Green Thumb – Pruning Tips for Healthier Shrubs

September 24, 2020

Most of us take an interest in pruning when we become first-time home owners. In my case I thought it was my duty to maintain the shrubs like the previous owner. After all, cones, boxes and meat-ball spheres were the landscaping shapes we saw down the street. On Saturdays you heard the buzz of those electric shears.

I soon found out that pruning is not about time-saving power tools, but about why, when and how you prune. Without some basic knowledge you’ll never see flowers on your lilacs, and some of your bushes might be a thin veneer of green leaves over stubby brown twigs.

The fundamentals of pruning shrubs

1. Timing

Winter, during dormancy, is a good time to reshape the non-flowering shrubs or those that flower in the summer or fall, because they bloom on new growth. Spring flowering shrubs, on the other hand, should not be pruned until after they bloom or you’ll remove their spring buds.  They bloom on old wood.

2. Tools

Bypass handpruners make a clean cut and will take care of stems up to 1/2 inch in diameter. Lopping shears work best on ½ to 1 inch diameter stems.  Anything much larger than about an inch needs a pruning saw. Hedge shears can give a formal hedge a haircut, or trim off soft growth where hand-pruning would take a long time. Make sure the base of a hedge is wider than the top.

3. How much to cut

This depends upon what you want to achieve. There are three basic methods:

Rejuvenation pruning renews a plant all at once. You do this on multi-stemmed shrubs by cutting them down to within 4-6 inches. This stimulates new growth to come up from the roots. I do this to keep ‘Anthony Waterer’ Spirea from growing tall and loose.

Renewal pruning cuts out the oldest stems at ground level. New growth will come up when sunlight and air penetrate the interior of the shrub. Don’t remove more than one third of the stems each year to avoid stressing the plant. The oldest stems are usually the tallest, so this method shortens the shrub. This works on larger shrubs like lilac, viburnum and ‘Bridal Wreath’ spirea, after the flowers fade.

Heading-back pruning cuts a branch back to an outward facing bud. It is used to control the size and shape of a bush and opens up the plant to sun and air. Leave at least a quarter of an inch between the cut and a new bud. The longer you wait to prune, the more growing points you will have to cut back.

Some annual pruning is always in order once a shrub reaches its mature size, but try to preserve the natural shape of the plant. Shrubs with a mounding habit are easiest to maintain if the plant fits its location.

When a branch gets long, reach in and cut from deep inside. This will keep its more natural look.

Cane growers, like forsythia and kerria come up from the ground.  If you head them back they will no longer fountain out. Cut the oldest canes back to the ground, but remove no more than one third a year. Boxwoods can be pruned any time before late summer.

Any time is right to prune out dead wood. It’s important for the health of the shrub because it helps to manage insects and diseases. If you do some thoughtful pruning on a regular basis your place will always look loved.

Janice M. Weber – University of Illinois Extension, DeKalb County Master Gardener