Winter pruning can dominate the gardening year as there is more time during that season and, for deciduous plants at least, the shape of plants is more discernible, thus aiding pruning to shape. However, this risks losing spring flowers and inducing vigorous regrowth.
Deciduous plants transfer their food reserves to their roots for winter before relocating them to the canopy in spring. Plants trimmed in winter have much more resources for regrowth than ones pruned in summer and can rebound quite dramatically. This leads to large handsome leaves, which is desirable in many cases – Sambucus, for example – but the vigorous shoots can take several years to come back into flower.
Winter pruning makes plants smaller, although it sometimes does not seem like it, but summer pruning greatly reduces regrowth and plant size by removing foliage whose photosynthesis drives plant growth.
Another important factor is that in summer, wounds heal much more quickly, limiting exposure to diseases such as silverleaf that infect through wounds.
Excessive summer pruning, however, can weaken plants by starvation if insufficient foliage remains to sustain the roots. Staging heavy pruning over several years avoids this danger. Typically, pruning need only begin three years after planting.
Spring-flowering shrubs do not always need pruning. If there is enough space for them, they remain full of flowers and they are not trained to walls and fences, they can be allowed to spread.
Plants that flower in late summer, Buddleja, Caryopteris and deciduous Ceanothus, would lose all their flowers if pruned now.
However, plants that have flowered since winter, Deutzia, Ribes (flowering currants) Weigela and winter jasmine (Jasminium nudiflorum) for example, can benefit from pruning now – removing older, flowered growth to where fresh green growth is already developing.
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Where needed, they can be kept to size by judicious reduction without loss of flowering next spring. This might involve removing some shoots, typically one in three or four, to near ground level. The sooner they are pruned after flowering the better as this maximises the time for new flowering shoots to develop.
As with any pruning, remove damaged, dead and diseased shoots entirely. Focus on congested areas to promote uncrowded and, therefore, less disease-prone flowering shoots. Cut out the thinnest, weakest shoots as these are unlikely to contribute to flowering and just deplete plant resources otherwise available for flower production.
Spiraea that flower in early spring (bridal wreath Spiraea arguta, S thunbergii) respond well to removing one stem in three to near the base. Forsythia, another very early-flowering plant, is also treated in this way. Sometimes Forsythia is clipped to make a ball, or a hedge or trained against a wall. These can be clipped after flowering and usually flower well enough. If any remain unpruned, treat as soon as possible.
On the other hand, later flowering shrubs such as Diervilla and Philadelphus won’t be ready to prune until later in June. Diervilla flowers on new growth and can be cut back in spring, but June pruning will keep it bushy. Philadelphus grow relatively vigorously and spread widely and although pruning in summer, removing one in three older stems to near ground level, won’t successfully make larger cultivars smaller.
Spring flowering evergreens are pruned in early summer as soon as they have finished flowering, Choisya, for example, needs no regular pruning but can be shortened after flowering to restrict its size with removal of a third of the oldest branches at near ground level to check its spread. Other evergreens to which a similar treatment can be applied include evergreen Berberis such as B darwinii, Camellia and evergreen Ceanothus. The latter can resent heavy pruning but tolerates pruning of younger shoots after flowering as long as cutting into older wood is avoided.
Wall-trained evergreen Ceanothus is pruned to outward growing shoots after flowering to prevent unwanted spread. Only new shoots should be pruned.
Cherries and plums need little pruning unless trained as fans against walls. As these are highly susceptible to silverleaf fungus and subsequent dieback, summer pruning when silverleaf spores are fewest before mid-July is good practice.
Later still in July or August, once the shoot tips have hardened summer pruning of apples and pears and also wisteria, stimulates bud formation and by removing much foliage controls vigour. In the case of apples and pear, the fruit on pruned trees has access to more light which improves ripening and colour.
