Weed growth around sidewalks, curbed areas, and parking lots presents unique challenges as the goal for sidewalks and curbs is typically a sterile, plant-free environment. These areas are open to the environment, so wind, rain, and a multitude of other factors inevitably bring soil and plant seeds from yards, gardens, construction areas, etc., into the cracks and crevices of sidewalks and parking lots. Weeds, being the opportunists that they are, have learned to exploit these niche environments.
One alternative is to go from gray to green. Consider converting or reducing some hard surfaces to green landscaped surfaces. This will reduce adverse impacts like pollution and flooding risks from stormwater runoff. For more on the harmful effects of stormwater runoff, see the City of South Portland’s manual, Why Worry About Stormwater.
Here are some products and practices that can help to maintain hardscapes without resorting to the use of hazardous pesticides:
There are a wide variety of organic herbicides that have shown efficacy in controlling weed problems on hardscapes. Herbicides need to be applied as soon as possible after the weed sprouts. For full effectiveness, make sure to coat the entire target plant. Try to apply in warm weather, and check if a repeat application is necessary for larger weeds.
For a complete list of organic herbicides that you can use to battle sidewalk weeds while complying with South Portland’s pesticide law, see Products Compatible with Organic Landcare.
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Vinegar is a good example, though unlikely to work unless concentrations with high acetic acid are used. In 2002, the U.S. Department of Agriculture studied the efficacy of vinegar herbicides on common weeds, including lamb’s quarters, giant foxtail, smooth pigweed, and Canada thistle. The agency found that vinegar concentrations of 5-10% only killed weeds that were in their first two weeks of life. Older, larger plants needed to be hit with concentrations of 20% acetic acid or more. These concentrations showed an 80-100% kill rate.1 These higher concentrations of vinegar have not, at this time, been registered as a pesticide and therefore these products cannot be sold as a pesticide or labeled with pesticidal claims. Even if they are exempt from registration under federal law, they must be registered in Maine. As the market emerges, these products may become more widely available.
Physical and mechanical removal
Your best bet is to mow down weeds early in their growth stages. After that, weed whacking large plants repeatedly over several weeks will eventually exhaust their energy stores. You could also go the simplest route and hand-pick any hardscape weeds. There are some great products, such as the Garden Weasel and the Diggit Duck, which make hand-pulling or scraping out weeds easier.*
One popular option is the use of “flame weeders,” which use propane and a torch wand to literally burn down weeds. The Red Dragon is a popular flame weeding product, and it comes in mini, full sized and backpack kits (for large areas).* Weeds should be flamed early, before they reach 4 inches, or repeated flamings may be necessary. One quick second is enough to kill weeds – they do not need to be burned to a crisp. Flame weeding is safest when it is wet outside. Make sure to be careful around sensitive flower beds and gardens, as well as conifers which are very flammable.
Steam weeding machines are highly effective. These machines use a jet of superheated steam to take care of weeds, and are ideal for hardscaped areas, around garden and landscape beds, as well as general weed management. Steam weeders do not damage soil biology, weeds decompose in place and do not need another pass by workers to be removed, the product can be used in all weather conditions, and its use will also germinate weed seeds and thus exhaust the seed bank.
* Note: mention of these products does not imply endorsement.