Summer is a great time to enjoy the garden and a good time to prune many trees, shrubs and vines.

I am still amazed how many people think that pruning deciduous trees in spring and fall is healthy for the trees. Pruning during leaf formation in spring can cause a tree to bleed sap, something that is definitely not healthy for the tree. If the sap-bleed does not weaken the tree, then the pest and disease organisms that feed on the sap can injure the tree in its weakened sap-bleeding state. Pruning during the fall when deciduous trees are preparing for their winter sleep is also unhealthy for the tree, primarily due to the potential for disease infection on the freshly cut wound at a time of year when temperature and moisture levels in the air are optimum for disease growth. Summer by comparison is an ideal time to prune many trees for two main reasons: Firstly, the summer sunshine acts like a disinfectant, killing most disease spores that land on the pruning wounds and the sunshine dries the wound to make disease germination less likely. Secondly, the dry air of summer also helps prevent disease spores from growing on the pruning wound. Trees that have thin bark and that are prone to disease infection such as birch or dogwoods are best pruned during June, July and August. Trees that run their sap early, such as Japanese and North American maples, are also best pruned during summer. Oak, walnut, hazel, black locust, magnolia and most other deciduous trees can be pruned safely and effectively during summer. There is another advantage to pruning during summer: vigour control. Pruning during the dormancy of winter will result in invigoration and the growth of watersprouts or suckers from the pruning wounds. Pruning during the summer causes a suppression of the tree's growth response and therefore no or very few watersprouts or suckers will occur and the tree's vigour can be controlled. Therefore, summer pruning is effective in controlling the size and vigour of many trees, shrubs and vines.

Pruning during summer for crown raising (removal of branches to lift the crown higher) and crown thinning (removal of selected branches to thin the tree's canopy to allow more light penetration under the tree's canopy) is best done during summer for the previously mentioned reasons and to allow an accurate gauge of the light enhancement under the tree. I am generally against the modern arboriculture technique of spiral thinning, due to the unnatural appearance of the tree after the fact and because many of our native conifers respond poorly when they are exposed to excessive spiral pruning and strong winter winds. The prevailing wisdom that spiral thinning of conifers allows the wind to pass easily through the tree's canopy fails to consider that many spirally pruned conifers suffer more branch damage because the branches are not able to rub against each other to absorb wind energy and they therefore bend past their natural breaking point.

Conifers are not as responsive to invigoration or suppression pruning and therefore pruning Douglas fir, western hemlock and pine during summer is not as critical as pruning maple or magnolia. Japanese maples -- whether they are the upright or low growing forms -- are best pruned during summer, to prevent unnatural regrowth and to prevent disease attack. The low growing forms of Japanese maple that undergo Penxing or Bonsai are always pruned during summer for all the reasons mentioned herein.

Several vines respond favourably to summer pruning including wisteria, kiwi, some early flowering honeysuckles, ivy, Holboellia, ornamental grapes and akebia. Summer flowering vines such as campsis, Tropaeolum speciosum and late flowering clematis should not be pruned during summer due to their summer or fall flowering habit.

Before you begin pruning, have a reason for pruning. Pruning without a reason is an unfocussed waste of time. The reasons for pruning are classified into one of the three following categories: to remove dead, diseased or damaged wood; to direct, control or modify growth; to improve fruit and flower production. The most common reason to prune -- it's too big -- is usually a consequence of the wrong plant in the wrong place. Proper plant selection during detailed design to choose the right plant for the right place is the only lasting solution for this problem. Nevertheless, circumstances and our desires often overtake our sense of reason and so excessive pruning maintenance seems inevitable in some gardens. Since gardens develop into maturity over time, some adaptive pruning maintenance is expected but it need not be onerous if you have a good garden design and a reason for pruning.

Todd Major is a journeyman horticulturist, garden designer, writer, lecturer and organic horticulture teacher. For advice, contact him at stmajor@shaw.ca.