"Noxious" weeds are so called because they are harmful (from the Latin, nocere, to harm). But the term is an inexact one, since a so-called noxious weed may not be harmful to everyone; plus people will disagree on what criteria should be used to measure harm. In the broadest sense, quite a variety of unwanted plants can be classified as noxious weeds, so we must approach the subject from a number of perspectives.
- The amount of money wasted by farmers in trying to battle them, as they encroach upon farmland
- The number of native plants threatened as such noxious weeds encroach upon their wild habitats, out-competing them for resources
In either of those two cases, after enough data is in, the noxious weeds in question will end up on one of the popular plant "blacklists," often organized by state (in the U.S.). Some states have even banned the importation of certain invasive plants, including Oriental bittersweet (Celastrus orbiculatus).
But there are other senses in which unwanted plants may be classified as "noxious weeds." To name just three of them:
- If they are poisonous plants
- If they are plants that are irritants
- If they produce a lot of pollen, as does common ragweed
Use the pictures of noxious weeds presented here to aid you in identification, starting with kudzu vine. Kudzu vine is so notorious that it has earned the right to serve as the poster child for this topic. It has also earned the nickname, "the vine that ate the South," due to the way that it has voraciously spread through the Southeastern U.S. Known botanically as Pueraria montana, this plant is native to the Far East. Kudzu vine can reach 35 to 100 feet in length and was introduced into the United States for, among other reasons, erosion control.
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The berries do not all ripen at the same time, meaning that a bittersweet nightshade plant can bear green, yellow, orange, and red berries all at one time. This makes for a colorful display, a display that could easily tempt young children into plucking them and eating them.
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You've heard the rhyme about Toxicodendron radicans, "Leaflets three, let it be." It's a rhyme to help you remember what poison ivy plant looks like. You will also hear the version, "Leaves of three, let it be." It means the same thing, but some sticklers prefer to refer to the individual parts of a compound leaf as "leaflets," while others simply call them "leaves."
Regardless of which version of the rhyme you like better, the plant clearly does have leaflets or leaves of three parts. But other types of plants also have "leaves of three." So knowledge of this feature will get you only so far in truly being able to tell poison ivy from other plants.
The next step for those interested in being able to differentiate this weed from vines that look like poison ivy is to study what poison ivy looks like at different times of the year (and, consequently, at different stages of growth).1:27
8 Facts About Poison Ivy
03 of 12
Poison sumac (Toxicodendron vernix) has a lovely autumn leaf color. Look, but do not touch!
No need to become paranoid, though. Of the noxious weeds covered here, poison sumac is perhaps the one that the average person is least likely to encounter in the yard because it tends to grow in swampy areas.
If it's plants with great fall foliage you wish to grow in your yard, you can make a safer choice than poison sumac. If you are still tempted by the colorful hues of poison sumac, try the non-poisonous types of sumac such as Tiger Eyes (Rhus typhina 'Bailtiger') or plant other shrubs for fall color. Others will prefer to grow fall foliage trees.
04 of 12
People interested in planting herbs may grow stinging nettle (Urtica dioica) in their gardens for culinary purposes, but most gardeners consider it a noxious weed.
On the culinary side of the ledger, consider making a stinging nettle soup. You can also eat stinging nettle (when the leaves are young and tender) as a boiled green, the way you would eat spinach.Continue to 5 of 12 below.
05 of 12
Spotted knapweed (Centaurea maculosa) is an invasive biennial or perennial wildflower whose flower looks a bit like a thistle. The genus name will sound familiar to flower gardeners. There may well be plants in your landscaping classified as Centaurea.
Farmers hate this plant for the negative impact it has on crops. Not only does spotted knapweed produce enormous amounts of seed, but, like the walnut tree, it is also allelopathic.
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Not only is bull thistle (Cirsium vulgare) an invasive plant, but it can also just plain hurt to run up against its barbs. Some birdwatching enthusiasts do grow the plant, however, due to its ability to attract goldfinches.
Those of you familiar with the flora of overgrown pastures may know bull thistle as a common inhabitant of such areas. But this Old-World native can also take advantage of disturbed ground along roadsides, etc. No shy wallflower, you'll know this robust noxious weed when you see it: Plants can reach a height of 7 feet under ideal conditions.
Bull thistle is a biennial. It develops rosettes the first year, succeeded by flower stalks the following year. Plants are covered with spines, making the sort of spontaneous removal that you may practice with other noxious weeds a bad idea for bull thistle. No, this is the type of plant you come prepared to remove, wearing heavy gloves and long sleeves/pants and wielding a shovel. Dig bull thistle out by the root. Don't let it go to seed.
The reason why letting it go to seed is such a bad idea is that the seeds of bull thistles, like those of dandelions (Taraxacum officinale) and common milkweed (Asclepias syriaca), are easily transported across the air. Each seed is attached to thistledown, so it can travel far away from the mother plant when the wind blows.
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Japanese knotweed (Polygonum cuspidatum) is the most difficult invasive plant to eradicate that you are ever likely to encounter. Other common names you'll find used for this plant are "Mexican bamboo" and "Japanese bamboo."
When its "bamboo shoots" first appear in spring, they can be considered edible weeds.
The tender Japanese bamboo shoots eventually harden as they grow taller, then die in winter, becoming brittle canes (the root system lives on underground, unfortunately). Even as new shoots in spring, though, they have great strength: They will exploit cracks in concrete in urban areas to push there way up through sidewalks, driveways, or paved parking lots.
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Morrow's honeysuckle bushes (Lonicera morrowii) are indigenous to Eurasia, according to Marilyn J. Dwelley, author of Trees and Shrubs of New England (1980). It's hard to go anywhere in parts of New England without seeing them. They make this noxious weeds list on the basis of their invasiveness. Don't confuse them with Japanese honeysuckle vines (Lonicera japonica).
Reaching 5 to 8 feet in height, Morrow's honeysuckle shrubs readily exploit disturbed soils to naturalize along roadsides and in thickets and open woods.
Tatarian honeysuckle bushes (Lonicera tatarica) are, says Dwelley, native to Europe. The novice at plant identification has to take great pains to distinguish Morrow's honeysuckle bushes from the Tatarian version; these two shrubs are very similar in appearance. But if you see such a shrub with pink flowers, that plant is a Tatarian honeysuckle, because the flowers of Morrow's start out white and then become a creamy yellow. Both plants bloom May to June and are invasive.Continue to 9 of 12 below.
09 of 12
Autumn olive may be confused, by the casual observer, with honeysuckle bushes. The two also share the same habitat.
A shrub native to the Far East, autumn olive (Elaeagnus umbellata) is an invasive plant in North America. Effective at erosion control and salt tolerant, autumn olives were introduced into the U.S. for roadside plantings. They later escaped from cultivation.
Autumn olive is very similar in appearance to another invasive shrub or small tree, Russian olive (Elaeagnus angustifolia). Both have silvery leaves, but autumn olive's leaves are usually silvery only on the underside. Another way to distinguish between autumn olive and Russian olive is to inspect the shape of their respective leaves. Russian olive bears a narrower leaf than does autumn olive; thus its species name, angustifolia, which means "narrow-leafed" in Latin.
The species name for autumn olive, umbellata, refers to the "umbels" of flowers borne by autumn olive shrubs (an "umbel" being a flower head wherein the individual flower stalks are of about the same length, radiating out of the center like the spokes of an umbrella).
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Tree of Heaven
Tree of heaven is the common name for Ailanthus altissima. This invasive plant is also known as "Chinese sumac," because it is native to China and it resembles the sumac native to North America.
Tree of heaven, a tree sometimes ubiquitous in urban areas, was the inspiration behind A Tree Grows in Brooklyn, by Betty Smith. One person's noxious weed is another's inspiration.
Not that you should be totally surprised that tree of heaven could have inspired A Tree Grows in Brooklyn. Noxious weed or not, the seed clusters on tree of heaven are rather pleasing to the eye.
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While this resource on noxious plants has focused on weedy plants, that does not mean that "legitimate" landscape plants are always beyond reproach. If growing such a specimen were to result in the poisoning of a child, the parents of that child would understandably consider it a "noxious plant" of sorts.
Monkshood (Aconitum) is no weed but, rather, a fairly popular landscape plant. It is a perennial commonly used in flower borders, valued for its purple flowers.
But monkshood is presented here not to discuss its beauty but to warn you about its toxicity (making it a "noxious plant" on one level).
"Monkshood" is a reference to this perennial's flower. Those imaginative folks who have given plants their common names over the centuries fancied that the shape of the flower mimics that of the hood on a garment traditionally worn by monks.
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Dogbane (Apocynum cannabinum) goes by a number of other common names, including "hemp dogbane."
While Apocynum cannabinum is the type native to North America, there are other kinds of dogbane native to the Old World. It has small flowers that offer white blooms in the late summer months. Despite being an annoyance to gardeners, it works perfectly in a wildflower or native plant garden, as it attracts butterflies.