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What Is a “Noxious” Weed?

Of the approximately 250,000 species of plants that exist worldwide, only about 3% behave as weeds that we don’t want in cultivated areas. And just a handful of these are what we call “noxious,” which isn’t just a word that we made up. This is an official plant classification that we apply to a small number of plants that are so harmful that we need to destroy or contain them to limit their spread.

According to the WSSA (Weed Science Society of America), a “noxious” weed is “any plant designated by federal, state, or local government officials as injurious to public health, agriculture, recreation, wildlife, or property.”

These aren’t garden-variety weeds. Just because crabgrass has stepped on your prize rose bed doesn’t make it a bothersome weed for everyone. In your own garden or lawn, no one decides what a “weed” (undesirable plant) is except you.  See what weeds tell you about your soil.

But think about the word “noxious.” Some synonyms for it are deadly, virulent, harmful, dangerous, toxic, environmentally unfriendly, poisonous, nasty, awful … well, you get the general idea. Noxious weeds are a danger to our environment and the economy. They can take over entire ecosystems, destroy natural habitats, damage agricultural production, and cause losses worth millions of dollars. As they spread to natural areas, they harm wildlife and plants and can be impossible to eradicate.

Many weeds are also classified as “invasive” weeds. These are non-native invaders that lack natural competitors or enemies to curtail their growth, which allows them to overrun native plants, displace species, and alter ecosystems. Some weeds are not only noxious but also aggressively invasive—and big trouble if they find their way into your garden!

How to Get Rid of Six Noxious or Invasive Weeds

We’re not going to sugarcoat it. If you are 100% opposed to using any type of garden herbicide, this is not the article for you. The Old Farmer’s �������� advocates minimal use of herbicides and recommends natural remedies as a general rule. However, some weeds simply can not be controlled organically. We believe that the best strategy is to use a limited application of appropriate chemicals that can quickly address the issue without harming the plants nearby.

Note: Photos below are courtesy of Dr. Lambert McCarty, Clemson University, Clemson, SC.

1. Field Bindweed

A single yank might get you a handful of bindweed, but make no mistake: It will be back! Imagine a plant that has roots that are 10 feet deep. Each fragment of root will grow into a new plant!

Classified as “noxious,” field bindweed (Convolvulus arvensis) looks like a pretty perennial vine with white flowers that resemble wild morning glories, but it’s an invasive, noxious weed from Eurasia. It snakes its way up, over, and through your garden plants. And it spreads quickly! A single field bindweed plant can spread radially more than 10 feet in a growing season. It spreads by seeds that can sprout after 50 years and roots that can grow 10 feet deep. You can try digging it out, but any bit of root left in the ground will sprout into a new plant.

How to Kill It: Simply digging it out doesn’t work. In fact, fragments of vertical roots and rhizomes as short as 2 inches long will form new plants.

1. Kill this nemesis as early as possible—ideally, within 3 to 4 weeks of germination in the springtime. After this period, perennial buds form, and by summer, it’s not possible to get to all of the roots.
2. If field bindweed is growing by itself, you can spray it with this , which is ideal for larger properties. This weed killer efficiently enters the plant through leaves and then moves down through its vascular system. Visual symptoms of its effects usually appear in 1 to 3 weeks. People and pets can be in the area as soon as the spray has dried.
3. If your field bindweed is tangled in other plants, you’ll have to carefully paint the chemical onto only the field bindweed leaves. You’ll miss lots of leaves, so repeat applications will be necessary.

2. Bull Thistle

Bull thistle (Cirsium vulgare) is a robust and spiny plant found in every state in the U.S. and listed as a noxious weed in western states. It spreads entirely by seed and can produce 100 to 300 seeds per flowerhead, with one to more than 400 flowerheads per plant! As a non-native, it will outcompete all native plants once established. It’s not palatable to wildlife or livestock.

As a biennial, bull thistle needs two seasons to reproduce. During its first season, the plant forms a “rosette” up to 3 feet in diameter. This is a taproot with a cluster of leaves that are long, deeply lobed, and bearing coarse, prickly hairs on the top and woolly hairs beneath—as well as long, sharp spines along the edge! In the second year, the rosette sends up a flowering stalk with pink-magenta flowers that bear spine-tipped bracts below the petals. 

How to Kill It: We recommend an integrated approach to controlling bull thistle.

1. Repeated mowing throughout the growing season can be helpful, provided it’s not done too early. It’s best to mow after the species sets seeds but before it’s in full flower. However, mowing doesn’t prevent bull thistle seeds from blowing in from other areas, so mowing must be combined with treatment. Tilling often just makes the situation worse, unearthing weed seeds.
2. Treatment in the rosette growth stage in early spring provides better control than later applications. The younger the rosette, the better the results! Once the plant is in stalk form or flowering, it’s more difficult to kill. Most of the herbicides used for control also kill pasture legumes. So, spot-spraying individual plants or patches rather than broadcast-spraying the entire pasture uses chemicals efficiently and also spares the beneficial legumes. Use this  that is safe for people and pets once the spray has dried.

Bull thistle requires long-term, consistent management to keep the thistles in control. One bad year, and it starts all over, so stay on it!

3. Yellow Thistle

Yellow thistle (Centaurea horridulum) is appropriately nicknamed “horrible thistle.” One early botanist called this spiny plant “one of the most terribly armed plants in the genus.” A winter annual and sometimes biennial, yellow thistle bears long leaves with thick, sharp spines along the edges. Their aggressive and spiny growth shades out grasses and clovers and deters cattle from grazing the infested areas.

Often found along the edges of salt marshes from Maine to Florida, it is also a pasture weed in the South, where its flower heads are frequently red-purple instead of yellow. The clustered flower heads have spiny, feathery bracts that almost enclose each flower’s entire head. Stems are covered in fine hairs and hairlike projections. Yellow thistle spreads by seeds attached to tuffs of soft white hairs.

How to Kill It: Yellow thistle is a persistent weed that can sneak up on you and quickly take over a pasture or property.

1. Mowing offers limited control. Mow when thistles reach the late bolting stage, when flower buds begin to emerge. However, new shoots will emerge from buds in the leaf axils below the mowing heights and will flower and produce seed. 
2. The best time to dig or hand-pull thistles is when they have bolted but not yet flowered. Although many of these thistles do not produce creeping roots, you should remove as much of the fleshy taproot as possible to prevent new shoots.
3. Herbicide treatment can provide the most effective thistle control. It’s best to apply  during the “rosette” stage when the plant emerges in early spring, before flowers bloom in late spring.

4. Horseweed

Horseweed (Conyza canadensis), also called mare’s tail, is an annual weed emerging in late spring. It’s a strong competitor for water, grows rapidly, and is a big problem in agriculture and gardening because it resists many weed treatments. We’ll help you to knock back this troublesome weed!

Seedlings develop in a basal rosette with long, narrow leaves with scalloped edges. As a plant matures, it forms a single, hairy stem up to 6 feet tall with spear-shaped leaves about 3 to 4 inches long. At the top of the plant, the stem branches to produce tiny white daisy-like flowerheads. (Note: Annual fleabane looks very similar but has leaves shaped more like eggs than baseball bats and shorter hairs held closer to the leaf.)

How to Kill It: Control of horseweed is more effective when plants are in the rosette stage or less than 2 inches tall. If the plant gets above 5 inches tall, it can survive most herbicide applications. 

1. Tillage can be helpful. Horseweed is a prolific seed producer, but the seeds are not long-lived, so plowing when the plant is young will help to eliminate seeds. However, once plants are established, this practice is discouraged due to the opportunity for new weed seed emergence upon soil disturbance.
2. Control of horseweed is more effective when plants are less than 2 inches tall. If the plant gets above 5 inches tall, it can survive most herbicide applications. To prevent seed production, ideally, it should be applied in the rosette stage in early spring. Many weed killers do not work on horseweed. Use this that’s also safe for kids and pets once the spray has dried.

5. Common Ragweed

Imagine a plant that can produce up to 62,000 seeds and remain viable for 5 years or more! That’s common ragweed Ambrosia artemisiifolia, the plant that we know as a serious cause of hay fever and allergies. Controlling this summer annual in early spring is essential for reducing weed seed production. Classified as noxious in many states, ragweed is found throughout the U.S. and can thrive in extreme environments where other weeds would not survive. 

Look for small, deeply lobed leaves with small hairs on their underside; the emerging stems are also hairy. This plant grows very quickly, reaching 3 to 5 feet tall within the season. Leaves grow up to 8 inches long and are divided into many narrow, toothed segments, making for a fern-like appearance. After the summer solstice, the plant flowers for 30 days; the flowers are small and grow in slender, elongated spikes that are green-yellow. In another 50 days, the seeds mature.

How to Kill It: Ragweed becomes a concern when you want to use land that’s covered in the weed; this will require an herbicide program combined with tillage, mowing, and other preventative practices.

1. Ragweed’s weakness is that it needs light to germinate and needs to be within 2 inches of the soil surface, so burying the plant obstructs its access to light and inhibits germination. Tillage can bury the seeds so deep that they will not emerge; however, this can unearth previously buried seed, as well. Therefore, aggressive tillage is most effective in late spring prior to planting.
2. Seeds always seem to find safe harbor on roadsides for later reinfestation, so mowing on your property’s perimeters near roadways could also decrease the prevalence of common ragweed.
3. An effective ragweed herbicide program begins early in the spring to reduce the seed bank.  that allows you to reseed after 3 weeks.

6. Creeping Buttercup

If creeping buttercup (Ranunculus repens) finds its way into your gardens, you’ve got trouble with a capital “T”! The yellow petals may look pretty, but creeping buttercup is one of the most aggressive invasive plants in North America. One plant can colonize so rapidly that it covers more than 40 square feet in a year. In addition, creeping buttercup is very toxic to animals.

To identify this perennial weed, look for dark green leaves that are three-lobed (the middle lobe has a stalked base) and have pale patches. Both the leaves and stems are somewhat hairy. Starting in March, the plant bears bright yellow flowers with five shiny petals. 

Creeping buttercup spreads by seed and runners that root at the nodes. Normally, plants form more runners through branching. However, when nitrogen is limited, the plant responds to its environment! The runners grow longer and unbranched so that the plant can find a suitable new home; once there, the weed spreads and depletes all nutrients in the soil, killing all nearby plants.

How to Kill It: Unfortunately, cultivating or disturbing the soil increases the buttercup population because it can sprout from nodes along stem and root fragments. In addition, plants are at ground level and resist mowing; they quickly resprout if cut.  

1. In lawns and pastures, the best prevention is to promote healthy grass by overseeding and adding lime. However, lime won’t control Buttercup, which is already well established. 
2. It also helps to improve soil drainage, as this weed thrives in wet areas. Reduce compaction by aerating and avoid trampling when soils are wet.
3. However, the only real way to eradicate mass areas of creeping buttercup is the right herbicide, such as this. It will probably take at least two or three applications to eradicate creeping buttercups because of the seed bank and because some mature plants will generally recover.

We hope that this advice helps you to identify some of the toughest weeds on Earth. You can indeed prevent and control them on your own property, as well as deny their spread to natural lands where they will destroy native vegetation and wildlife. With the right information, we can all do our part to be stewards of the environment.