In my opinion the three most pernicious weeds in this area are common Bermuda grass (Cynodon dactylon), purple or yellow nutsedge (Cyperus sp.) and creeping oxalis (Oxalis stricta and O. corniculata). I choose these three is because they seem most difficult to get rid of. Chemical controls are available for all these problems, and if properly applied according to package directions may offer success. However for the organic gardener, non-toxic controls may be the only choice.
In Southern California mulching, hoeing, and handpulling are the three best ways to control most weeds in the home garden. A thick layer of organic mulch is additionally beneficial since it helps retain water and as it breaks down it can improve fertility of the soil. However, in the case of Bermuda grass, it will grow up through mulch, no matter how thickly it is applied. Hand pulling is also an ineffective control for Bermuda grass because the roots create stolons which cannot be pulled out. Thus hand-pulling only increases the proliferation of this weed. This is true also with nutsedges but for a different reason. Nutsedges produce tubers on the roots. When you pull off the tops, the tubers stay in the ground to re-sprout. However, in the case of nutsedges, repeated pulling will eventually weaken the tubers. It is possible to win in the end by sticking it out and pulling repeatedly, but this requires great vigilance.
A better system for getting rid of both Bermuda grass and nutsedge is to use shade. Neither of these weeds can grow in solid shade, such as can be provided by black plastic or a layer of newspapers covered in both cases with mulch. When using black plastic or newspapers, be aware that irrigation water may not sufficiently penetrate newspaper and thus drip systems had better be installed beneath the plastic or newpaper. When water is from rainfall or above-ground irrigations systems, a better solution is to apply the shade by putting down a layer of landscape weed cloth that is permeable by water and then covering it with a thick layer of mulch. A fine-textured landscape, weed-barrier, cloth allows water to penetrate, but it will not allow most weeds to get through it. Make x’s in the landscape cloth through which to plant your permanent shrubs, vegetables, or flowers. In flower gardens and shrubberies this system will work well to kill off nutsedges. However, Bermuda grass will travel long distances under the barrier and try to sneak out the ends. In these cases, try treating the escaping stems with white vinegar. Or repeatedly pulling, or bending the stems down and covering with more cloth, thus cutting off all light. Solid shade will eventually kill off the Bermuda grass, but the system takes time and assiduous watchfulness.
Several kinds of oxalis are pests in the garden, some of which also make tubers, thus making them more difficult to control. Pulling oxalis repeatedly and adding organic mulch around plants are probably the least toxic and most effective ways of keeping it under control in flowerbeds. In walls, and cracks between pavement, try spraying with white vinegar. White vinegar can also be used to spot-treat weeds in lawns. A strong solution of urea and water will also kill weeds when used as a spot-treatment in lawns, in cracks in pavement, or in walls, in the same way that dog urine will kill out a little patch of lawn. In lawns, it will take a while for the spot to heal up, so after the weed has died, follow up with water and gypsum to help wash the vinegar or fertilizer away and then put in a plug of fresh grass.
There is one other non-toxic way to get rid of weeds, which is soil-solarization (wetting the ground and covering with clear plastic in full sun during a whole season.) If your whole sunny garden is overrun with any one of the above weeds, you may never be able to get rid of it without resorting to soil-solarization over the entire area it infests. This method will kill off most or all weed seeds and tubers, but it is likely that they will return eventually from seeds brought in by birds or blown in by the wind or in the soil mix of nursery plants. It should also be noted that whereas soil-solarization gets rid of harmful pests and diseases such as nematodes, white grubs, and harmful fungi, it also kills many beneficial organisms such as beneficial nematodes, earthworms, and bacteria in the soil including rhizobia, that increase fertility by adding nitrogen to the soil. Thus the shade method is safer and in the long run more effective.
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