Pruning: Why and How

Many gardeners never prune their plants because they are scared of damaging them. The reason for this unfounded fear is that pruning has been made to appear more complicated than it really is. There’s no great mystery to pruning, particularly pruning ornamentals, and there’s really very little that can go wrong.There is a difference between pruning and trimming. Pruning is a training method; the cutting makes the plant grow in a desired direction or shape or encourages the growth of a particular type of wood. Trimming is simply removing excess growth and reshaping an already existing structure.The theory of pruningThe main reasons for pruning are to promote strong new growth that rejuvenates the bush, to produce a well-shaped plant, to maintain plant health and to produce good crops of fruit or flowers. Pruning is also important in maintaining good ventilation, which reduces fungus problems, and allowing light to penetrate to the centre of the bush or tree.When shaping a plant you must have an understanding of the way it develops. Severe trimming and topping are damaging in most cases. Not only do they spoil the shape of many plants they can also weaken the plant, especially if the repeated at regular intervals. Regular pruning lessens a plant’s photosynthesis ability and depletes its stored reserves.Heavy pruning can also produce branches that grow at acute angles. These are more easily damaged by wind or may eventually break under their own weight. Careful trimming and thinning, however, can strengthen a plant by removing weak branches and enabling the plant to channel its energies into stronger growth.Most woody plants grow primarily from the branch tips. That’s where the new spring growth will start then it will progress back down the branches. Growth hormones are more concentrated in the growing tip and this tends to inhibit lateral branching. This is known as apical dominance and is seen at its most extreme in the strong suckers or water-shoots that occasionally break, usually from the base, and race ahead of the main growth.Apical dominance remains even if the branch tip is removed. With most trees and shrubs, you can expect that it will be the bud immediately below any cut that will be the first to start into growth. That initial growth spurt will often suppress the growth of buds lower down. This is important because it enables you to determine the direction of a branch’s growth.Consider the ultimate shape of the plant before you cut. Bearing in mind that any branch will tend to shoot from the bud immediately below a cut it’s clear that if the centre is to remain open you must cut to buds that face away from the centre of the plant These are known as outward facing buds. In some instances you may wish to leave a few inward facing buds to fill in the centre of an otherwise loose growing shrub.Many times a plant is just too dense and twiggy to prune it with precision. In these cases and all over trimming and thinning will usually suffice. There’s no point in attempting to cut to outward facing buds on something like a Hebe or a low bushy conifer.The practice of pruningWhen is the right time to prune? The answer to that depends on the type of plant and the severity of your winter climate. Hardy deciduous plants are usually best pruned in the winter. They are unlikely to be damaged by the cold and will be less likely to bleed (ooze sap) during winter. Frost tender plants are best left till spring; cutting back over winter only exposes the vulnerable cut stems open to frost damage. Spring pruning will still allow for an entire season’s growth before the next winter.It is generally recommended that the best time to prune most hardy plants is late winter. This is mainly because the branch structure is more easily seen when the plants have few leaves but there’s no reason why you shouldn’t trim and thin in summer if you wish. Don’t cut back too early as spring pruning can lead to cuts that refuse to properly heal. Shaping the plants during the growing season, when you can readily see the effects, is often easier than trying to envisage exactly how the growth will develop after winter pruning. Hedges may need two trims during the growing season.How far to cut back? That’s the question that always that leads to confusion. You can find all sorts of theories about how hard to cut back and why but it all comes down to the initial reasons for pruning; renewing vigour, maintaining health and shaping. As mentioned earlier too severe a trimming may actually lead to reduced vigour and a poorly shaped plant.Ensure that your secateurs (and saws too if heavier weapons are needed) are sharp so that all the cuts are clean. Trim so that water runs away from the bud. When trimming side branches remember to first make an undercut on the lower side to avoid the bark tearing as the branch falls.The exact method of cutting of side branches is open to debate. The old school of though was that you cut as flush as possible to the main stem then sealed the wound with a pruning paste or paint. Recent research suggests that it may be better to leave a stub, or crown, and only use pruning paste (preferably one with anti-bacterial agents) on plants that are prone to infection, such as apples and pears. This appears to encourage a more rapid formation of callus tissue and therefore quicker healing.The general methods for pruning shrubs and small trees are as follows. First look for strong branches of the previous year’s wood. Having identified these main stems completely remove any old, diseased or spindly branches so that just the main stems remain.The primary objective is to maintain health. You have already removed the old, diseased and spindly wood, now take out any overlapping branches, water-shoots and basal suckers. What is left will be a healthy but unshaped plant.Cut back each main branch to the first outward facing bud you find the stand back and look at the shape of the bush. It will grow well but is it going to be the shape you want? If not trim back to lower outward facing buds or those that will grow to the shape you require, making sure they are on good wood, and remove any misplaced branches. Does it have too many main branches? If so thin out some of the branches, you really only need four or five main stems. Is it still too tall? If so cut lower down.To summarise, the procedure for winter pruning is as follows.Completely remove any diseased, damaged or weak wood. Remove suckers and overly vigorous water-shoots. Locate the healthy main branches formed during last season’s growth. Cut back to healthy outward facing buds. Assess the results and adjust as necessary. Always remove any fallen debris and spray with a fungicide after pruning. You may well have disturbed fungal spores that will find an easy entry to the plant by way of the freshly cut stems.A pruning paint or paste can be used to seal the cuts but as mentioned above there are doubts about its long term merits. It may prevent immediate fungal infection but it could also slow down or prevent proper healing.Pruning berry fruits, vines, canes and fruit treesThe correct pruning of fruiting plants is a very important part of ensuring that you get the best crop possible. You’ll always hear stories along the lines of: “I’ve got an old plum tree, never prune, and it always has a huge crop of plum. Good ones too”. It may be so but it wouldn’t pay to bank on your trees performing the same way.Fruit pruning, particularly fruit tree pruning, is a little more complicated than pruning ornamentals. It’s a combination of thinning, shaping, maximising the plant’s potential and improving its crop. Improving the crop means getting the best fruit not necessarily producing the largest possible quantity.The most important of pruning fruit crops is understanding where, and how, the fruit forms. Some plants, such as plums and cherries, flower and fruit on the previous season’s growth. Others, such as black currants and grapes flower and fruit on the new spring growth. Both types may produce fruiting “spurs”, that is a special type of growth, distinct from the normal wood, on which flower buds form.Berry fruitsThe pruning method depends on the type of wood on which the fruit forms. Raspberries, boysenberries and loganberries fruit on the wood of the previous season. Remove any canes that have already fruited immediately after the crop is harvested. Cut back the fresh canes in August to encourage branching.Black currants fruit on the first year’s wood so they may be cut back hard each winter. Gooseberries, blueberries, red currants and white currants flower and fruit on new side shoots so some old wood must be retained. Thin out tow and three year old branches to encourage fresh new growth.Fruiting vinesMost fruiting vines flower on the new growth so they can be cut back hard each winter. With grapes cut the lateral growths nearly back to the main trunk in winter. Leaving too many fruiting spurs will result in overcrowded undersized fruit that may fail to ripen properly. Be prepared to sacrifice some quantity to raise the quality. Thin any very large bunches of grapes: nearly always there will be a few grapes that are obviously going to be of poor quality, they are the ones to remove.Stone fruitPeaches and nectarines flower on young wood so remove any branches more than two years old. Plums, apricots and cherries flower and fruit on older wood too, which allows greater leeway when pruning. After the initial framework of the tree is established little pruning should be needed. Any pruning should be done as soon as possible after harvest to lessen the risk of silver leaf disease.CitrusMost pruning can be done when harvesting the crop. Cut the fruit from the tree rather than picking it. Cut the stem back to a firm outward facing bud and thin out any weak growth. Other than this little or no pruning is required or desirable. If you do have to cut back overgrown old trees do it in spring after any risk of frost damage has past.Other fruitIn most cases deciding when and how to prune is a matter of determining whether the fruit forms on the old or new growth and pruning accordingly.Apples and pearsApples and pears have distinct fruiting spurs that are instantly recognisable once you know what they look like. All wood above the spurs can be removed as can any weak or unproductive shoots. After the initial training the framework of the tree is easily maintained by cutting back to the fruiting spurs.AvocadoAvocados should be kept cut back so they remain compact. They flower and fruit over quite a long season so any pruning is bound to affect the crop to some extent. Nevertheless, some cutting back is usually desirable. Do not prune if there is a risk of frost damage.FigWhen trained against a wall aim to keep about five to eight strong shoots. The oldest of these should be removed each year to make way for new wood. Figs often produce crops in spring and autumn. Any pruning must be delayed until the spring crop is ripe, which may mean some of the autumn fruit has to be sacrificed.

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