One of the most effective ways to control perennial weeds in a organic garden is to use a paper barrier (Newspaper-and-Mulch Method). The action is quite simple:
the newspaper acts as a physical barrier which prevents perennial weeds from growing above-ground, which over time starves the plant, since it is unable to photosynthesize. The results are very good if it’s done properly.
This method should not be used around some shallow-rooted shrubs (particularly some plants in Ericaceae). Digging dogs and squirrels can undo the barriers as well, so the treated areas need to be monitored carefully if these animals are present in the garden.
Tools and Materials
Newspaper or cardboard: A large amount of newspaper of cardboard is needed, since to be effective the barrier beeds to be at least 1 cm (about 1/3 inch) thick when wet. Newspaper can often be attained from local recycling centers, cardboard from supermarkets (call the market first and ask them to save it).
Plastic or metal bins: Shallow, flat bottomed containers to hold water, large enough to fit a folded newspaper section. This might not be useful when using cardboard, depending on the size of the boxes.
Covering material: Mulch, compost, soil, or gravel needs to be put on top of the paper to keep it from blowing away (and to hide it from sight).
Once all the materials are together, it’s time to prepare the area for application. If it is a new bed (without ornamental plants already growing), start by mowing or weed-wacking as close to the ground as possible. Tilling is also an option, as long as you can get the soil good and smooth afterwards, and compact it a bit with a rake. Make sure that the outermost 6 inches or so of the bed are as weed-free as possible, so plants can't just grow around the barrier.
If there are already ornamental plants growing, more care is needed. Shrubs and small trees with shallow root systems will not appreciate having paper on their roots, so you’ll need to carefully hand-weed within their driplines. Outside the dripline, cultivate just as you would on a new bed. Perennials are usually OK with the newspaper laid within 4” of their crowns.
If the bed is badly invaded with running weeds like bindweed, thistle, goutweed, it’s usually better to just cut everything down to the ground, transplant and quarantine the shrubs, and apply newspaper to the entire area.
Prepare the newspaper by thoroughly soaking it in water to give it weight and help it bind together. The easiest system for doing this is to have two tubs of water, so one tub can be used for soaking while you pull paper out of the other one.
Once the paper is thoroughly wet, begin laying it down on top of the soil. Use whole sections, not unfolded, and let them overlap by 1/2, so that the entire soil surface has 2 sections on it, at least 1/4 inch thick (1/2 inch is even better). Once the paper is down, pat the sections together so they’ll start to form a continuous layer. This layer will keep perennial weeds or turf from poking through, effectively smothering the weeds and denying them light. Leave at least 2 inches on the outer portion of the bed to allow for edging without having to cut through the paper.
Once the paper is down, and before it starts to dry, start laying down mulch on top. Don’t dump wheelbarrows on the paper, but instead place it by hand, to avoid disturbing the newspaper as much as possible (unlike mulching without a barrier, it's actually better to go from front to back, rather than back to front). Put on enough mulch to thoroughly cover the paper, as this will not only look nicer, but will also prevent the wind from blowing the paper around the yard.
If it’s a “must plant” location, use 6 inches of compost instead of mulch, and grow annuals and shallow rooting vegetables on top (keep in mind that you’ll need to water the bed more often than the surrounding soil, because soil moisture will not wick up through the paper barrier as much as it would if the paper wasn’t there). With that much compost sitting atop the bed, it’s a good idea to use some logs on the edge (on top of the paper) to keep it from washing into adjacent areas.
As a rule, you’ll want to leave the bed alone for about a year to ensure that the perennial weeds are completely smothered. Use the area in the meantime for containers, ornaments, to store new plant acquisitions in, or as the quarantine site for any shrubs you might have moved.
Make a regular inspection for tenacious weeds that might try to pop through the paper, and if you see any, brush the mulch aside, find the hole they’re using, and add a patch layer of newspapers to seal it up.
Once you’ve had a full year without any weeds poking through, it’s safe to till the bed and start growing in it. Keep in mind that there will still be weed seeds in the soil, but the pernicious perennials will be history, as long as you keep on top of the new seedlings. If using a tiller, remove any large pieces of paper and add them to the compost pile, as they are a carbon source and may divert nitrogen from the soil.