Weeds are often spread by contaminated mulch, making weed control harder than if no mulch was used. Black plastic, a solid sheet of polyethylene, effectively controls annual weeds. The disadvantage of black plastic is that water and oxygen cannot pass through this material. The soil should be moist prior to laying this synthetic material. Check the soil under black plastic during the growing season to ensure that the soil contains adequate moisture.
Black plastic warms the soil, which is an advantage in the spring but can be harmful in the summer.
Clear plastic, which increases soil temperature more than black plastic, will not control weeds since sunlight can reach the soil surface. Perennial weeds may be suppressed by black plastic, but plants like yellow nutsedge will push through the material in places. The new geotextiles, also called weed barriers, are woven or spun-bonded fabrics containing polypropylene or polyester and may come in black or white. These fabrics are more expensive than black plastic, but they allow water and gases to pass through the fabric. Research generally indicates good control of annual weeds, but annual weeds may be able to germinate above the fabric and send their roots through the fabric.