When plants grow so close together that the ground between them is shaded, sun-seeking weeds, such as pigweed and purslane, don't have a chance. Use double rows rather than single ones whenever possible in your vegetable garden. In flower beds, place flowers in closely spaced groups. As plants need more room to grow, thin them gradually so weeds get only a fleeting chance at good light.

Plants with broad leaves, such as squash and cabbage, do a good job of crowding out weeds. Vigorous lawn grasses that form a tight turf naturally crowd out weeds. To keep turf tight, apply a slow-release organic fertilizer during your lawn's most active season of new growth. The recommended cutting height varies with different species of grass, but with any type of grass it's a good weed-preventive strategy to mow high and often. Long blades of grass often do a good job of shading out germinating weed seeds.