Pruning Trees and Shrubs
Good pruning keeps trees and shrubs healthy, safe and good-looking; bad pruning can shorten their lives. Here’s when to prune in Lethbridge, how to make a proper cut, the elm pruning ban, dealing with storm damage and diseased branches, and when to call in a professional instead.
On this page
Why prune at all?
A city tree grows over sidewalks, roofs and driveways, takes the full force of our wind, and can drop a broken limb on a car or a person. Pruning guides it to fit its space safely.
- Safety: removing dead, broken or weakly attached branches before they fall.
- Health: taking out diseased wood, and branches that rub and wound each other.
- Structure: shaping young trees so they grow a strong framework that stands up to wind.
- Clearance: keeping branches off the roof and above sidewalks, and sightlines clear at driveways and corners.
Pruning is not a cure-all. Every cut is a wound the tree has to close, so the goal is to make as few cuts as needed, of the right kind, at the right time. A mature, healthy tree often needs very little.
When to prune
| What | When | Why |
|---|---|---|
| Most deciduous trees (ash, linden, maple, birch, oak, ornamental crabapple) | Late winter, while dormant: roughly late February through March, on a mild day. | With no leaves, you can see the branch structure. Wounds start closing as soon as growth begins in spring, and many insects and diseases are inactive. |
| Elms | Only October 1 to March 31. | Alberta’s elm pruning ban runs April 1 to September 30 (see below). |
| Spring-flowering shrubs (lilac, double-flowering plum, forsythia-type shrubs, early spireas) | Right after they finish flowering. | They set next year’s flower buds over the summer. Prune in winter and you cut off this spring’s flowers. |
| Summer-flowering shrubs (potentilla, late spireas, hydrangea that blooms on new wood) | Late winter or early spring, before growth starts. | They flower on wood that grows this year. |
| Evergreens (spruce, pine, juniper, cedar) | Lightly. Pines: snip back new “candles” in late spring. Spruce and junipers: light trimming in spring or early summer. | Most evergreens can’t regrow from bare, old wood. Cut back too far and the spot stays bare. |
| Dead, broken or hazardous branches | Any time (but not elms during the ban, except as the rules allow). | Safety first. |
What it means for you: a calm, mild day in March is pruning season for most yard trees in Lethbridge. Avoid heavy pruning in spring just as leaves are coming out, when the tree has spent its stored energy on new growth. Maples and birches pruned in late winter may “bleed” sap; it looks alarming but doesn’t harm the tree. Avoid heavy pruning in late summer and early fall too, which can push out tender growth that winter kills.
Elms: the pruning ban and wood rules
Lethbridge has many elms on streets, in parks and in yards, and Alberta works hard to keep Dutch elm disease out. The disease (DED) is spread by elm bark beetles, which are attracted to fresh pruning wounds and breed in dead and dying elm wood. To protect the trees, the province has rules for everyone, not just the City.
- Do not store elm firewood. Elm wood can’t be stored unless the bark is removed and it has been properly kiln-dried. Dispose of elm wood promptly.
- Do not move elm wood around, except to an approved disposal site. Don’t bring elm wood into Alberta, and don’t transport firewood when you go camping.
- Dead or dying elms may be removed any time of year, as long as the wood is disposed of right away.
- Clean your tools before moving from one elm to another.
- Report suspect trees. If an elm has branches that suddenly wilt or die back in summer, report it. In Lethbridge, call 311; you can also call the STOPDED hotline.
In Lethbridge, the City’s yard waste sites accept disease-free elm prunings; diseased elm branches go to the Waste & Recycling Centre. The City’s green cart page says elm wood should not go in the green cart; it can be bagged, tied and put in the black cart. Check lethbridge.ca or call 311 if you’re unsure.
What it means for you: if you have an elm, plan its pruning for fall or winter. Keep it healthy with deep watering from spring until mid-August, then a good soaking before freeze-up. The province advises stopping watering in mid-August so the tree can harden off for winter.
How to make a proper cut
Where you cut matters more than almost anything else. Look at where a branch joins the trunk or a larger limb. You’ll usually see a slightly swollen ring of bark, the branch collar, and above it a raised ridge of bark in the crotch, the branch bark ridge. The collar holds the tissue that closes the wound.
- Cut just outside the branch collar, not flush with the trunk. A flush cut removes the collar and leaves a bigger wound that closes poorly and invites decay.
- Don’t leave a stub either. A long stub can’t close over and slowly rots back into the trunk.
- When shortening a branch, cut back to a side branch or a bud facing the direction you want new growth to go, about 5 mm above the bud, sloping slightly away from it.
- No wound paint. Pruning paints and sealers don’t help the tree close the wound and can trap moisture. A clean cut in the right spot is the best treatment.
The three-cut method for heavy limbs
A heavy limb cut from the top in one go will often peel a long strip of bark down the trunk as it falls. The three-cut method prevents that.
- Undercut. About 30 to 40 cm out from the trunk, cut upward from underneath about a third of the way through the limb.
- Top cut. A few centimetres farther out than the undercut, cut down from the top until the limb drops. The undercut stops the bark tearing back toward the trunk.
- Final cut. Remove the short stub that’s left with a clean cut just outside the branch collar.
Know your limits. If a limb is too big to hold with one hand, too high to reach from the ground with a pole pruner, or over a roof, fence or wire, it’s a job for a professional. Never use a chainsaw from a ladder. Keep sharp tools sharp: clean cuts close better. See sharpening and edge maintenance.
How much to take off
As a rule, don’t remove more than about a quarter of a tree’s living crown in one year, and less on mature trees. Several small cuts are better than one big wound. Young trees tolerate more than old ones.
What to remove first
When you stand in front of a tree or shrub wondering where to start, work through this order and stop when the job is done.
- The three Ds: dead, damaged and diseased. Dead wood is brittle, has no buds, and often has flaking bark. Cut back to healthy wood.
- Crossing and rubbing branches. Where two branches rub, remove the weaker or worse-placed one before the rubbing wears through the bark.
- Suckers and water sprouts. Remove shoots coming up from the roots or the base of the trunk, and straight, upright shoots growing from main limbs.
- Clearance. Raise low branches over sidewalks and driveways, and take back branches touching the house or roof.
- Structure (mostly on young trees): choose one strong central leader and well-spaced side branches with wide angles. Narrow, V-shaped crotches are weak and tend to split in wind.
Never “top” a tree. Topping means cutting the main stems back to stubs to make a tree shorter. It starves the tree by removing much of its leaves, leaves big wounds that rot, and triggers a mass of weak, fast-growing shoots that break off easily in wind. A topped tree often becomes more dangerous, not less, and never regains its natural shape. If a tree is too big for its spot, the answer is selective thinning by a professional, or replacing it with something that fits.
Shrubs, hedges and fruit trees
Rejuvenating an overgrown shrub
Many old lilacs, dogwoods, spireas and honeysuckles end up as a tangle of thick, woody stems with flowers only at the top. Rather than shearing them, renew them gradually using the one-third rule:
- Year one. Cut about a third of the oldest, thickest stems right down to near ground level.
- Year two. Cut another third of the remaining old stems, and thin the new shoots that came up, keeping the strongest.
- Year three. Remove the last of the old stems. You now have a shrub made of young, vigorous stems.
For spring bloomers like lilac, do the renewal cuts right after flowering.
Hedges
Shape a hedge so it’s wider at the bottom than at the top, with sides that slope slightly inward as they rise. That lets sunlight reach the lower branches so the hedge stays leafy all the way down. A hedge trimmed wider at the top shades its own base and goes bare and twiggy underneath. A wider base also sheds heavy snow better. Trim formal hedges once or twice in early summer, and leave fall trimming alone so the new growth isn’t tender going into winter.
Fruit trees
Apples, crabapples, pears, plums and cherries are pruned in late winter while dormant, to open up the centre to sun and air, remove crossing branches and keep the tree a manageable height for picking. Dwarf sour cherries and bush fruit like haskap and saskatoons are pruned more like shrubs, removing the oldest stems. The fruit page covers varieties that suit southern Alberta, and the trees and shrubs page covers choosing trees that fit your yard.
Black knot and fire blight
Pruning is the main home control for these two common prairie diseases.
Black knot
Hard, black, lumpy swellings on the branches of mayday, chokecherry, plum and cherry. Prune out every knot while the tree is dormant, from late fall to early spring, when knots are easy to see. Cut at least 15 to 20 cm below the knot, back to a side branch or the trunk.
Fire blight
A bacterial disease of apple, crabapple, pear, mountain ash and cotoneaster. Shoot tips wilt and turn black, curling into a “shepherd’s crook,” and leaves look scorched but stay on. Cut well below the visible damage; the City of Edmonton advises at least 46 cm (18 in) into healthy wood. Prune in dry weather, ideally while the tree is dormant.
- Clean tools between cuts. Wipe or dip blades in rubbing alcohol (70%) or a bleach solution after every cut on a diseased tree, and before moving to another tree. Bleach can rust steel tools, so rinse and oil them afterward.
- Don’t compost diseased wood or keep it as firewood. Black knot galls can keep releasing spores for months after they’re cut. Seal diseased material in a bag and put it in the black cart, or take it to the Waste & Recycling Centre.
More on tree and shrub problems, hail damage and pests is on the pests and problems page.
After wind, hail and heavy snow
Lethbridge trees take a beating: strong chinook and westerly winds, summer hail, and the occasional heavy, wet spring snow that lands on trees already in leaf. After a storm:
- Look up and around before you step under a tree. Broken branches can hang in the crown (they’re sometimes called “widow-makers”) and fall later.
- Stay away from any wire. Assume every downed or sagging line is live. Keep well back and call your electric utility or 911.
- Shake off heavy snow gently while it’s still soft, by brushing upward from underneath with a broom. Don’t whack frozen branches; they snap.
- Tidy up broken branches with clean cuts back to the collar or a healthy side branch, so ragged stubs don’t rot.
- Hail-torn bark usually closes on its own. Don’t paint it. Give the tree water and time, and watch for dieback next spring.
- Split trunks, large torn limbs or a leaning tree need a professional assessment.
Never prune near power lines. Electricity can arc to a tool, a ladder or a branch without you touching the wire. Only qualified utility line-clearance workers should work on trees near overhead lines. If a tree on your property is growing into a line, contact the electric utility. In Lethbridge, start with 311 if you’re not sure who to call.
Watch for wind warnings and heavy spring snow in the forecast. Before a big wind, clear away dead branches you already know about. Our preparedness page covers getting ready for severe weather.
When to hire an arborist
Call a professional for any job that involves climbing, a chainsaw above shoulder height, limbs over a house, fence or wire, large dead trees, or a tree that leans or has a split trunk. Look for an ISA Certified Arborist: someone who has passed the International Society of Arboriculture’s certification exam and keeps up with continuing education.
Questions worth asking:
- Are you an ISA Certified Arborist? Can I see the certification?
- Do you carry liability insurance and Workers’ Compensation (WCB) coverage? Can you show proof?
- Can you give me a written estimate that spells out what will be cut, cleanup, stump removal, timing and total price?
- For elms: will the work be done between October 1 and March 31, and how will the wood be disposed of?
Red flags: anyone who offers to “top” your tree, uses climbing spikes to prune a tree that isn’t being removed, or knocks on your door after a storm offering a bargain on the spot. Reputable tree services are usually busy enough without canvassing.
City trees, digging and yard waste
Boulevard and City-owned trees
The City of Lethbridge prunes and maintains many trees along streets, boulevards and in parks, with winter pruning crews working through the dormant season. Who is responsible for a particular boulevard tree in front of your house, and whether you may prune it yourself, isn’t something to guess at. Check lethbridge.ca or call 311 before cutting a tree that may be on City property, to request service on a City tree, or to report a damaged or hazardous one.
Stump grinding and planting: Click Before You Dig
Grinding out a stump or planting a replacement tree disturbs the ground, and there may be gas, power, phone or cable lines just below the surface. In Alberta, request free utility locates through Utility Safety Partners (Click Before You Dig) at utilitysafety.ca, at least three full working days before you dig. Any ground disturbance counts, whatever the depth, including planting trees and digging out stumps. Utility locates don’t cover privately owned lines, such as an irrigation system or a power line to a garage; those need a private locate.
Yard waste
- Green cart: the City’s green cart takes branches smaller than 5 cm across and 1 m long, plus leaves, trimmings and grass. Pickup is weekly from May to October and every second week from November to April.
- Yard waste sites: the City’s seasonal yard waste sites at the depots (open April to mid-November) take branches up to about 1.8 m (6 ft) long and 20 cm (8 in) across. Roots and stumps aren’t accepted.
- Diseased wood and elm: bag and tie it for the black cart, or take it to the Waste & Recycling Centre, as described above.
- Christmas trees: the City has run an annual curbside Christmas tree collection in early January, with trees chipped into mulch; trees can also be dropped at the Waste & Recycling Centre. Dates change each year.
Programs and limits change, so check lethbridge.ca or the City’s Waste Wizard first.
A tree keeps a diary. Every year, a tree grows a new layer of wood just under its bark. In spring the wood grows fast and is pale; in late summer it grows slowly and is darker. Each pale-and-dark pair is one year, called an annual ring.
Wonder: would a tree grow a wider ring in a wet year or a dry year?
Try this: when a grown-up cuts off a thick branch, ask for a slice of the end. Use a magnifying glass to count the dark rings from the middle out. That’s how many years old the branch was. A safe job for you: gathering small twigs into the yard waste bag while the grown-ups do the cutting, staying well back from saws and falling branches.
Sources
- Government of Alberta, “Dutch elm disease prevention: what you can do” (pruning ban April 1 to September 30, elm wood storage, transport and disposal, STOPDED hotline, tool cleaning, watering).
- City of Lethbridge, news release “Elm tree pruning ban begins April 1” (reporting through 311, STOPDED hotline).
- City of Lethbridge, Composting and Green Carts (accepted branch sizes, collection schedule, elm wood and diseased branches).
- City of Lethbridge, Dropoff Depots (yard waste sites: season, branch size limits, elm prunings).
- City of Lethbridge, Christmas tree collection page.
- City of Edmonton, Black Knot and Fire Blight pest pages (pruning distances, tool disinfection, disposal).
- International Society of Arboriculture, consumer brochures “Why Hire an Arborist,” “Pruning Mature Trees” and “Why Topping Hurts Trees.”
- Utility Safety Partners, “Click Before You Dig” and Damage Prevention Process: Homeowners Edition.
- Alberta Agriculture, Pruning in Alberta (Agdex 270/24-1), for further reading.