barrowful of well-rotted organic matter for each square meter and a 5cm layer of grit, river sand or fine gravel to one spade's depth. These materials are cheapest when bought from a landscape supply company. Don't use building sand because the small particles make it pack down too closely, and it contains too much lime. Mulch with organic matter in spring and summer.

If you have wet soil but you want to grow plants that need good drainage, such as herbs, rock plants and some perenni­als, you will need to build a raised bed that is wide and deep enough for the soil not to dry out too readily.

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