Tom Lanini, a professor of plant sciences at the University of California Davis, said mulch is the most important factor in preventing weed growth. Nearly any barrier that blocks light works as a mulch. Bark and other decorative mulches work, but dried leaves, cardboard and newspapers are also effective.
“I think organic mulches are definitely the way to go. They have benefits beyond weed control.” Hartzler said.
Lerner notes that organic mulches improve soil structure, and add nutrients, particularly when used near the end of the growing season. They also keep the soil cool and reduce water loss to evaporation.
Straw and hay are among the cheapest mulches, but they must be free of weed seeds to be effective. Like other methods, mulch may be less effective on established perennial weeds. They are better at blocking smaller annual weeds.