The Worst Invasive Plants and Garden Weeds

Our list of worst invasive plants and garden weeds will have you covered for the summer gardening season.

Purple Loosestrife

Purple Loosestrife is an invasive weed.

iStockphoto.com/Nancy Nehring

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They’re the schoolyard bullies of the plant world – bigger, meaner and pushier than anything else in the garden. They’re frequently the first to show up in spring. They compete relentlessly for soil nutrients, moisture and growing space intended for your crop. And they’ll fight back if you try to subdue them. You can hoe ’em, pull ’em out by the roots, smother ’em with compost, or spray ’em with chemicals, but these invasive plants and garden weeds just keep coming back.

Of the thousands of weed species inhabiting North America, here are 10 of the meanest you’ll encounter.

Most menacing. Found in all 48 contiguous states and Hawaii, dodder is a parasitic plant with yellow or reddish filaments that wrap themselves around flowers such as salvia, petunias, mums and geraniums, and garden plants including potatoes, onions and blueberries. Also known as strangleweed, devil’s guts and hairweed, dodder uses suckers to draw water and nutrients from the host plant. As it continues to spread from host to host, it flowers and sets seeds. As soon as each seed germinates, it begins searching for a host plant, and the process continues. If dodder gains a foothold in your garden, you may have to cut the affected plants down to the ground, collect and burn them.

Hardest to kill. Forget hoeing out a bindweed infestation. Even a fragment of a remaining root or rhizome is capable of regenerating a new plant. Because a single plant can produce up to 200 vertical roots that may grow as much as 30 feet deep, you’d need a backhoe to get ’em all. You can try smothering a bind-
weed patch with plastic sheeting, but some gardeners
report vines that have run as far as 40 feet under landscape fabric and mulch to emerge in the sunlight. Intense tillage a few days after each emergence helps, but since the seeds remain viable in the soil for up to 50 years, you may have to till your garden continuously for two or more years to bring it under control.

Fastest growing. The fastest growing weed may be kudzu, called the “foot-a-night vine” because it literally can grow a foot each day. Imported from Japan in 1876, this vine infests more than 7 million acres throughout the Southeast United States, and has even been found in the Pacific Northwest. It’s known to kill trees, destroy forests, pull down power lines and overwhelm houses. Agronomists say it can take seven to 10 years of repeated herbicide use and cutting and chopping to subdue an infestation.

Most prolific. The champion could well be the purple loosestrife, with each plant capable of producing 2 to 3 million seeds annually. Introduced to the United States from Europe in the 1800s for ornamental and medical uses, the purple loosestrife has invaded wetlands, crop fields and pastures in virtually every contiguous state in the nation.

The common purslane, which reportedly produces as many as 1.8 million seeds, is a close runner-up for the most prolific title.

Most miserable. One ragweed plant produces as many as 1 billion grains of pollen, making life miserable for the 35 million Americans who suffer from ragweed allergy. Ragweed pollen has been found as high as two miles in the atmosphere and 400 miles out to sea. Hoe out ragweed seedlings while they’re small, but never allow them to flower or produce seeds.

Tallest growing. Ragweed, reaching 16 to 20 feet tall, may hold this title as well. Pigweed is another tall-growing weed, standing up to 10 feet tall and capable of producing up to 100,000 seeds. Hoeing or cultivation can remove small weeds, but some glyphosate-resistant strains have been identified.

Most invasive. It’s a toss-up between purple loosestrife and leafy spurge, which infests rangelands in 35 states and Canada. Widespread throughout the Dakotas, Nebraska, Montana and Wyoming, leafy spurge
displaces native vegetation and produces plant toxins that defeat other plants. Dense infestations reduce rangeland carrying capacity from 50 percent to 70 percent. And here’s a neat trick: Leafy spurge seeds expel-led from seed capsules can fly 30 feet through the air!

Hardiest. One of the hardiest weeds is Canada thistle, a perennial found throughout North America. It actually originated in Eurasia, not Canada, and it produces a deep root system that can extend 10 feet or more into the ground. Breaking off or hoeing the top just leaves the root to regrow the next season.

Most poisonous. The Weed Science Society of America says poison hemlock is one of the deadliest weeds in America. It looks like parsley, but ingesting even a tiny amount results in rapid respiratory collapse and death. Their list also includes water hemlock, oleander, bittersweet nightshade, common pokeweed or pokeberry, pennyroyal, meadow death camas, foxglove, groundcherry, and jimsonweed. Don’t eat ’em!

Most painful. If you’ve ever stepped barefoot on a puncturevine seed pod, you know why this category is included. Several weed species, including the cocklebur, sandbur, burdock and buffalo bur, produce burs with hooked spines that can be caught up in the hair coats and tails of horses, cows and other animals. The Spanish needle produces a long, slender seed armed with three prongs, each of which has a sharp curved hook that can penetrate clothing and skin. 

Jerry Schleicher works hard to keep his property in Parkville, Missouri, free of weeds – mean or happy.