Bark and Mulch: Locking Moisture Into Dry Soil

Of everything in this series, mulching gives the best return for the least ongoing effort. It is a job you do once or twice a year that then works continuously, without a timer, a tap or a decision.

It is also the one most commonly done wrong, in a way that makes things actively worse. So the order of these sections matters more than usual.

What it does, mechanically

A layer of loose material over the soil surface does four separate things.

It blocks evaporation. Bare soil in sun and wind loses water from the surface continuously, and that loss is a large fraction of what a garden gets through in a dry spell. A layer of bark breaks the connection between wet soil and dry air.

It keeps the soil cooler. Surface temperature on exposed soil in full August sun is far higher than it is a few centimetres down under a mulch, and cooler soil holds water better and keeps roots and soil life functioning.

It stops the surface capping. Bare soil hit by heavy rain or a hose forms a hard crust that then sheds water rather than absorbing it — which is why a downpour onto baked ground so often runs off and does almost nothing. Mulch takes the impact and lets water in gently.

It suppresses weeds, which are competing for exactly the water you are trying to conserve.

None of that requires an expensive material. It requires depth and coverage.

The rule that matters most

Only ever mulch onto damp soil.

This is not a refinement. It is the whole thing. A mulch is a barrier, and a barrier works in both directions: laid over dry ground it will keep that ground dry, shedding light rain before it ever reaches the soil, and keeping the drought in place under a layer that looks like you have done something helpful.

In the middle of a dry August, that means one of two things:

• Wait for proper rain — not a shower, but enough to wet the soil to depth — and mulch immediately afterwards, while it is still damp. This is the better option.
• Or water thoroughly first. A deep, slow soak of the area, checked with a trowel to confirm the moisture has actually gone down rather than sitting in the top centimetre, and then mulch on top of it the same day.

If you cannot do either, wait. Autumn is the traditional mulching season for exactly this reason: the soil is still warm from summer and has been rewetted by autumn rain, which is the ideal combination. A bag of bark sitting in the shed until October is far more useful than the same bark spread over dust in August.

Depth

Five to seven and a half centimetres — two to three inches — is the working figure for moisture retention. For serious weed suppression you can go to ten.

Thinner than about five centimetres and it does not do much: light penetrates, weeds come through, and evaporation continues. Much thicker than ten and you risk keeping rain off the soil entirely and starving the roots of air, particularly on heavy ground.

The most common error is spreading a bag too thinly over too large an area because it seemed a shame to stop. Half the border done properly beats all of it done at two centimetres. If you are short, mulch the plants that need it most — new plantings, thirsty subjects, anything in full sun — and leave the rest for next time.

What not to mulch onto

This is the section worth reading twice, because most mulching damage is caused by putting it in the wrong place rather than by using the wrong material.

Not against stems and trunks. Leave a clear collar of 5–10cm of bare soil around the base of every shrub and tree. Mulch heaped against bark holds moisture against tissue that is meant to stay dry, and causes rot at the collar — which kills plants slowly and from a cause nobody thinks to check. Around a tree, the mulch should look like a doughnut, never a volcano. It is a very common sight and it is doing real harm.

Not onto dry soil. As above. This is the big one.

Not onto waterlogged or frozen ground. Both times you are locking in a condition you want to end.

Not on Mediterranean and alpine plants. Lavender, thyme, rosemary, sedums, artemisia, most silver-leaved things — these are killed by winter wet, not summer drought, and an organic mulch around the crown holds exactly the moisture that rots them. Mulch these with grit or gravel instead, which gives you the surface cooling and the weed suppression without holding water against the stems.

Not over seedlings or emerging bulbs, which will not push through it.

Not dug in, if it is fresh woodchip or uncomposted bark. On the surface it is fine and does no harm. Mixed into the soil, its decomposition ties up nitrogen in the root zone and the plants go yellow. Surface only.

Choosing a material

Any of these work; the differences are mostly about longevity and appearance.

Composted bark is the standard choice for ornamental borders. It lasts a season or two, looks tidy, and breaks down slowly into the soil.

Woodchip is coarser and lasts longer, and is excellent around trees, shrubs and along paths. It is often available free or cheaply from local tree surgeons, which makes it by far the best value if you have somewhere to put a load.

Garden compost or well-rotted manure is the most useful of all, because it feeds the soil as well as covering it. It breaks down fastest, so it needs replacing more often, and it is the best option for anything you are trying to improve rather than merely maintain.

Leafmould is free, takes a year or two to make, and is excellent — particularly in shade and around woodland plants.

Gravel, grit or slate is the right answer for dry, sharply drained plantings and for anything that resents winter wet. It is permanent, which is both its advantage and its drawback.

Straw works well on the vegetable patch and around soft fruit, and is easy to move aside.

Grass clippings can be used, but only in thin layers of a couple of centimetres at a time — piled on thickly they mat, heat up and turn slimy.

When to do it

Autumn is the best time in most gardens: warm soil, freshly rewetted, and the mulch is in place before winter.

Spring, once the soil has warmed but before it dries out, is the second-best time and the one that most directly buys you drought protection for the summer ahead.

Mid-summer only after rain or a thorough watering, as above.

Top it up rather than replacing it. Organic mulches are supposed to disappear — that is them being incorporated into the soil by worms, which is the long-term benefit rather than a failure. A border mulched annually for a few years develops noticeably better structure and holds water far better than one that has never been touched, and that improvement is cumulative and permanent.

The compounding bit

Mulch is what makes the rest of this series work harder. A deep soak under a mulch stays available to the plant for days rather than hours. A drip line laid under mulch loses almost nothing to evaporation. A newly planted tree with a mulched root zone establishes faster and needs less watering to get there.

And if you are planting into dry ground this autumn, the sequence is: plant, water in thoroughly, then mulch — in that order, with the watering done properly before the covering goes on.

If you are choosing what to put in first, our drought and dry soil listings show what nurseries currently have available for exactly these conditions. A mulched border of the right plants is close to a garden that looks after itself.