Digging deep: Anna Pavord answers a reader's query about coping with our changing climate, and gives some sound advice on what to plant and when to plant it

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'Like many others, I am anxious about the effects of global warming on my garden,' writes Mrs Jean Smith of Beaminster in Dorset. 'This summer some of my plants, particularly a Cornus controversa ' Variegata', have suffered in the hot, dry weather and I would appreciate help in choosing trees and other plants that will not be so affected by the new climate.'

Mrs Smith has asked a clear question and has every right to expect a clear answer. But I can't give it, doubting, as I do, whether even the most professional brains in the climate business are sufficiently well equipped to understand its complexities. We amateurs aren't helped by the fact that the brains so rarely seem to agree. Models are made, outcomes predicted, but they swoop about all over the place, depending on who has forgotten to put what into the mix.

We've had a hot summer, yes, but nowhere near so damaging in the garden as the summer of 1976. Some plants have certainly suffered more than others this year, but the key to survival is not only choosing the right plant, but planting it at the right time and in the right place.

There are several reasons why autumn-planted trees and shrubs have a better success rate than spring-planted ones. The soil is still warm from summer - especially this autumn - and provides a more inviting medium for roots to explore than the dank, chilly soils of spring. Leaves are falling now, or have already fallen, so the tree or shrub does not have to slog like a steam engine to pump up enough water through the roots to satisfy the greenery above. Lack of water is the biggest single reason for death in newly planted trees. Perhaps a third of them never make it through their first summer in the garden.

As the overhead network of leaves and shoots shuts down for winter, trees and shrubs divert their energy underground and rapid root growth takes place between leaf fall and the New Year, by which time the ground has cooled down. But a plant that has had the opportunity to get its roots well established during this time is much better equipped to survive during the explosion of growth in spring and through the potential stresses of a hot, dry summer to come.

There's another reason why I like to do the bulk of my planting in November and December. The nursery lifting season traditionally starts on Guy Fawkes Day, which means I now have the choice of buying plants bare-rooted rather than container-grown. A plastic pot is a measly thing for a tree to be reared in, when we know that its root spread ought to be as extensive as the spread of branches above. Roots in a container get horribly crowded, and the main anchors coil themselves in a heap which is very difficult to sort out at planting time.

Trees and shrubs that are field-grown and lifted just before dispatch have a much more generous and balanced root system as they have never been pressed for space. The roots are generally trimmed a little after lifting to make a manageable bundle, but even so, you end up with a much more generous and well-balanced root ball.

Even in the autumn, there are no short cuts to proper planting. First douse your tree or shrub with water (if it is in a container) or soak it in a bucket of water (if it is bare-rooted). Never leave a bare-rooted plant lying around uncovered. If we could hear it, it would be screaming.

Dig a decent hole for the plant. Decent in this context means large enough for the roots to lie comfortably spread out without any cheating on your part. This is important. If the soil is particularly thin or viciously unyielding, work some compost or well-rotted manure into the bottom of the hole, chopping it in with the edge of your spade.

I usually add a handful of bonemeal at this stage, as it is rich in phosphates which encourage root growth. Save other fertilisers for spring when the plant will be needing nitrogen to support leaf growth. Stand the plant in the hole and check that it is the right depth. There is usually a dark mark on the stem that shows where the final earth level should be. A stick laid across the planting hole will show whether the tree is sitting at the right depth.

Cover the roots with a little soil, wobbling the plant gently so that the earth settles down well round it. Then fill up the hole with earth, pausing at the halfway mark to tread down the soil carefully. Don't stamp. Roots need air as well as water. If you think your tree should have a stake, put this in before you plant, not after, and use a proper rubber tie with a spacer that you can adjust as the tree grows - otherwise you'll throttle it.

I've never had a Cornus controversa 'Variegata' but ordered one this year for the new garden. It's a fantastic eye-catcher, with branches whorling out in horizontal tiers, like a wedding cake. But it's slow growing and I fully understand why Mrs Smith has been so worried about losing it. Because they are slow, they are always expensive. As it happens, this variegated cornus, usually grafted on to the rootstock of a tougher dogwood, should not be planted in autumn. The safest time to plant is not while it is dormant, but when growth starts again in spring. The same applies to pot-grown magnolias, which are best kept slightly on the dry side through winter and planted out in spring, as they break their dormancy.

Michael Haworth-Booth, the fine plantsman who wrote Effective Flowering Shrubs, a classic of the Fifties, thought nothing of preparing a planting bed six feet in diameter for his Japanese and Chinese cornus ( Cornus controversa comes from those parts). 'Much can be done by regular mulching and top-dressing, it is true,' he wrote, 'but it is never the equal of a bed really well made at the start which is also kept mulched and thus provides a large 'sponge' affording a continuous supply of liquid nourishment, which is, after all, the only kind in which trees are interested.' He also pointed out that these Chinese and Japanese dogwoods hate being pruned and that any cut may result in instant death.

So having got all that stuff about planting in the right way and at the right time off my chest, next week I'll suggest some trees and shrubs that, given this help, might have a fighting chance of getting through another summer of heat and drought. Whether they will survive our winters, which the experts predict will be wetter than they have been, is of course another matter.