Three of the most common summer weeds in our area (spotted spurge, purslane and creeping wood sorrel or oxalis) are among the most difficult weeds to permanently eradicate. All three of these low-growing broadleaf weeds are prolific seed producers. Each spurge plant (annual, thick purplish mats, small oval leaves on stems that produce a corrosive milky sap) sets thousands of seeds over its long, April through October, growing season. Purslane (annual, thick succulent ovate leaves, thick stems) does the same. Oxalis (perennial, small clover-like leaves, small yellow flowers) sets seed pods that explode, scattering seed over a 10-foot radius. These three weeds grow too low for mower blades to cut off seed heads before they set seed.
Weed seeds can remain viable for years. Many weed seeds are carried into gardens on lawn equipment, tools and shoes. The spurge, purslane and oxalis you find in your planting beds and turf this year may have sprouted from seeds produced last season or even before in your neighbor's yard. Control for these weeds in turf combines herbicide applications including pre-emergents with good lawn care. Weeds thrive in poorly maintained turf. (UC Davis experts are experimenting with smothering oxalis in lawns by covering the weeds with heavy cardboard and topping that with a thick layer of mulch.)
The most effective means of control for these weed seeds in planting beds is soil solarization. This drought year, when planting beds are empty, provides a good opportunity to rid your beds of spurge, purslane and oxalis weed seeds by solarization.
Solarization uses the heat generated by the sun to raise soil temperatures high enough to kill weed seeds, disease pathogens, nematodes and harmful bacteria. During our hot summers, it only takes four weeks in June, July and August to heat the soil to a weed-lethal 140 degrees.
Here are the four easy steps for successful soil solarization:
First: clear the area to be solarized of all weeds, rocks, mulches and debris and level the surface.
Second: Wet the soil to a depth of 12 inches. Wet soil conducts heat better than dry soil.
Third: Lay down a 1.5- to 2-milliliter thick layer of clear plastic over the entire area to be solarized. Dig a shallow trench around the plastic and use the soil to seal the edges. Pull the plastic taut to flatten it against the soil and prevent air pockets.
Fourth: Don't disturb the plastic for four weeks. Remove it carefully to avoid spreading any still-viable weed seeds on the soil surface.
Solarization speeds up the decomposition of organic materials in the soil. The populations of beneficial bacteria and micro-organisms rise after solarization.