Garden bed edging: interlock, steel, plastic or a spade-cut edge, and which lasts in Ontario

Garden bed edging has to stop lawn grass creeping into the bed and, in Ontario, survive frost that lifts anything shallow or loose. Steel and an interlock border on a compacted gravel base handle both. A spade-cut edge is free but needs recutting once or twice a year, and plastic is the one most likely to be sitting crooked by April.

On our sod, edging and mulch project we set a curved interlock border between new sod and fresh mulch (the photo is at the end of this guide). It’s a good choice on garden bed jobs when you want a line that stays put.

Why do garden beds need an edge at all?

Because lawn grass here spreads sideways underground. Ontario nursery sod is grown mostly or entirely from Kentucky bluegrass, going by the Nursery Sod Growers Association of Ontario’s specifications.

Penn State Extension notes that Kentucky bluegrass spreads through rhizomes (underground stems) to form a dense sod. Without a barrier those stems run into soft, mulched bed soil every summer. Within a couple of seasons you can’t tell where the lawn stops.

An edge also keeps mulch in. Colorado State University Extension warns that without a defined edge, mulch spreads onto the lawn or walkway, where it becomes a mowing or trip hazard. And a clean line makes a bed look tended even when it’s due for weeding.

How do the main types of garden bed edging compare?

Steel lasts longest, a spade-cut edge costs least, and plastic gives the most trouble in frost. Here are the four common options, going by what extension services and manufacturers say about each:

Illinois Extension’s Rhonda Ferree sums up the trade-off well. Metal is the longest lasting but costs more and takes more work to install. Plastic is the most common homeowner choice because it’s cheap and easy, even though it holds up poorly.

Is a spade-cut edge good enough?

For many beds, yes. It’s free and it looks natural. Illinois Extension says an edge about 4 inches (10 cm) deep and wide can easily contain bluegrass.

The catch is that it needs recutting: once or twice a year according to Illinois, and seasonally according to Clemson’s Home and Garden Information Center.

To cut one:

1. Mark the line with a hose or rope and stand back to check the curves look right from the house.
2. Push a sharp flat spade or half-moon edger straight down along the line, about 10 cm deep.
3. Angle the spade in from the bed side and lift out the wedge of grass and soil, leaving a small vertical cliff on the lawn side.
4. Shake the soil off the grass and compost or discard the sod pieces.
5. Pull back the bed soil slightly so the mulch sits at or just below lawn level.

That last step matters. Colorado State Extension suggests dropping the bed soil about 7.5 cm (3 in) so the top of the mulch ends up level with the surrounding grade, or tapering the soil at the edge. Just make sure the bed doesn’t turn into a puddle that collects water from the yard.

Why does plastic edging pop out of the ground?

Frost pushes it up. Pavement Interactive’s frost action reference explains that water in fine-grained soil freezes into ice lenses. Those lenses draw in more water and lift whatever sits above them.

A thin plastic strip with a few short stakes has very little holding it down, so a freeze can nudge it up and it may not settle back. Low, wet spots are the worst.

Illinois Extension calls plastic probably the hardest edging to install so it lasts. Its fixes:

• Choose a round-top style, which doesn’t sink as easily as a flat strip.
• Or choose a grooved or lip style, which it says stops most frost heaving.
• Use steel anchoring stakes and take care with the install.
• For flat strips, nail a 1 x 1 inch strip of treated wood along the bottom with galvanized nails.

If you go with plastic, bury most of the height, stake it closely, and expect to tap a few sections back down each spring.

Is steel edging worth the cost?

If you want a thin, permanent line, usually yes. Illinois Extension rates metal as the longest-lasting edging and says it’s strong enough for places like driveway edges.

Colmet’s attached-stake edging, as one example, is a 10 cm (4 inch) tall strip of 14-gauge steel. It comes in 4, 8 and 10 ft lengths, with the stakes already attached.

It costs more per metre than plastic and it’s harder to put in, especially in stony soil. Illinois notes that steel can have frost-heaving problems too, so make sure the stakes go well into firm soil below the strip.

How much steel edging do you need?

Take a bed along the front of a house that’s 6 m long and 1.5 m deep. It needs edging on three sides, because the house wall is the fourth:

Four 8 ft steel sections give you 32 ft, which is tight once the joins overlap, so five is safer.

How do you stop an interlock border from heaving?

Set it on compacted gravel, not on the soil. In our project photo the border pieces sit low, close to lawn level.

Clemson says pavers or brick laid flush with the lawn make an attractive edge, and that low, flat edging lets a mower ride over it. But a row of stones dropped into a trench of soil can rise and tip with the frost, the same way pavers on a poor base do.

Paver manufacturers give the numbers for this, though for walkways rather than borders:

• Unilock calls for a minimum of 8 inches (20 cm) of gravel base for pedestrian areas on clay soil, because clay drains poorly and needs a frost-resistant layer.
• Techo-Bloc’s edge restraint instructions say to mount restraints directly on the compacted base, with the base extending past the edge so the spikes have something to grab.

Neither gives a separate figure for a lawn border, which carries far less weight than a walkway. Frost lifts a light border just as easily, so the base still matters. Our guide on what goes under interlock explains the layers in more detail.

Gravel works because it drains. Pavement Interactive notes that clean sands and gravels with few fine particles aren’t frost-susceptible, while soils with a lot of fine particles are.

Stone edging should also be big enough to do the job. Illinois says at least 7.5 cm (3 in) thick and 10 cm (4 in) wide to block grass.

What are the common edging mistakes?

Edges usually fail for one of these reasons:

• The edge is too shallow, so grass runs straight under it. Aim for about 10 cm.
• Plastic laid on the surface or barely buried, with stakes only at the ends.
• Stone or interlock set in soil with no gravel base, so it heaves and tips after one winter.
• The edging sits too high above the lawn and the mower catches it.
• Mulch heaped above the edge, so it spills over every time it rains.
• Tight, wiggly curves that look fussy and are hard to mow around. Long, gentle curves are easier to live with.

When is it worth having it done for you?

A cut edge or a short run of plastic is a reasonable weekend job. It’s worth calling someone when the edging is long, or when you want interlock or stone (the base work is the heavy part).

The same goes when the bed itself needs weeding, reshaping and mulching at the same time. That’s the job we do as part of garden beds, planting and mulching across Markham, Richmond Hill and nearby.

If the bed needs more than an edge, our guide to low-maintenance garden beds covers planting and weeds. The mulch guide shows how much mulch to order once the edge is in. Edges are also a natural part of a spring cleanup.

Text a photo of your beds to (437) 684-2535 and we’ll arrange a visit and a free quote.