How much edging do you need?

Edging is bought by the running foot, so the number you want is the distance around the bed and not the area inside it. Here is how to measure it, the depth that actually stops grass getting in, and why the cheapest edging on the shelf is the one you will be replacing.

Perimeter, not area

For a rectangular bed it is two times the width plus the length:

A 10 by 20 foot bed is 60 feet around. Doubling the bed to 20 by 40 quadruples the planting area, from 200 square feet to 800, but only doubles the edging, to 120 feet. The same relationship that makes a big yard cheap to fence makes a big bed cheap to edge.

Then take off the sides that do not need it. Edging separates a bed from grass, so a bed running along a fence, a wall or a driveway only needs edging on the sides that meet lawn. That 10 by 20 bed set against the house needs 40 feet, not 60, and nobody ever notices the missing side.

Curves are where estimates go wrong, because a curve is always longer than the straight line you pictured. Lay a hose or a rope along the line you actually want, then measure the hose.

How deep it has to go

This is the whole job. Edging that stands proud of the ground and barely enters it looks like a border and stops nothing: lawn grasses spread by runners below the surface, and a two inch strip is something they go under rather than through.

Set the top at grade or a whisker below it, never above. Edging level with the lawn lets the mower wheel ride straight over the join, which is the entire point: an edge that stands proud has to be trimmed separately by hand, and you have bought yourself a chore rather than removed one.

What to buy

• Steel. The default worth paying for. Usually 4 or 5 inches deep, sold in 8 or 10 foot lengths with stakes, and it bends to a curve by hand. It rusts to a dark brown and stops there. Thicker gauge stays straight; thin gauge ripples.
• Aluminium. Steel's behaviour without the rust, lighter to handle and dearer. Worth it where the edging will be visible rather than buried in planting.
• Plastic and poly roll. The cheapest thing on the shelf and the reason so many people think edging does not work. It is shallow, it lifts as the ground freezes and thaws, and within a couple of winters the top is riding above the lawn in waves.
• Paver or brick. A mowing strip rather than a barrier: a course of brick set level in the ground, wide enough for a mower wheel to run along. Handsome and slow to lay, and it will not follow a tight curve.
• Stone border. The most expensive per foot and the least effective at stopping grass, because runners go straight between the stones. Choose it for how it looks and plan to edge behind it anyway.

There is also the option of no edging at all. A spade-cut trench, a clean vertical face on the lawn side and a shallow slope into the bed, looks better than most manufactured edging and costs nothing. It just needs recutting two or three times a season, which is the trade you are making.

Laying it

• Cut the trench before you order the mulch. Digging a trench through a fresh mulch layer mixes the two and undoes both.
• Steel sections overlap at the joins rather than butting, so add about 5 per cent for the overlaps and a little more if the run is all curves.
• Stake it from the bed side, so the stakes are hidden and the lawn side stays clean.
• Drive the stakes below the top of the edging. A stake head at mowing height is a stake head that meets a mower blade.

Draw your beds in the planner

Lay edging as a run along a bed's edge and the planner labels each stretch with its length as you draw it, curves included, so the total comes off the shape you actually drew rather than the rectangle you would have estimated.