When and how to prune without doing harm

Friday, January 29th, 2010

Steve Whysall

Gerry Gibbens, senior gardener at VanDusen Botanical Garden, uses a lopper to prune a long-established Japanese flowering dogwood (Cornus kousa). Photograph by: Bill Keay, PNG, Vancouver Sun

It’s easy to get carried away when you get out in the garden with a pair of loppers and start slicing chunks off your trees and shrubs.

This is the time of year gardeners get back into the garden in earnest and start cleaning all the decaying remnants of last season out of borders and beds and begin to snip away with pruners.

The danger, however, is that we can get addicted to the power of snipping and slicing and end up doing more harm than good.

The key is to always keep in mind why you are out there doing this in the first place. The goal in pruning is always to make a plant healthier, shapelier, more attractive, more productive.

What you are there primarily to do is remove ugly, dead branches or ones broken by heavy snow over winter.

You’re also there to rejuvenate by clipping in a way that promotes healthy new growth or restores the plant to a beautiful, shapely, manageable specimen, without damaging its natural form.

This sounds straightforward enough, but you’d be surprised how many people, mostly inexpert gardeners, get carried away and somehow zone out in a crazed clipping frenzy. When they come out of it and step back, often all that is left is a horrible, misshapen, hobbled tree or shrub.

There is a rule in pruning that you first need to wander (walk around the tree or shrub and look carefully), then ponder (think about what you might do, and if necessary, get someone to pull a branch to one side to give you a better idea of what the plant will look like afterwards) and then prune (cut, but do it the right way, cutting close, but not into the collar without leaving awful stumpy bits that poke out and tell everyone an amateur did this).

In many ways, pruning is all about controlling and redirecting the flow of sap and opening the way for growth in the way you want it to go.

It’s a bit like being nature’s traffic cop — stopping sap from flowing one way, waving it on here, slowing it down there.

Remember, if you remove all the lower branches of a tree, you are giving sap permission to race unimpeded to the top of the tree, where it will fuel masses of new growth.

If you cut to a bud pointing out on the left, you are asking nature to produce growth in that direction. Cut to a bud on the right, and you are forcing growth that way.

So it’s important to think about where the growth will happen once you have made cuts.

Unless you have significantly reduced the root system, the tree or shrub will always seek to produce growth above ground equal to its structure below ground.

Not everything requires the same attention.

Buddleia, for instance, always needs to be cut down to size because it is such a rigorous grower and will rebound from a hard clipping, even if it is cut close to the ground.

C-type summer-flowering clematis, which you will find is already showing new growth, can also be dramatically clipped back to within a few feet of the ground, if needed.

Roses are best left until you see the yellow flowers of forsythia in bloom, when you will easily see new buds swelling and where the dead and diseased stems are.

The basic rule of when to prune something is to know when it flowers. Then you can decide whether you want to sacrifice a few blooms and prune before it flowers or wait to prune it until after it has finished flowering.

Generally, it is recommended that spring-flowering shrubs be pruned after they have flowered, but some experts think this is not always practical because it is harder to see the branch structure of a shrub once it has leafed out.

Deciduous trees are pruned while they are dormant, usually in January and February, before the sap starts to flow and when the frame structure is easily visible. This is the time to open up the canopy to allow in more light and better air circulation. It is also the time to consider strategically removing a branch here or there to allow more light in to flower beds or lawn areas that will do better with more sunshine.

Sometimes pruning is not the answer at all. For instance, if a large rhododendron has been planted below a window, it will forever be wanting to grow and block the view. Why fight nature? Better to lift the plant and relocate it to a spot where it can thrive and fulfil its genetic coding.

Here’s a guide to how to prune some popular plants.

– Clematis: Most summer-flowering clematis can be pruned back to within a few feet of the ground. Exceptions are B-types such as Nelly Moser. (For a complete list of B-type clematis, go to www.homeofclematis.com.)Early-spring flowering varieties should be pruned after flowering.

– Wisteria: Lateral and side shoots can be nipped back to two or three buds. After it flowers in July or August, snip them back again to four or five buds close to the main branch.

– Buddleia: Prune hard down to the main frame or lower in February.

– Fruit trees: Prune out dead, diseased and damaged branches and open up the canopy to allow in more light before buds begin to swell in March.

Mophead and lacecap hydrangeas: Cut stems back to a pair of fat healthy buds in early March. Take out weak, spindly stems to improve the shrub’s basic framework.

– Roses: Prune hybrid teas and floribundas when you see the yellow blooms of forsythia in March-April. Reduce the size of bushes by a third to a half, cutting back to a healthy outward-facing bud. Prune climbers, cutting laterals back to main canes. Trim shrub roses back by a third.

– Rhododendrons: It is better to move a large rhodo to a roomier spot. But if pruning is the only option, do it over three years, cutting a little each year after it flowers.

– Cane fruits (raspberry, blackberry, loganberry): Remove old unproductive canes to make way for new ones. Cut autumn-fruiting raspberries to the ground, remove only old canes of summer-flowering ones.

– Bush fruits (gooseberry, blueberry: Prune in March when buds can be seen. Cut for shape and to promote vigour. Keep the centre of the bush open to allow good air circulation. Prune again in summer to restrict growth and maintain shape and structure.

– Grape: Prune shoots in February back to one or two strong swollen buds. Routine pruning in summer involves shortening excessively long shoots.

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