Potting soil, compost or mulch: what is each one for?

Potting soil, compost and mulch are sold side by side and look a bit alike. But each one has a different job:

• potting soil is for filling pots;
• compost feeds the soil;
• mulch covers the soil.

Using one instead of another is a common reason plants struggle.

Potting soil: for pots only

In a pot, the plant has nothing but potting soil around its roots. So the potting soil has to do everything: hold water, let the excess drain and keep air around the roots. That's why it's light (peat, perlite). That's perfect in a pot, but not in garden soil.

Use potting soil for pots, planters and repotting. For seedlings, use a seed-starting mix instead, finer and without fertilizer. Otherwise, young seedlings can rot at the soil line.

Compost: it feeds the soil

Compost is organic matter that has already broken down. It feeds earthworms and the micro-organisms in the soil, which then release nutrients for plants. This mostly happens when the soil is warm: compost does little in April and a lot in July.

It also improves the soil right away: it makes clay less compact and helps sand hold water.

Two rules:

• Use mature compost: dark, crumbly, with no recognizable pieces. Compost that isn't broken down enough can burn fine roots and take nitrogen from the soil instead of adding it.
• Never plant in pure compost. It keeps breaking down and shrinks. The plant sinks with it, and its base ends up buried, which makes it rot. Compost is mixed into soil; it doesn't replace it.

Mulch: it covers the soil

Mulch goes on top of the soil, never into it. It keeps moisture in, protects the soil from heat and cold, and stops many weeds from sprouting. Before spreading it, pull out quackgrass, horsetail and bindweed: these plants grow right through.

How thick?

• 5 to 7 cm in a flower bed;
• up to 10 cm at the base of trees.

Less than 5 cm, light gets through and weeds grow. More than 10 cm, water and air have trouble getting through.

In winter, mulch protects perennials planted that year. Repeated freezing and thawing can push them out of the ground. Spread it only once the soil surface is frozen, often in late November, sometimes in mid-December. Put down too early, on soil that's still warm, it attracts voles.

Fresh wood mulch should never be mixed into the soil of a planting hole: as it breaks down, it would take the nitrogen the roots need. On the surface, it's not a problem.

Don't fill the planting hole with potting soil

This is the most common mistake. A plant bought in a pot already has a root ball of potting soil. Filling the hole with more potting soil just makes the pot bigger. The plant looks fine the first year, then, after a few years, it grows poorly, dries out quickly and leans in the first strong wind. Why?

• Water builds up. Water has trouble moving from light potting soil into finer soil, like clay. It stops at the bottom of the hole, which fills up like a tub with no drain, and the roots run short of air.
• The roots go in circles. They'd rather stay in the loose potting soil than push into the harder soil around it. They circle in the hole just like in a pot, and the plant stays poorly anchored.

How to plant, step by step

• Dig wide, not deep: two to three times the width of the root ball.
• Open up the root ball. The roots of a potted plant often circle around. Firmly score the sides of the root ball from top to bottom, and cut any root that wraps around the trunk. Otherwise, it will keep circling and can strangle the trunk years later.
• Find the root flare, where the trunk widens into roots. It's often hidden under a few centimetres of potting soil. Scrape the top of the root ball until you see the first large root. That root should end up at ground level once planting is done (or 2 to 5 cm higher in heavy soil, because the soil settles). A buried root flare rots.
• Set the root ball on undisturbed ground so it doesn't sink.
• In clay, roughen the sides of the hole with a fork: the shovel makes them smooth and water can't get through. Don't put stones at the bottom: water stops there instead of draining away.
• Backfill with the original soil only for a tree or shrub. Put compost on the surface: earthworms will work it in. In a flower bed, you can improve the whole area (one shovelful of compost for three or four of soil), but not a single hole.
• Water in two steps: plenty of water when the hole is half filled, then again when it's full. Count 20 to 40 litres for a tree. Water removes air pockets better than stamping the soil down.

Then, throughout the first season, water slowly and deeply once a week if it doesn't rain, rather than a little every day. For conifers and shrubs that keep their leaves in winter, give one last big watering in October, before the ground freezes: in Quebec, winter kills them more often through lack of water than through cold.

Mulch should never touch the trunk

Mulch piled against the trunk keeps it damp: the bark softens and diseases get in. In winter, it also hides voles, which chew the bark at the base. A trunk chewed all the way around dies, and you only find out in spring, when the buds don't open.

• Leave about 10 cm free of mulch around a shrub, and 15 to 20 cm around a tree.
• To properly protect a young trunk, put on a wire mesh guard (6 mm mesh) for the first five years, and at all times on fruit trees. Put it on in the fall, pushed 3 to 5 cm into the ground and tall enough to stand 60 to 80 cm above the snow. Loosen it in spring so it doesn't squeeze the trunk.
• For perennials, summer mulch goes between the plants, never on the crown. In winter, once the ground is frozen, cover them with 10 to 15 cm of straw, shredded leaves or conifer branches (no bark or wood chips). Remove this cover in April.

In short

• Potting soil is for pots. In the garden, it makes the pot bigger instead of freeing the plant.
• Compost feeds the soil: mixed into it or spread on top, never pure.
• Mulch covers the soil: 5 to 10 cm, never against the trunk.
• When planting: open the root ball, plant at the root flare, backfill with the original soil and water in two steps.
• A planting mistake rarely shows the first year. That's what makes it costly.