Let's face it - gardening on clay soils can be tough going. Soil which is wet and sticky in winter and then dries out like concrete in summer isn't exactly the favorite choice of most plants. And it can nearly you drive to despair when just about every tree at your local nursery carries a label saying: "needs friable, free-draining conditions." What about trees for clay! I hear you cry. Well there are trees that will grow in heavy clay and some hardy types will actually thrive in the stuff.
Deciduous trees often do better than evergreens when it comes to surviving in clay. This is because many deciduous trees don't come into leaf until late spring (by which time the soil is warmer and drier) so they're less likely to suffer from wet feet due to cold, wet conditions.
Three great deciduous species for heavy clay soils are Oaks, European Ashes, and Liquidambars.
European ashes are well-suited to clay soils because they can not only handle wet, sticky conditions in winter, but also have good tolerance of hot, dry conditions in summer (ashes are a member of the olive family). Not only that, but they grow quickly and look green and healthy all summer long. However, Asian and American ashes are not as clay tolerant as the European ash, so be careful when choosing varieties.
Oaks come in various forms and have a high tolerance of hot summer winds. Pin oaks, which are also known as swamp oaks, come into leaf in late spring and are particularly good for low-lying spots which don't drain very well in winter.
Liquidambars have similar leaves to maples and put on a fine display in autumn with their colourful foliage. They have higher moisture requirements than ashes and oaks, but are fast growing and also have pretty good tolerance of strong summer winds and hot conditions.
To make sure your trees get off to a good start, be sure to plant them correctly. In clay soils it's necessary to dig wide planting holes (two to three times the width of the root ball of the tree is recommended). This will allow the roots to spread out into the surrounding soil and help the tree get established faster. Don't be tempted to add lots of soil amendments like compost or sand since this may turn the planting hole into a water sump that attracts water from the surrounding soil during wet periods.
After planting, you'll need to pay some attention to watering and mulching, especially during the first few years. Water trees deeply during dry weather to encourage them to put down deep roots. Mulching is essential around new trees in summer as it helps preserve moisture and stops the surface of the soil from baking hard and restricting the growth of surface roots. The best time to lay down mulch is in the spring when the ground is still moist.
Use organic mulches like straw or rotted compost, and apply them thickly. At least 7-8 cm is necessary to conserve moisture as mulches quickly rot down in hot weather. If using lawn clippings make sure to add some nitrogen containing fertilizer like urea or sheep manure as lawn clippings temporarily take nitrogen from the soil as they rot down. Organic mulches also help to condition the soil by increasing its humus content and encourage worm activity, which helps to aerate the soil.
Once established, these tough deciduous trees will only need occasional watering during particularly dry conditions. Otherwise they usually need little attention as they don't tend to suffer from many pest and disease problems.
Michael H Carr is freelance journalist and a keen gardener with experience in developing new gardens on heavy clay soil subdivisions. He currently lives in New Zealand.
He has produced an e-book entitled Clay Soil Gardening, a no-nonsense guide to growing plants on heavy clay soils in temperate climates.