Anna Pavord: 'Plants need to pruned to flourish - and now is the time to do it'

Pruning is an art that is worth learning, says our gardening correspondent

Rose stems pulled down to arch against walls or supports will flower more freely (Alamy)

There are two great moments to prune shrubs and one of them is now. Pruning is an art and it is one that is worth learning. The first lesson is that pruning means more than just cutting back. Cutting back is what you do when something in the garden is getting in the way. You aren't thinking about the essential nature of the thing you are attacking. You just want certain bits of it gone.

Pruning is a way of persuading the plants we have to perform even better (in our eyes) than they would if left alone. It may also cut down the space that the plant takes up, but that shouldn't be the first reason to do it. When you put in a tree or a shrub it's only fair that it's given the space it needs to express itself in its true form.

First – pruning clematis, because that seems to be the thing that gardeners, especially new ones, worry about most. Don't waste time on worrying. There are roughly three groups of clematis: early flowering, big-flowered mid-season ones, and late clematis. Of these, only the last group needs regular pruning. And when I say "needs" I don't mean that it will die if it is not pruned. It just won't display itself as well as it could. Flowers will appear in a bundle of growth high up, leaving you to look into a bird's nest tangle of bare stems.

The late-flowering group includes clematis of the Viticella kind and a guide such as The Plant Finder will helpfully tell you which group is which. "Etoile Violette", "Royal Velours", and "Venosa Violacea" are all members of the Viticella gang, which is perhaps the most useful kind of clematis to combine with an earlier flowering shrub, or to accompany a rose. If the clematis does well, it may try to completely obliterate its host. By clearing out the clematis growth each season, you give the supporting shrub a breathing space. It can get on with its own performance before the clinging clematis smothers it again.

So you need to track back to the point where the clematis starts and cut back all the stems coming from this point to about 45cm/18in off the ground. Then pull away all the top growth. You feel bad doing this if the clematis has already burst into fat bud on these stems, but it will bud up again and you will then have a chance to train the growths in the direction in which you want them to grow.

If you have planted a clematis to grow with a rose on a pergola or against a wall, the rose itself will probably need pruning now. So that means you also have to do something about the clematis. I've been thinking about a "Constance Spry" rose that needs dealing with. Although described as a shrub rose, it can easily get to 7m/20ft if it has support. It's a modern rose, bred by David Austin in 1961, but it looks like an old one, with big cabbagey flowers of a not too sick-making pink. It's beautifully scented.

It's been flowering, though, at bedroom-window height and needed attention. In this situation the best thing to do is to pull down the stems, arching them against the wall or support as near horizontal as they will go. This brings the bulk of the rose down to eye level. It also persuades it to flower more freely than it would if the stems were left in the upright position. Not all roses are amenable to this pulling down. "Constance Spry" has quite pliable stems. A rose such as "Compassion" does not.

The clematis growing with "Constance Spry" was "Venosa Violacea", a Viticella. So that meant it was easy to deal with both at once. If C.macropetala or C.alpina had been its partner, it would have been almost impossible to prune the rose without temporarily ruining the clematis. Those kinds both flower early, in April and May, and are already bursting with growth. Cutting them back now would not have killed them, but it would have ruined the chances of them flowering later this spring.

Rules are made to be broken though. If an early-flowering Clematis montana gets out of hand, as they often do, you can prune it immediately after flowering to reduce its bulk. Conversely, if I always followed the rules, I would prune our yellow, September-flowering C.orientalis every year, but I don't. It does a good job covering a trellis in front of a laurel hedge and does not get in the way of any other plants. So I sometimes leave it for five or six years before tackling it. It's the first pruning job on my list this month. If it ever stops raining.