Even if you start with a clean plot, weeds will continually try to invade. As well as ways of discouraging them from taking hold, there are ways of removing them easily when they appear.
Sinking barriers that are impervious to weed roots can help to stop troublesome weeds coming in from your neighbour’s garden. Seedlings will always be blown in by the wind, although growing a tall hedge will help. Creeping weeds can be kept out by digging a 15cm (6in) deep trench, lining it with heavy-duty polythene and filling it with . The roots of couch grass and ground elder, however, can pierce polythene, so to discourage these weeds line the trench with gravel boards or breeze blocks instead; sink them to a depth of 30cm (1ft) and fill the trench with soil. Very deep-rooted weeds like bindweed are difficult to keep out; they tend to push under most barriers eventually.
Loose biodegradable mulches such as straw, leafmould, bark and shreddings put down on clean ground between plants can stop weed seeds germinating and can smother small annual weeds. However, they will not prevent established perennials from pushing through.
A 10cm (4in) layer is usually recommended for the best control as this will prevent light reaching the soil surface, but even a thin layer helps because it makes the soil surface more friable and so the weeds are easier to pull out. Top up the mulch annually as it will gradually decay into the soil. For properties and application of these mulches see Mulching to Improve Garden Soil. Their effectiveness for weed control can be increased by putting a layer of newspaper or cardboard underneath as for clearing the ground.
Non-degradable mulches made from plastic and polypropylene sheeting are durable and can be very effective at long-term weed control —on paths, for example. A sheet mulch such as this can also be put down around widely spaced shrubs. Disguise it with bark or shreddings.
For small, closely spaced plants the sheet must be put down before planting. Anchor it by pushing the edges into the soil or weighting them with bricks. The soil must always be moist before this type of mulch is put down. Cut a small cross into the sheet so that you can make a hole with a trowel to plant in the normal way. Remove plastic mulches on annual crops carefully at the end of the season so that they can be reused; throw-away plastic mulches are not acceptable in an organic garden.
Apart from black plastic sheeting, most of the materials sold for mulching are porous —that is to say, they allow air and water through to the soil. This makes them suitable for permanent use on large areas. Black plastic is good for tree mats (see below) and on narrow beds because moisture can seep sideways enough to benefit mature plants and soil life. Initially water by hand through the planting holes or through seep hose laid beneath the sheet.
Liquid feeds can be applied through a nondegradeable mulch, but there is no way of adding organic matter. Before the mulch is laid the soil must be moist and prepared with sufficient organic matter to last the whole time the mulch is down. This restricts the use of nondegradeable mulches in an organic garden.
Cardboard (covered with old straw or hay)
Biodegradeable. Lasts a growing season. Cheap. Can be put down on weedy ground.
Examples of use: round fruit bushes and trees; between rows of raspberries; as tree mats.
Biodegradeable. Lasts two to three months. Newspaper is available free. Rolls of recycled paper mulch are relatively cheap. Can be put down on weedy ground.
Examples of use: between widely spaced vegetables (cover with grass mowings); on paths between vegetable beds (cover with shreddings or straw).
Wool matting (brown felted)
Biodegradeable. Lasts one growing season in a damp place, longer if dry. Expensive. Do not put down on ground containing perennial weeds, especially couch. Easy to plant through. Not unattractive.
Examples of use: tree mats; shrubs;; other fruit.
Black plastic (300 gauge or thicker)
Non-degradeable, non-porous. Lasts two to five years; lifetime improved if covered with loose material to prevent damage by wind and sun. Relatively cheap. Can be put down on weedy ground. Easy to plant through. Warms the soil, and keeps air-above it warm at night.
Examples of use: tree mats; hedges (cover with shreddings or hay to disguise and protect plastic); vegetable beds (widely spaced crops can be planted through the mulch; take care to clear the crop very carefully so the plastic can be used another time).
Black/white polythene (black underneath, topside white)
As for black polythene except that the white surface reflects light and heat. This keeps the soil cooler and helps fruit to ripen.
Examples of use: round summer fruiting crops such as, , .
Woven black plastic (sometimes sold as ground cover)
Non-degradeable. Porous. Usually lasts for at least five years. “Stabilized” to stop it degrading in sunlight. More expensive than ordinary plastic. Cut edges will fray unless they are heat-sealed and strands can tangle round plants and wildlife. Keeps down some perennial weeds but is pierced by couch grass.
Examples of use: to stand pots of plants on outside the greenhouse — cover with gravel to disguise mulch and help keep surface even. It can also be used underneath gravel, bark or wood-chips on paths.
Black polypropylene (sometimes sold as black fleece)
Non-degradeable. Lasts about four years. More expensive than plastic. Porous. Do not put down on weedy ground, especially if couch grass is present. Easy to plant through. Warms the soil. Examples of use: strawberries planted through the mulch; round shrubs (cover with shreddings or bark to disguise); shrubbyor alpines (planted through mulch, disguised with gravel or stone chippings); under the gravel or hard-core on paths to stop soil mixing with the surfacing materials and weeds rooting through. Grey polypropylene can be used similarly where it can be covered to exclude light.
If vegetables are planted close together in blocks rather in than rows (see Bed System for Growing Vegetables), they can smother small weeds and will prevent other weed seeds from germinating. Some crops are more effective at this than others, for instance,
onions, with their slender, slow-growing leaves, have little effect whereas quick-growing crops with broad leaves such as spinach soon form a close canopy. Vegetables that need a wide spacing but grow slowly can be intercropped with a quick-maturing crop to cover the ground: two rows of Brussels sprouts planted approximately 60cm (2ft) apart could be intercropped with a row of lettuce down the middle, for example.
The same principle applies to planting annual, and even perennials if sufficient plants are available — perhaps if you have propagated them yourself from seed or cuttings. For example, plant five small sage plants in a group approximately 23cm (9in) apart; they will grow up to look like a single bush, but will cover the ground more quickly than just one plant.
Ground cover plants
Bare areas of soil can sometimes be kept weed-free using ground cover plants — low, spreading, vigorous plants that are able to suppress weed growth.
Grass is one of the commonest ground cover plants and it can be used effectively between trees and specimen shrubs. The disadvantage of grass, however, is that it needs regular mowing, although some areas can be turned into wild-flower meadows. Other effective ground cover plants are low evergreen shrubs or herbaceous plants which keep a covering of foliage over winter. As well as suppressing weeds, they provide food and shelter for wildlife.
One disadvantage of using ground cover is that if weeds do become established between the plants they are very hard to get rid of. It is essential to clear the ground thoroughly before planting and to keep it weed-free between the plants until a tight canopy has formed. Hoe or mulch the plants, or plant them through wool matting or another bio-degradeable sheet mulch that will eventually disappear.
On bare areas for future cultivation, use a suitable type of green manures as ground cover (see Green Manuring and Green Manure Crops).
- Spade the edges of a black plastic or polypropylene mat down into the soil.
- Put bark or shreddings on top to disguise it, keeping the material clear of the trunk.
If you are planting a tree or shrub into grassland, use a mat to keep clear an area at least 1m (3-1/4ft) square at its base. This stops competition from weeds and keeps the ground moist so the tree will establish and grow much more quickly.
Black plastic works well, even where the ground is very weedy. You could also use cardboard or newspaper. Wool matting or black polypropylene, would be suitable where the ground is reasonably weed-free to start with. In ornamental areas, the mat can be disguised with bark or shreddings, but keep loose material away from the trunk.
Leave the mat on for at least two growing seasons until the tree is well established. Trees on dwarfing rootstocks and shrubs which are not vigorous growers may need permanent mats.