In the fall, after mowing, sprinkle a little new grass seed over the whole lawn – apply 25-50 percent of the recommended rate for new lawns listed on the grass seed label. An all-purpose mix or a shade mix for the Willamette Valley are good choices, depending on how much sun your lawn gets. De-thatching before you seed can help the new grass take root and crowd out your weeds.
Avoid “weed and feed” products that promise to control weeds, as these contain hazardous herbicides that can pollute local rivers and streams. If your lawn looks pale or sparse, apply some slow-release fertilizer (you can find organic ones at your local nursery) when you over-seed.
Then tuck it all in with a thin (1/2-inch) blanket of compost if your soil is compacted or needs an extra boost. Avoid using home compost on your lawn since it is likely to have some weed seeds.