Grass and weeds growing out of pavement cracks in sidewalks, driveways, and patios is a common annoyance. Often, plants seem to grow better in these tiny crevices than they do in the lawn and garden. This defies all logic, since pavement surfaces are brutally hot and dry places where you might think that nothing could survive. Yet not only do these tenacious grasses and weeds survive, they seem to positively thrive in this no-man's land of blistering hot pavement.
And grasses and weeds that sprout up through the cracks in pavement are very hard to control. It's easy enough to pluck the top of the grass off at pavement level, but without extracting the entire root, the plant often simply sprouts up again.
Why is it that some plants seem to like these crevices?
- Driveway and sidewalk cracks hold a surprising amount of soil and organic matter, a perfect bed for grass and weed seeds.
- Moisture that seeps into driveway and sidewalk cracks may remain a lot longer than in other parts of the landscape. The driveway holds moisture beneath the surface, much the same way mulch does, and any plant that sends its roots down below the slab has access to this trapped moisture.
- Some grasses and weeds thrive in heat. Crabgrass, for instance, is a warm-season annual grass that thrives in driveway and patio cracks. Its seeds are very tiny and can penetrate the smallest cracks. Quackgrass is even more diabolical because it is a perennial weed that can survive even if just small pieces of root remain beneath the slab. If the exposed portion of the grass is removed, a new shoot will pop up in no time at all.
- In cold weather, a dark-colored driveway (asphalt) absorbs sunlight and keep the soil beneath warmer than the surrounding landscape. As for the salts in ice-melt products, some grasses and weeds can easily tolerate them. Fescue, for instance, is a cool-season grass that is somewhat salt-tolerant and might have a good chance of surviving a winter driveway. Sedge is a grass relative that tends to stay green in winter. And then there are the cold-happy weeds such as chickweed that seem to scoff at temperatures at which other plants would have long disappeared.
To kill grass and weeds in the driveway, patio, and sidewalk cracks, here are some home remedies for broadleaf weeds and grasses (but probably not on woody plants). These solutions are likely to kill moss in sidewalk and patio cracks as well:
- Boiling water. If the unwanted grass is near the kitchen door, pour leftover boiling water from the stove on weeds rather than down the drain. Don't worry if there's salt in the water; salt helps kill many weeds. (But don't throw water with oils or meat leftovers from cooking.) After a few visits from boiling water, most broadleaf weeds and grasses give up.
- Kitchen vinegar concoction. A mixture of 1 cup salt (about 228 grams) and one gallon (about 3.8 liters) of white vinegar (5% acetic acid) on hardscape will kill most weeds and grasses. To make it even more caustic, add 1 cup (about .28 liters) of lemon juice. To increase sticking power, add 2 tablespoons (about 28 grams) of dish soap. (Pickling vinegar is more acidic than regular white vinegar and probably more effective.)
- Propane torch. An ordinary propane torch can be used to burn weeds sprouting up through pavement cracks. Many weeds tolerate a fair amount of heat, but not the 2,000 degrees produced by a propane flame. Some manufacturers now offer long-handled weed torch tools specifically for this purpose.
- Horticultural vinegar. It may be hard to find in local stores, but it can be ordered online. This vinegar is 20% acetic acid. Mix it with some orange oil and a bit of phosphate-free dish soap. Acetic acid burns the plant's top growth, depriving it of the ability to photosynthesize. Don't forget protection for hands and eyes!
- Non-selective weed killer. Chemicals should be a last resort, but if other methods fail, spot-treating grasses and weeds with a weed-killer containing glyphosate (such as Roundup) will kill the plant, roots and all. Any chemical product should be used carefully, but glyphosate does not linger in the environment the way the chemicals in some other weedkillers do. Note that most weed killers will not kill grasses, only broadleaf weeds.
- Seal cracks. Plants can't sprout up if there aren't cracks for seeds to penetrate. Inspect your pavement annually and fill any cracks with mortar or a mortar caulking product. Vacuum out the cracks first, then fill them with mortar or masonry caulk to seal them.
Caution: Limit mixtures with salt to hardscape areas and don’t allow them to run into lawns and gardens.
Weeds may come back, especially perennial weeds with strong roots. And there may be lots of weed seeds waiting in the cracks for their chance to sprout. Be prepared to apply these mixtures more than once.