The idea of woodland gardening is so appealing, isn’t it; a place perhaps for primulas, trilliums, ferns and hellebores and little treasures such as Vancouveria and Disporum. For gardeners living in the deserts of East Anglia moist woodland can be the sweetest of dreams.
But what is “woodland”? If you go into most woods the richest flora is usually around the edge of open glades where there is most light and moisture. In the constant dark of solid tree cover and in competition with hungry, thirsty roots, plant-life can be much less striking.
So it is in a garden too. Shade gardening is better done in the shadowy hinterland of trees rather than directly under their canopy. The soil will be more moist; you will be able to dig it and plant into it without constantly encountering roots that are taking the goodness out of it. On the other hand, if the only place you have to grow shade-loving plants is directly under trees, then which are the best trees to garden under?
Surprisingly, the trees you think of as light and airy are not necessarily the best. Silver birch, for example, is a shallow-rooted tree often found on sandy soils. It is a pioneer species of newly opened spaces, and puts out its roots fast to stake its claim on the space. On soils that are naturally dry, gardening under a birch can be difficult, although when birch grows in wetter climates things become easier, even if there are still shallow roots to contend with. There you can produce a fresh spring display so long as you keep feeding the soil with compost to build up humus and offer nutrition.
The same problem is found with robinia and its golden form ‘Frisia’. The idea that airy foliage is kind to the ground below is simply not true. The roots are shallow and hungry and, worse, when they are damaged by gardening they throw up suckers. Spiny suckers at that. So robinia and birch are trees best gardened behind rather than under, where they can provide dappled shade but not too much competition. Robinia, like the ash, is at least late into leaf and early out of it, which means that if it is light shade you want, then these are suitable species.
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The ideal tree under which to garden has a deep taproot and little surface root, and that is exactly what you get from the English oak, Quercus robur. You need only grow one from an acorn to see how true it is. The fat taproot comes out of the acorn like a greyhound from a trap, and shoots downwards almost faster than the green shoot grows upwards. Whereas a conker tree does the same and then makes a dense mat of fibrous surface root, an oak manages to get by with remarkably little surface root.
Oaks bring their problems. Sometimes they fall prey to mildew, which looks shabby, and aphid attack can mean honeydew drips on to foliage below causing moulds. On the other hand you can keep cutting off the lower branches until you create a high canopy suitable for tall woodland shrubs. The tree does not mind a bit, and those tall straight trunks can be a delight. If you fancy something with more extravagant autumn colour, try red oak Quercus rubra, the scarlet oak Q. coccinea, or the pin oak Q. palustris.
Steer clear of the evergreen oak Quercus ilex, however — its shade is as heavy as its thirst. (Scots pine, on the other hand, can be good to garden under, especially when trained to a high canopy.) Avoid eucalyptus too; they draw every last drop of life out of the soil. Keep off willows and alders and poplars, as their thirst is prodigious, and off hungry planes and limes. Avoid walnuts, as they chemically inhibit plants growing under their canopy.
Some trees resent the disturbance of people gardening beneath them. Magnolias are one, the handkerchief tree, too, Davidia, and cherries which, as well as being thirsty, also throw up suckers when the roots are damaged.
What’s left? Well there are smaller trees that are happy to be gardened under. Laburnums work well, and some of the less vigorous maples such as Acer nikoense, davidii and rufinerve. Many of the rose family are excellent: rowans, apples, amelanchier, the ornamental thorns, the wild pear with its creamy blossom, black corrugated bark, and golden fruits and yellow foliage in autumn — why do we not plant it more?
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Whatever tree you find yourself gardening beneath, you can never add too much compost. Sweep up leaves and pile them among the plants. Tip out spent summer containers of compost. Anything just to keep your woodland floor plants fed and watered.
Suppliers
Bluebell Nursery, Smisby, Ashby de la Zouch, Leicestershire (01530 413700; www.bluebellnursery.com).
Chew Valley Trees, Chew Magna, Bristol (01275 333752; www.chewvalleytrees.co.uk).
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Langthorns Plantery, Little Canfield, Great Dunmow, Essex (01371 872611; www.langthorns.com)