Training exercise

To prune or not to prune, that is the question. Will cutting back an overgrown shrub just make it grow even more vigorously? Will chopping your clematis leave you without any blooms for next year? For the new or even seasoned gardener, pruning is a topic with endless questions and baffling answers. Sadly, there is no one pruning regime that fits every plant, but a little know-how can go a long way.

As a garden designer and panellist on Radio 4's Gardeners' Question Time, John Cushnie has answered more than a few queries about pruning over the years. "It can be daunting, but it shouldn't be," he says. "The first thing you need to know is what you're pruning – you need to know the names of your plants." Once you've got a name, you can do a little research and find out the best course of action.

Cushnie points out that even an informal wildlife garden needs a certain amount of maintenance if it's to look its best. Many plants, when left to their own devices, become overgrown and untidy, producing ever fewer flowers and fruit. By pruning, you are promoting growth – a useful thing when you are building up a framework of branches on a new shrub or tree, but less helpful when you're trying to stop a plant from outgrowing its allotted space. "The other important message to convey is that the correct time to prune often depends on the time of year when the plant flowers," says Cushnie. "Without that knowledge you may well cut off the very branches that would have justified the plant's existence."

For spring flowering shrubs such as forsythia, pruning after flowering gives the plant time to make healthy new branches where blooms will be produced the following year. Shrubs such as weigela that flower in summer or early autumn on stems produced that year should also be pruned when flowering is finished. A large number of shrubs, such as escallonia, rhododendron and laurel, can cope with severe pruning. If such a plant is old and overgrown, with all its flowers at the top, cutting all the main stems to knee height will rejuvenate it (although it's best to do this over a two or three-year period, removing one third of the oldest branches from different areas of the plant in late winter or early spring).

Identifying the need for clear information about how and when to prune, Cushnie has written How to Prune, a book that covers everything from choosing the right tools to creating the perfect fan-trained apple tree.

If you're faced with an overgrown garden that you want to get back into shape, Cushnie suggests you start by pruning the "three Ds" – diseased, damaged and dead branches, which should be cut back to above a suitable (healthy) bud, side shoot or branch.

"If someone has taken over an established garden and they don't really know where to begin, starting with the three Ds would get them well on their way to having a good plant," says Cushnie. "If you get rid of dead, diseased and damaged wood and keep the centre of the plant open, really you can get away with waiting another two years before you learn how to prune properly, and meanwhile you'll do no harm."

Keeping the centre of the plant open by strategically removing branches means you're allowing light and air movement into the heart of the plant. Cushnie explains that over the space of a year some fast-growing shrubs will end up with branches crossing from one side of the bush to the other. These will cause congestion and possibly damage by rubbing against other branches. So it's a good idea to remove the entire branch or shorten it to a side shoot that is heading in the right direction.

"The big tip I would give everybody is that whenever you're pruning, if there's a space where there isn't a branch and there should be, it doesn't matter if it's an apple tree or a rose bush, if you prune to a bud pointing in that direction, a branch will grow in that direction," he says. "It's simple; it'll fill in and you'll get a nice complete shape."

Of course, not all plants require this sort of careful pruning. Herbaceous perennials that die down in winter just need the dead stems pulled or cut off at ground level, either in winter or spring. Deadheading spent flowers while the plant is in bloom will ensure a longer flowering season and improve the look, although plants such as bush roses have such attractive rosehips that you might want to leave them in place.

One highly effective form of pruning is coppicing, where plants such as willow or dogwood are cut back hard in spring, leaving stumps with two buds, resulting in vivid new growth that provides winter interest in the garden.

"The ones that I really love pruning hard are things like eucalyptus and polonia," says Cushnie. "You can cut those down every spring and with the eucalyptus you get the lovely round leaves rather than the sickle leaves and it's such a vigorous plant, it grows six feet every year."

Some plants – roses and clematis, for example – have very specific pruning requirements, depending on the species and variety, so consult a good reference guide before delving in with the loppers. As for mature trees or shrubs, Cushnie says, know your limits.

"I would say to the average gardener that if you have to leave the ground, if you have to stand on a ladder or climb a tree or something like that, really you're much better off getting a professional in," he says.

Pruning can seem fiendishly complicated and it's tempting just to hack at everything, but once you know the correct tactics for any given plant you'll be well on your way to producing a more attractive garden. sm

• How to Prune by John Cushnie is published by Kyle Cathie, priced £19.99.