Updated: 21:00 GMT, 21 August 2009
I have been cutting my hedges in readiness for winter. The evergreens - yew, box and holly - need slightly different treatment from the deciduous ones - hornbeam, hawthorn and field maple - as they'll keep on growing all winter if the weather permits, which means there's a real chance of tender new shoots growing in a mild spell, and then being damaged by frost.
Deciduous hedges can be cut at any time, although it is a bad idea to give more than a light trim during the bird-nesting season, between the beginning of March and July.
However, when you choose to do your hedge cutting will have a profound effect on its growth, whether evergreen or deciduous.
Clip round the year: Immaculate yew hedges at Longleat
This is because, in effect, you are pruning each hedging plant and, according to when and how you prune, this will either stimulate or restrict growth.
I know this seems counter-intuitive. To prune means to cut back and therefore to restrict. But plants are not passive: they respond to the weather, climate, soil and damage in the way that will most benefit their survival and reproduction.
A healthy tree or shrub has two hormones that regulate growth - auxins, produced by the leaves and buds, and cytokinins, produced by the roots.
Strong upward growth in a shoot will suppress buds breaking on the stem below it. The most obvious example of this being the 'leader' or topmost shoot on a tree.
If you prune this away you remove the repressive auxin signal and a mass of buds will break. In the same way, if a hedge is clipped regularly, it will respond by becoming increasingly twiggy and dense and the regrowth will be a mass of small shoots - until one of them dominates and grows away and stifles the growth of its fellows.
The auxins travel down to the tip of the roots, where they limit root growth but are also destroyed in the process. But if the roots are curtailed too much, the increasingly vigorous leader shoots outgrow their source of nourishment.
Did you know? The base of a hedge should always be wider than the top. If you cut the sides straight, then the top will shade the bottom
However, cytokinins are produced at the root tip, encouraging the roots to grow constantly out, seeking more moisture and food. Then the signal goes back up the tree to the tips of the new leading shoots, limiting their growth.
But, just as the
auxins cannot survive the journey down the plant, so the cytokinins perish when they complete their voyage up it. Thus, the tree regulates its own growth in accordance to the sustainability of root and branch growth.
The roots start to grow before the leaves and continue after they stop, storing food to fuel spring growth. So if you cut between October and April, you will stimulate root growth.
When spring brings new buds, the plant will burst into growth like a runner out of the blocks. Cut the same plant in mid-summer and the regrowth will be much less enthusiastic.
So, if you want to rejuvenate a deciduous tree, hedge or shrub, prune in winter. For all-over growth, cut in summer.
The other consideration is when the plant in question flowers. As a rule, plants that flower in the first half of the year do so on wood that has grown in the previous growing season.
Prune these between midsummer and spring and you'll have lots of new growth, but no flowers. The opposite is true for plants that flower in the second half of the year.
Their buds form on new growth, so you can be ruthless in late winter - or spring - and cut them back hard in the knowledge that this will instigate vigorous new growth, which in turn will carry all the flowering buds.
Prune any plants that produce their flowers on new growth or which need a new burst of energy, and overgrown hedges, in winter, while evergreens, shrubs that flower on old wood and established hedges should be pruned in the summer months.
Sometimes you harness both of these growth patterns on the same plant.
For example, in winter I prune my espalier pears to encourage new shoots where I want to train them, and I know that the harder I cut them back, the stronger the growth will be the following summer.
In July and August I reduce new growth upwards from the fruiting spurs and any other growth that goes outside the limitations of the three parallel rows of the espalier. Another example is hybrid tea roses.
In winter I cut back hard to stimulate floriferous new stems, and in summer I dead-head, to stimulate side shoots and thus more flowers.
Points to remember: sharpen pruning tools regularly; the more you trim the sides of a young hedge, the denser it will grow; and a young or unhealthy hedge should have a foot of bare soil either side - mulched at least once a year and watered well.
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