A reader once wrote, “It was mid-January, and there--peeking through a bit of snow--was pretty green grass! Why does grass grow better in the driveway than in the lawn?”
I have also heard from others who puzzle over grass growing in asphalt driveways at 100 degrees.
That grass in the crack may be teaching us a lesson in microclimates. Consider these points, and then read on for some DIY solutions:
- 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.
- Some grasses and weeds thrive in heat. Crabgrass, for instance, is a warm season annual grass that thrives in driveway and patio cracks in the area where I live.
- 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 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 where other plants would have long disappeared.
To discourage grass in the driveway, patio, and sidewalk cracks, here are some home remedies for broadleaf weeds and grasses (but probably not on woody plants):
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 oil or meat leftover 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.)
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!
Weeds may come back, especially perennial weeds with strong roots. And there may be lots of weed seeds waiting in the cracks. Be prepared to apply these mixtures more than once.
These solutions are likely to kill moss in sidewalk and patio cracks as well.
Caution: Limit mixtures with salt to hardscape areas and don’t allow them to run into lawns and gardens.