Garden & Landscape Tips

Weeds are thriving as a result of a range of changes and trends that benefit them over less competitive plants.
© George Weigel

You’re not imagining things if it seems like weeds are worse than ever these days. According to the Weed Science Society of America factors ranging from warmer weather to people driving more are increasing weed populations throughout the United States. Weed control is a growing concern mainly because of the trait that makes a weed a weed – its ability to out-compete other plants in changing and less-than-ideal conditions.

Among the trends and changes leading to higher weed populations:

Extreme Weather

Both the erratic floods and the droughts that much of the country has been experiencing lead to worsening weed problems Weed seeds can move from one location to another in flood water, and wet soil is good for weed-seed germination and its subsequent growth. On the other hand, drought can kill planted gardens and lawns, opening the door to bare soil that’s often re-colonized quickly by opportunistic weeds.

A Warming Climate

Most noticeable to gardeners is the northward creep of weeds that previously died out in colder winters. Kudzu (Pueraria lobata), the so-called “vine that ate the South,” is a prime example, having already worked its way as far north as the Canadian shore of Lake Erie. Harvard University scientists recently studied the plant life and temperature changes at Massachusetts’ Walden Pond, where Henry David Thoreau meticulously catalogued plants in the 1850s. The scientists found that average annual temperatures are now 4.3 degrees warmer, a change that benefited species with the most adaptable germination habits and flowering schedules. Those species were primarily non-native (a.k.a. exotic) and invasive ones that were not even present in Thoreau’s day. Of the species that Thoreau saw, 27 were now extinct.

Ragweed pollen counts rise sharply as carbon dioxide levels in the air increase.
© George Weigel

Rising Carbon Dioxide

Along with warmer weather has come an increase in carbon dioxide in the air — a trend that research has found benefits weeds even more than ornamentals and crop plants. Researchers at the U.S. Department of Agriculture’s Agricultural Research Service found that weeds growing in warm urban settings, where carbon dioxide levels were higher, on average grew four times taller than the same species in country settings with cooler temperatures and lower levels of carbon dioxide. Worse yet, common ragweed quadruples its pollen output when carbon dioxide levels double, suggesting increasing problems for hay-fever sufferers if the current trend continues.

An earlier study by USDA found that poison-ivy plants also have been growing larger, and developing more potent rash-causing oil in the last 50 years as carbon-dioxide levels have risen.

Ride-Hitching Weeds

A 2011 study by Montana State University researchers found that another underrated way that weeds spread is on the tires, bumpers, wheel wells, and undersides of cars and trucks. That’s been an increasing problem as Americans have become more mobile. The researchers counted weed seeds on vehicles and found that seeds from roadside weeds are readily picked up, especially in wet weather and in fall. The seeds often travel 160 miles before falling off. In cases where the seeds are caked in mud, typically they stay on until washed off. ATVs and other off-road vehicles picked up 20 times as many weed seeds as on-road vehicles. The researchers concluded that frequent vehicle-washing was the most effective way to control hitchhiking weeds.

Plant Stow-Aways

Yet another overlooked problem in weed management is weed seeds spreading via plants bought in nurseries, stores, and garden centers. USDA researchers in Alaska bought more than two dozen plants from 29 different nurseries and then incubated the soil in a greenhouse to see what would sprout. They found 54 different species of weeds, especially chickweed, hairy bittercress, groundsel, La Plata sandspurry, and Canada thistle. They also found more weeds in balled-and-burlapped trees than potted plants, more weeds in pots with soil-based mixes than in soilless potting mixes, and a big difference between growers using good weed control and weed prevention vs. those not controlling weeds.

Regardless of where weed seeds are coming from or what’s fueling their abundance, effective weed control includes mulching, filling beds with desirable plants (a way to deny that space to weeds), and applying a weed preventer to stop weeds from germinating in the first place. Preen Garden Weed Preventer is guaranteed to block weeds for 3 months around your flowers, trees, and shrubs. Apply it anytime throughout the growing season. Preen Mulch Plus is a great way to apply Preen Weed Preventer and mulch at the same time, while Preen Natural Vegetable Garden Weed Preventer is a natural alternative to preventing weeds.

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