Most weed seed germinates when exposed to light – this means that only seed near to the soil’s surface will grow. A mulch makes the soil’s surface darker & suppresses weeds. But perennial weeds can still grow through this, so must be dug out, hoed off for a season or sprayed, before a mulch is applied.
- At least a two-inch depth of organic material is essential to inhibit seed germination. Use well-rotted compost or bark chippings (such as from B & Q or Rolawn). Top up the layer each year with a small amount of material.
- Some weeds will still appear, from seeds blown onto the top of the mulch, but these are easy to pull out. If your garden compost has not heated up enough, weed seeds could still be present and grow. If you think your compost heap won’t kill weed seeds, don’t put flowering weeds (or roots of dandelion, couch etc) onto the heap, but bin them (in the council’s garden waste bin if you have one).
- An alternative mulch for a large area is old carpet, cardboard or thick black polythene (anchored down) which will suppress weeds, kill any already growing, and buy you some time until you can deal with the area. However, this does encourage slugs.
- In areas already planted up, you could use sheets of Plantex (or similar rot-proof woven mulching fabric that allows water though) beneath bark or compost mulch, to prevent worms from dragging the mulch into the soil.
- Growing a mulch – another possibility is to plant a green mulch ie something that will spread but that will be easy to cut down/mow off or pull up when you want to use the area. Geranium sanguineum (a cranesbill) is one we use because it spreads quickly, is ornamental (evergreen and has spring flowers), but is easy to pull up and doesn’t self seed. Growing green manure is an extension of this idea, where you choose a plant that will help lock nutrients into the soil, and dig in the whole plant once you are ready to use an area. You need to choose leafy plants that will be easy to bury near the surface and that will decompose easily. Look under ‘green manure’ in catalogues to see what’s available. Crocus has a handy green manure comparison table of various plants suitable, some of which are sold by Thompson & Morgan, either listed under green manure seeds or specific names.