What to Do About Weeds in the Cracks of Old Cement?

Weed Types

• A weed is any plant that grows where it's not wanted. Annual weeds are those that sprout from seeds and die within one year once they bloom and set seed. Winter cold or summer drought can kill annual weeds. Biennial or perennial weeds are more durable and difficult to control, since they have stronger, more extensive root systems that sprout new leaves. These plants' life cycles include seed germination followed by at least two years of returning growth. Perennial weeds may be herbaceous or woody.

Prevention

• The concrete itself does not provide a medium for weed seeds to germinate and grow. The crack allows collected dust, grit and organic debris to build up and create a substrate for seeds to sprout. Sweeping concrete areas regularly prevents dirt and leaf clippings from forming a thin layer from which wind-deposited seeds can sprout. Any weeds that do sprout need to be promptly killed when small. As roots and stems grow, they can extend subtle pressure against the crack's side. Coupled with fluctuation in moisture and temperatures, the roots exacerbate the widening and flaking of concrete in the crack. The weeds themselves catch wind and debris to create more of a soil layer atop the crack.

Non-Chemical Methods

• Hand-pulling is the most basic way to rid concrete cracks of weeds, but is best done when weeds are tiny so entire root systems are pulled up. Cutting back or mowing weeds is another option to control size, but fails to prevent perennial weeds from expanding their lower stems and roots, making the cracks larger over time. Pouring boiling water or burning weeds with a propane torch are other non-chemical means to kill both young and established weeds. Once weeds are killed, thoroughly sweep or power wash the area to remove debris in the crack that provides a niche for new weed seeds to collect and sprout.

Chemical Control

• Spraying herbicides on weeds in the crack of concrete also can effectively stop infestations. Chemical products may be non-selective -- killing all plants with which the spray comes into contact -- or selective, such as a broadleaf herbicide, which kills all non-grass weeds. Ideally, spray and kill weeds before they flower and produce seed. Once dried up, pull or snap away the weeds and sweep or wash the area to remove the habitat for new seeds. Granular preemergent herbicide may be sprinkled into cracks to inhibit seed sprouting over the next two to 10 months.