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To prune or not to prune? That is the question

Linn County Master Gardener

OSU Linn County Master Gardener.

To prune, or not to prune? Why would we prune shrubs and trees in winter? 

It’s easier when we can see the structure without the leaves. Choosing what to remove is more clear. While the foliage is minimal is a good time to remove broken, damaged or diseased limbs.

Winter pruning stimulates vigorous growth when it comes out of dormancy, which is good if we want more fruit in summer. Mild winter temperatures during dormancy are good but avoid pruning in extreme cold when the plant is already stressed. 

Pruning does not correct human errors, like planting a large tree in a small space. It can help to build a strong framework or pleasing shape. Pruning can open a plant up to let in air and light, encourage bushiness, or trim excess growth so energy will be directed to better flowers and fruits. 

Heading cuts remove the tip of a shoot or branch, increasing the number of new branches at the cut end. The shrub or tree becomes bushier when growth resumes. 

Thinning cuts remove an undesired branch or twig by cutting at the point of origin, whether at ground level, from the trunk or from the parent branch. Thinning cuts result in a more open plant and doesn’t stimulate excessive new growth. 

Pruning removes current or future leaves, which through photosynthesis, produce plant food. Experts recommend limiting the amount removed to no more than one-third each year. Avoid making major cuts in trees and shrubs in winter. 

When growth begins in the spring, sap flows actively and major cuts may lead to “bleeding” or heavy sap excretion out of the cut. 

Spring flowering shrubs should be pruned after flowers fade in late spring or early summer. Trimming them now will remove the developing flower buds. New branches that develop in response to pruning will provide flowers next year.  

Summer-flowering shrubs (including roses) bloom on new branches, so trim them before new growth begins to encourage more flowers. If a bushy plant is not our goal, only thinning cuts should be made. 

For established trees and shrubs, summer pruning is a viable alternative to winter pruning but will likely reduce the plant’s vigor. Summer pruning stimulates little or no growth. If you choose to prune in the summer, prune lighter than you would in the winter.  If the plant size needs to be reduced, summer pruning is a better option. 

For disease management, boxwood, cherry, peach and plum trees should be pruned only in the summer when the air is dry to discourage infection. There is less stress on the plant when temperatures are moderate. 

Fall pruning can result in lots of new growth that are vulnerable to winter cold. 

OSU Publications on pruning are available online at https://extension.oregonstate.edu/catalog.

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