July 13, 2013
The best time to prune any plant is usually when it is dormant. This can vary greatly among different plants. Still, there are some rules of thumb to follow. Unless there is concern about particular pests and diseases, you should remove "the three Ds" anytime you see them: dead, diseased and damaged wood. You also should never remove more than 25 percent of the photosynthetic portion (green leaves) of a plant at any one time.
Other guidelines include:
- Maximum pruning: Ten percent to 15 percent maximum pruning is suggested for mature trees in less-than-ideal circumstances (this is most trees), 20 percent for vigorously growing trees in fairly ideal circumstances and 25 percent for saplings and other small, vigorous trees that often need training.
- Making the cut: Always make good clean cuts where the plant can seal off the wound quickly, making either heading back cuts to a lateral branch of sufficient size or a thinning cut at the growth ring. It is important to consider what the function of the plant is, what hazards might be avoided or created by pruning, and what the reaction of the plant will be for each cut.
- Controlling growth: When it comes to food crops such as fruit trees and vines, summer pruning can help reduce the size and vigor of the plant. Most home gardens do not have the space for full-sized fruit trees. Even when on a dwarfing rootstock, many fruit trees can outgrow a typical residential yard space quickly. Summer pruning does not take the place of winter dormant pruning, but it can help reduce the amount necessary as well as promote shorter trees that can be more easily harvested and maintained. Another reason is to inhibit diseases and pests by opening up the canopy to allow for more airflow and sun penetration to lower fruiting branches and spurs. The pruning also allows easier access for natural predators to eat pests.
- Disease prevention: Apricot and cherry trees are best pruned in the summer to reduce the spread of Eutypa dieback disease, a fungus that causes limbs or twigs to wilt and die suddenly in late spring or summer with the leaves still attached. It infects fresh pruning wounds when rainfall occurs two to six weeks after pruning, hence the dry season is ideal for doing this pruning. Summer pruning on fruit trees also promotes more blossoms the following spring. If sudden oak death is of concern, they can be more confidently pruned when it is dry for similar reasons.
- Structural work: Summer is also a great time to do some structural work. Suckers, water-sprouts and crossing branches can be easily removed during the summer and energy will be directed toward more strategically located branches and flowers and/or fruit production. By using string or spacers, young vigorous branches can be trained to create more symmetry in the branching structure, fill in voids in the canopy and promote wider, stronger crotches that are more resistant to splitting. By cutting back to lateral buds, the gardener can promote more branching as well as decide the direction new growth will take.
- Keep a canopy: It is important not to allow too much sun exposure to trunk and scaffold branches, which can scald the living cambium layer just under the bark and create large wounds that can open up the wood to decay organisms as well as jeopardize the strength of the wood. This can be avoided by leaving enough canopy to shade the bark or by treating the bark with a whitewash. Care should be taken not to cut back tender plants too late. As pruning can invigorate new growth, it is important that any hard pruning gets done early enough for the resulting new growth to harden off before colder weather sets in.
- Late-summer dormancy: Many perennials, woody plants and most of our native plants naturally go dormant later in the summer when water is scarce. They prefer to be thinned or reduced in size before the late fall and winter rains. Many late-spring flowering plants, if pruned just after bloom and before setting seed, can produce a second bloom. Any time you remove ripening seed from a perennial it will store more energy for the following season. Directing new vigorous growth after bloom on aggressive plants such as wisteria can save a lot of winter work and keep them in bounds.
Learn more about summer pruning at "Essential Summer Pruning and Preparing Your Garden for Autumn" from 9 to 10 a.m. July 20 at the Falkirk Cultural Center at 1408 Mission Ave. in San Rafael. Bring your gardening gloves and pruners, and, if you wish, do some hands-on work in the garden following the workshop.