How, why and when to prune your garden flowers and shrubs

The deadliest thing that can enter any garden... is a gardener armed with a pair of secateurs — pruning clippers — but no sound idea of what to do with them. There are many reasons to prune: to remove dead and diseased parts of the plant, let light into the middle of an overly-dense shrub, and simply for aesthetics since pruning even the rattiest aged shrub can make it look like a sparky teenager again. Pruning produces an almost Zen-like feeling; you are 'in the zone' when you're doing it properly.

Why do it

Pruning to a plant, is like getting a splendid new hairstyle — it will look and feel better for the doing of it. You are aiming to have the perfect form of the plant so it must be done strategically.

• Crossed branches will rub against each other so this is always a good place to start cutting back. 
• Diseased or dead parts of plants will be obvious too, and once you cut out these sick parts of a plant, get the bits out of the garden completely and clean your pruners so they don't spread disease.
• Things can get crowded inside a shrub so prune out dense interiors by carefully removing no more than one third of the branches inside the plant. That means taking out no more than one third of the actual branches not shearing off the exterior of the plant by one third. The latter will end up ruining the shrub. 

What you'll need

Start with good equipment. I like Felco's #2 secateurs because they fit comfortably in my hand. I use them every day, which means I never have to sharpen them, and a little denatured alcohol at the end of each session keeps them clean. Use loppers for bigger jobs, and if you have branches with a thickness of more than 5cm, call a certified arborist (be sure to ask for references). 

When to do it

My overall advice is to prune when you've got time, and heed these general guidelines which can help a beginner gain confidence.

Prune any plant after it has finished flowering. Spring bloomers (such as clematis) and other vines and shrubs (such weigela), thrive if pruned in late spring or early summer. The plant's energy returns after the hard work of producing buds and blooms is past, and they can put that energy into healing themselves.

For summer blooming plants, wait until autumn to nip, and stop pruning about six weeks before the local frost date. Japanese maples and birches should be done in mid summer when they have finished their heavy sap periods. Truthfully, I often ignore this and run the risk of harming the plant irrevocably, so my advice is: if you don't know what you are doing, stick to the rules.  

July is a great time to cut back unruly perennials such as asters, heleniums, lespedeza and sedums that tend to start flopping in August. The controlling hand can use either scissors or pruners successfully here. You don't have to be scientific about it, but make sure the kitchen scissors are sharp and can execute a clean cut.  

How to do it

The ideal way to figure how you should prune is to lie on the ground under the plant in question. That is how you will see exactly what needs to be eliminated (crossing branches, dead bits and the ugly). It's a great idea until you can't do it because you can't get under there. If lying on the ground is impossible, just keep in mind why you are pruning, and you will probably see what needs to be done. 

Before your first attempt, I highly recommend looking at The Shigo 3-Step Tree Pruning Method but here are the basic rules to follow:

• Always slant the secateurs slightly away from the bark of the tree or shrub so that you aren't ripping the cambium (the outer woodie layer which protects the plant) when you make the cut. 

• Never leave a stub behind (a projection sticking out from a branch). A plant heals itself by being able to form a collar around the cut, to keep out disease. A stub doesn't allow for this, and if left, will be an entry point for the ravages of fungal diseases. 

• Start by cutting back to an outward facing bud along the branch. This is especially important on perennials such as roses. Cut the dead part of the branch right back to such a bud, and the branch will continue growing along that branch, toward the outside of the stem or branch.

So, armed with these tips, feel free to always carry your secateurs in the garden. Nip at things as you go along, keep unruly growth under control, and never give up on a shrub until it is definitely declared dead, as you can do dramatic things to make it recover. Pruning is about renewal and we can all use a bit of that.

Marjorie Harris has been gardening for the past 40 years in her own backyard; written 15 books about Canadian gardening including Botanica: North America: An Illustrated Guide to Our Native Plants, and most recently Thrifty Gardening From The Ground Up. She is a garden and plant consultant, garden writer and columnist for the Globe and Mail. She lives and makes gardens in Toronto. See: www.marjorieharris.com and marjorieharrisgardens.ca. Follow her on Twitter: @Marjorie_harris.