What is mulch for in the vegetable garden?
A mulch is a layer of well-rotted compost, well-rotted manure or straw, at least 5 cm thick, laid on moist, weed-free soil and kept off the stems. In winter it covers empty beds and buffers frost; in summer, once the soil is warm, it holds water.
By region
| Season | North | Centre | South |
|---|---|---|---|
| Winter | At least 5 cm of compost or well-rotted manure, from late autumn to late winter, on moist soil that is neither frozen nor sodden. | As in the north. | Frost matters less. Cover bare soil if it drains; if it stays sodden, wait. |
| Summer | On heat-loving crops, once the soil is already warm, to hold water. | As in the north. | In the hot months you almost always need it: straw or well-rotted compost, water at the base, not on the leaves. |
The RHS depth: at least 5 cm, ideally 7.5. Keep it off the stems.
The RHS gives at least 5 cm (2 in), ideally 7.5 cm (3 in), from mid to late spring and in autumn, on moist soil, after weeding, and never on frozen or sodden ground. On tomatoes, University of Minnesota Extension mulches once the soil is already warm, with 7.5–10 cm (3–4 in) of straw or seed-free clippings, to hold moisture and limit blossom end rot. Empty beds in winter are also in the winter vegetable garden.
What to spread
Well-rotted garden compost, well-rotted manure, leafmould, seed-free straw. Grass clippings go on in a thin layer, and only if they carry no weed seeds, or the bed inherits them. Bark and gravel cover the soil but do not feed it. On a vegetable plot, compost, well-rotted manure and straw are the ones that leave organic matter as they break down.
Black plastic, for the RHS, is used on marrows and to grow potatoes without earthing them up. University of Minnesota Extension mentions it on tomatoes to warm the soil. It does not add organic matter. Earthing up and cover for potatoes are in when to plant potatoes.
How to lay it
Moist soil, weeds removed, an even layer. Not against the stems: the RHS leaves about 10 cm (4 in) clear around woody plants; on vegetables, simply keep it off the neck, because damp material rots it. On tomatoes the RHS leaves a gap around the stem. A drip line can run under the layer: less evaporation, and the tubing out of the sun. How much water is still the job of the watering guide.
In a raised bed the soil dries faster, and mulch matters more: that is in the raised bed guide. In December, well-rotted compost and a cover on bare beds for spring are in the December calendar.
What it does not do
It does not replace water in a dry summer, and it does not cure a plant that is already diseased. Debris from tomatoes or potatoes hit by late blight is not a mulch and does not go on the compost: take it out of the garden, as under pests and diseases.
The RHS describes it as frost protection and as a check on water lost from the soil. On tomatoes, University of Minnesota Extension lays it once the ground is already warm.
In the app
The app takes account of recent rainfall in your area and adjusts your watering reminder to match.
Frequently asked questions
What material do I use on a vegetable bed?
Well-rotted compost, well-rotted manure or seed-free straw. Bark and gravel cover the soil but do not feed it.
How thick should it be?
The RHS says at least 5 cm (2 in), ideally 7.5 cm (3 in). On tomatoes, University of Minnesota Extension says about 7.5–10 cm (3–4 in) of straw or clean clippings.
Is black plastic all right?
The RHS uses it on marrows and for potatoes grown without earthing up. University of Minnesota Extension mentions it to warm the soil for tomatoes. It does not feed the soil: compost or well-rotted manure does that.
When do I lay it in the south?
In summer, almost always, once the soil is already warm, to hold water. In winter, cover bare soil if it drains; if the ground is sodden, wait.
Can I mulch tomatoes?
Yes, once the soil is warm, and not against the stem. It helps keep moisture steady and limits blossom end rot. Don't use debris from diseased plants.