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This story was originally published in May 2020.
Every gardener has that one weed that they can never seem to shake, no matter how many times they pull it up. After the umpteenth time it crops up again, there is temptation to reach for a chemical spray that will get rid of it once and for all.
Even if you’re at your breaking point, natural remedies have many advantages over synthetic chemical herbicides.
“[You] don’t have to read any labels,” said Eric Gallandt, professor of weed ecology at the University of Maine. “[You] don’t have to worry about personal protective equipment.”
Plus, with herbicides, there is always a risk of hitting a plant that you do not want to kill.
“If there’s some drift where you’re spraying, you could hit something that you didn’t want to hurt,” said Sonja Birthisel, postdoctoral research associate at the University of Maine. “There’s [also] some complexity to choosing the right herbicide for the job.”
To properly address weeds, it is good to know what you are dealing with. In particular, you should be looking out for whether the weed is an annual or a perennial, and whether it has rootstocks deep underground from which another weed could grow.
“Look up the biology of the weed species that are most problematic for you,” Birthisel said. “The more you understand about the biology, the easier it is to choose the right tool for the job.”
To identify weeds, Gallandt recommended an app called PictureThis, where you take a picture of a plant and the app finds a match. The app requires a subscription fee of $19.99 each year.
“It’s kind of expensive, [but] it works so well,” Gallandt said. “At least for common weeds, it’s not going to miss any of them.”
Your local cooperative extension can also be helpful when it comes to identifying and managing tricky weeds.
“If you have a specific question about a specific weed species in context, reach out to your local cooperative extension agent,” she said. “They’re a really amazing resource whose job it is to provide more tailored recommendations.”
In general, there are a few natural remedies that stand above the rest in terms of weed removal. The primary one is mulching.
“I’m personally fond of mulch,” Birthisel said. “It not only suppresses weed emergence but [it can] add organic matter back to the soil.”
There are a number of materials you can use for mulching, including plastic mulches, newspaper, leaf litter from your yard and wood chips.
“Any of the above can be effective, [but] I would gravitate towards what is most easily accessible,” Birthisel said. “Which do you have laying around? Or are you trying to go for a certain look? Some people might feel that wood chips are a better look than newspapers.”
Gallandt prefers mulching made of organic materials, like leaves and straw (though he warned that there may be some lingering seeds in the latter).
“Organic mulches look nicer than plastic and you don’t have to worry about your carbon footprint and you’re adding organic matter to your soil,” he said.
While mulching is great for bigger crops like tomatoes and peppers, Gallandt warned that there are some crops that will not benefit from mulching.
“You’re not going to mulch your lettuce mix or radishes,” Gallandt said. “There’s quite a number of things that are not so amenable to mulching.”
Another way to prevent weeds is by covering your garden plot with a sheet of black or clear plastic prior to planting. The weed control is known as solarization, whereby the heat of the sun trapped by the plastic will kill the weeds and pests.
Birthisel suggested tilling the soil to encourage weed growth — as paradoxical as it may seem — and then covering the plot with a clear or black plastic tarp for a couple of weeks before planting.
Gallandt said that you can also keep the plastic on during the growing season and leave openings where you plan to plant, though you will need to continue to check for weeds in these spots.
Gallandt said that some perennials will grow through the black plastic, but this technique can be especially effective for caustic plants like poison ivy.
“Because it’s one that you don’t want to be accidentally playing around with, I would gravitate towards putting down a heavy layer of black plastic and put it down for a very long time,” Birthisel said. “It’s a perennial, so it’s going to take longer to kill.”
When it comes to homemade spot treatments for weeds — for example, using vinegar, soap or boiling water — Gallandt said that they will work to eliminate some annual weeds.
“Basically, it will burn off the vegetation [by] stripping the leaf of its oils [and the] cells will rupture,” Gallandt explained.
Vinegar is particularly potent, though the type generally recommended for weed control is stronger than what you can get at the supermarket.
“I have seen some compelling research on applications of high-strength vinegar,” Birthisel said. “This is a stronger vinegar than what you could buy over the counter, but you can order online. It can be effective as an herbicide for spot treatment.”
Some home remedies may also be more costly than they are worth in terms of effectiveness. For example, some websites recommend using vodka and other alcohols to kill weeds.
“I would think that you could get more bang for your buck through other means,” Birthisel laughed.
There is one that Gallandt recommends against: salt.
“Salt is what people would use to make land sterile so you can’t grow anything on it,” Gallandt said. “It makes that piece of soil saline.”
However, spot methods will be less effective on weeds with extensive root systems or rhizomes beneath the soil surface.
“It doesn’t move down into the growing point of the plant,” Gallandt said. “You can [still] generate new plant parts from a growing point.”
In a similar vein, such spot treatments will work better for annual weeds than perennial weeds.
“For perennial plants, if you spray it with vinegar you’ll probably burn the top off and it will regrow,” Gallandt said. “You want to use shallow soil disturbance [and] get as much of the root as possible, or use mulch.”
Sometimes, the best solution is just to pull up as much of the root as possible before you move to another remedy. This is especially true for perennial weeds.
“Perennial weeds are going to be harder to kill through some of these natural remedies because they often have carbohydrate stores that allow the plant to regrow,” Birthisel said. “Getting them out by the root can be the most effective way to go or over the course of a couple of treatments.”
Gallandt said to invest in a quality hoe, and to weed often when it is sunny and dry.
For persistent weeds, Birthisel and Gallandt both recommended addressing them while they are still young.
“We want to weed often so you’re targeting tiny weeds [that are] less than an inch tall,” Gallandt said. “They’re so sensitive then. Every day that they get bigger, they get marginally more difficult to kill.”
“[For perennial weeds], try to get them as soon as they get back up,” Birthisel added. “The more time you have green surface area, the more time they will have to put down more sugars.”
Planting your crops close together will also help prevent the need to weed.
“Think about spacing so they’re going to be competitive,” Gallandt said. “Plant them at a high enough density that they’re taking up the space. If you don’t take up the place with crop plants or mulch, weeds will take up the space and you’ll have to maintain the space with a hoe.”
Birthisel said that there is not going to be one catchall solution for dealing with every weed in your garden.
“There’s not one single tool or technique that’s going to work in every application,” she explained. “It’s important to consider the right tool for the job.”
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