A mulch is a layer of organic or inorganic material spread over the soil, where it performs several functions. Any mulch moderates soil temperatures, suppresses weeds, and helps to reduce moisture loss from the soil. Organic mulches, though, are broken down and carried beneath the surface by soil organisms, helping to improve the soil. Inorganic mulches of gravel, pebbles, or sheet materials such as geotextiles suppress perennial weeds but do not improve the soil structure.
How to Mulch
Mulch between mid- and late spring, when the soil is moist and is warming up. Clear all perennial weeds, then after watering or a spell of steady rain, lay the mulch. Cover the whole surface area of beds and borders, rather than mulching around individual plants. You should only need to top it off once a year in spring. Lay organic mulches 4 in (10 cm) deep to control weeds and retain moisture, leaving a gap around the base of trees and shrubs so their stems don’t rot; you can mulch closer to herbaceous plants, but avoid the crowns of emerging plants. Lay inorganic mulches 1–2 in (2.5–5 cm) deep.
Organic mulches will decompose, improving the soil structure and adding nutrients.
- Garden compost helps prevent weed seeds from germinating by excluding light, and breaks down slowly, supplying nutrients gradually.
- Leafmold is ideal for woodland gardens or shrub borders and is easy to make.
- Farmyard manure must be well rotted. It is a useful source of nutrients, and is very good for mulching roses and shrubs.
- Composted bark is the most nutritious of the bark and wood mulches. Use around trees and shrubs, particularly acid-loving plants.
- Chipped bark is low in nutrients and will deplete the soil of nitrogen at first. It is heavy and dense, discouraging weed germination, and lasts for years before it needs topping off.
- Wood chips are slow to decompose and initially take nitrogen from the soil. Use for paths or at the back of shrub borders, not around young or herbaceous plants.
- Composted straw is low in nutrients and may contain weed seeds, but is fine at the back of a border where it cannot be seen.
- Cocoa shells are decorative but costly. They decay rapidly, and need topping off annually. Water them to bind them together.
- Mushroom compost will supply some nutrients and is slow to decay. Not recommended for use around acid-loving plants like rhododendrons, because it contains chalk.
- Spent hops is low in nutrients and lightweight; it may blow around when dry, and rots down quickly. Lay a thick layer and water it.
These discourage mosses and prevent soil from splashing onto flowers and leaves. They provide good surface drainage, so are useful for plants whose stems and leaves should be kept dry.
- Gravel is decorative, and ideal for drought-tolerant plants or around rock and alpine plants.
- Coarse grit and stone chips are ideal for mulching small plants such as alpines or succulents in raised beds or terra-cotta pots.
- Cobbles and pebbles are attractive in most settings, especially around water features.
- Geotextile membranes are very useful around newly planted trees and shrubs, helping to retain moisture and suppress weeds.
- Black plastic sheeting can be laid around new trees and shrubs with a camouflaging mulch on top, where it will suppress weeds.