Canada Thistle

(Cirsium arvense)

Canada thistle is a tall weed — mature plants reach up to 5 feet tall — with purple, globular flowers. While the stems are smooth, the leaves are thick and spiny. 

Don’t let the name fool you, because Canada’s thistle is not native to North America at all. It actually comes from Europe and was most likely brought to North America in the 17th century as part of agricultural seed shipments.

Today, Canadian thistle is widespread in most of Canada and the United States. You can find these plants growing almost anywhere — common locations include ditches, pastures, gardens, unmown lawns, and lawns. 

Like many invasive plants, Canada thistle is largely a problem because it reproduces incredibly well. The root system is difficult to eradicate and is capable of producing new shoots at almost any time. Canada thistle also produces many viable seeds each year which help it spread. As a result, Canada’s thistle easily outcompetes native plants anywhere it grows.

While stubborn, Canada thistle can be removed by hand or using a chemical herbicide. A combination of the two is often the best strategy. At the very least, I suggest cutting off all flowers before they go to seed.

Musk Thistle

(Carduus nutans)

Musk thistle is a biennial weed with spiny leaves and stems. In the second year, musk thistle grows up to 7 feet tall and produces purple flowers. Once mature, the flowers fall to the side similar to sunflowers. Some people know this weed as a nodding thistle — a name inspired by the flowers’ drooping shape.

Like many other invasive thistles, this plant is native to Europe and Asia but has spread throughout North America since its introduction in the 1800s.

Musk thistle easily chokes out native plants in meadows and undeveloped plots of land. It is a prolific seeder — according to the University of Minnesota, one plant can produce 20,000 seeds! Livestock and wild animals also avoid eating it.

The best time to eradicate musk thistle is in the first year before the plant bolts and flowers. At this stage, musk thistle presents as a rosette of spiny leaves that grows low to the ground. Removing thistles in the first year eliminates the chance for seeds to develop and spread.

Bull Thistle

(Cirsium vulgare)

Bull thistle is easy to mistake for Canada thistle but generally easier to control. To distinguish bull thistles from similar-looking weeds, check for deep taproots and wing-like spikes on the stems. Bull thistle flowers are pinkish-purple and have spines all around their bases.

This is another biennial weed native to Eurasia. Regardless of where bull thistle originally came from, however, it is now widespread across North America and similar climates.

Livestock tends to avoid thistles in general but this is especially true of bull thistles. The thorny stems make grazing on this weed incredibly painful for nearly all animals.

Like musk thistle, this weed forms a low-growing rosette the first year and flowers the second. If you can identify and remove bull thistles in their first year, you can prevent potentially thousands of seeds from being spread.

Purple Dead Nettle

(Lamium purpureum)

Despite the name, purple dead nettle lacks stinging thorns or spikes. It bears fuzzy, heart-shaped leaves reminiscent of other members of the mint family. Purple-pink flowers resembling snapdragons emerge from the stem nodes. The leaves themselves fade from purple at the top of the stem to green at the bottom.

Purple dead nettle is a member of the mint family — not a nettle at all — native to Europe and Asia. While it is considered a weed throughout North America, only a few states and provinces categorize it as noxious.

Purple dead nettle spreads via seed, stem, and root. This often results in thick carpets of the weed overtaking native plant species.

Purple dead nettle is unique in that it can forage for tea or greens. The lack of toxic look-alikes makes it safe for even beginner foragers, though you should always take precautions when collecting plant matter.

With the right selective herbicide, purple dead nettle is relatively easy to keep at bay. I’ve also had success simply removing this weed by hand before it has time to overtake any particular area.

Devil’s Apple

(Solanum linnaeanum)

Devil’s apple is a shrubby perennial with long spines protruding from the leaves and stems. The stems turn woody in later years. The purple flowers are star-shaped and slightly tubular.

Devil’s apple is native to southern Africa and is part of the nightshade family — other famous nightshades include tomatoes, eggplants, and potatoes. It grows invasively throughout the Pacific region, including Australia, New Zealand, and Hawaii. It’s also prevalent in some areas of the Middle East. 

This plant produces poisonous, tomato-like fruit. The unripe fruit is green while the ripe fruit is yellow. Between the spines and toxic fruit, this plant is extremely unappetizing to livestock and grazing wildlife.

According to the Government of Western Australia, chemical herbicides are the most effective control against the devil’s apple. When pulling the devil’s apple by hand, be sure to remove the entire plant (including the roots). Burning the pulled plant matter is advised to prevent further spread.

Red Clover

(Trifolium pratense L.)

Red clover is sometimes called purple clover because of the color of its flowers. In rare cases, however, red clover flowers can be white. The flowers tend to be larger than other clover varieties found in lawns and gardens.

Red clover is native to Europe, Africa, and western Asia. Today, red clover is naturalized — meaning it freely grows in the wild despite being non-native — throughout much of North and South America. 

Whether or not clover is a weed depends on the context. Personally, I think clovers make wonderful alternatives to turf grass. However, non-native species of clover easily outcompete those that are native to a given area. 

You can eliminate red clover without harming your lawn by treating the weed with a selective herbicide. According to Pennsylvania State University, applying herbicide in the fall is ideal.

Purple Prairie Clover

(Dalea purpurea)

Purple prairie clover probably doesn’t look exactly as you’d expect. Rather than producing a blanket of short shamrocks, this clover grows up to 2 feet tall. There’s no such thing as a lucky four-leaved prairie clover, either. Instead, I think the foliage of this plant is more reminiscent of fennel. 

The purple prairie clover features thimble-shaped purple flowers at the end of its stems. It usually blooms in late summer in cool and temperate climates.

Purple prairie clover is native to North America. While many people in the United States still consider this to be a weed, purple prairie clover belongs in the average lawn more than most turf grasses!

If you can learn to appreciate purple prairie clover as a native wildflower rather than a nuisance weed, it makes a wonderful addition to any North American pollinator garden.

Self-Heal/Heal-All

(Prunella vulgaris)

Prunella vulgaris, which goes by names like common self-heal and heal-all, is yet another weed that is actually native to most areas it grows in. 

It typically grows less than a foot tall and produces a cluster of purple flowers in the summer months. Both the stem and flower spike of this plant are square-shaped. The flowers themselves loosely resemble those of a snapdragon.

Common self-heal is native to Europe, Asia, Africa, and North America and has naturalized in most other temperate climates. It is most invasive in the Pacific Islands, including Australia and New Zealand.

Self-heal’s invasiveness makes sense when you learn that this weed is a member of the mint family. Like its relatives, common self-heal vigorously grows and spreads in most environments. 

Despite its aggressive growth habit, common self-heal can be beneficial. Clouded sulfur caterpillars rely on the plant as a food source. All parts of the plant can also be foraged and eaten.

A thick, healthy lawn is the best defense against common self-heal. If the plant does invade your property, I recommend using a selective herbicide or pulling the entire weed by hand.

Violet Woodsorrel

 (Oxalis violacea)

There are many varieties of woodsorrel growing in the world. Some are more likely to be called weeds than others. But if your lawn or garden is being overtaken by a purple-flowered, three-leaved shamrock, there’s a very good chance violet woodsorrel is to blame.

Violet woodsorrel is a North American native that is frequently grown as an annual in outdoor beds. Though not as aggressive as some other woodsorrel, this native wildflower spreads quickly via underground rhizomes. It can easily escape garden beds and take over patches of lawns before you realize the extent of the problem. 

While we’re on the subject of weedy woodsorrel, another variety you should definitely watch out for is creeping woodsorrel or Oxalis corniculata. This species often has reddish-purple leaves, which many gardeners can mistake for purple flowers from a distance. According to Gardeners’ World, creeping woodsorrel is a nasty weed once established that is difficult to control without the use of herbicide!

Henbit

(Lamium amplexicaule)

Henbit is a close relative of both purple dead nettle and common self-heal. Most notably, all three weed species belong to the mint family. You’ll also notice several shared traits if you compare the appearance of henbit to either of these counterparts.

According to the University of Wisconsin, henbit may sprawl out or grow upright but almost always stays relatively close to the ground. Keep an eye out for the square stem typical of this and other mint family weeds. The clustered flowers are pink or purple with a somewhat tubular shape.

Henbit is a widespread annual native to Europe, Asia, and Africa. A common theory is that henbit originally appeared in the Mediterranean but has since naturalized across much of the world. Today, it is a common invasive weed in parts of North America and similar climates.

As is the case with many nuisance weeds, henbit is most likely to establish in disturbed areas such as construction sites. It is likely to overtake areas of exposed soil in your lawn that are caused by thinning turf grass or landscaping projects.

Dove’s Foot Crane’s Bill

(Geranium molle)

Dove’s foot crane’s bill, often known as dove’s foot geranium in North America, is an annual weed native to the Mediterranean region. It is considered to be naturalized in much of Europe, Asia, and Africa today.

While dove’s foot crane’s bill does bear a resemblance to the ornamental geraniums many of us plant in our annual beds, they are not closely related. (Common garden geraniums belong to the genus Pelargonium.)

From an ecological standpoint, the dove’s foot crane bill is not a huge threat. This weed does not outcompete native plants the way many other invasive species do. However, it is considered a nuisance by many homeowners in areas like the United States and the UK.

This weed is largely a problem because it thrives in dry soils with low fertility. So dove’s foot crane’s bill can easily overtake a patchy lawn. It also reproduces rapidly via bursting seed pods that spread far and wide.

Creeping Charlie

(Glechoma hederacea)

Out of all the weeds with purple flowers I’ve discussed so far, I am most intimately familiar with creeping Charlie. This persistent weed has plagued my lawn and garden since the day we moved in and I am far from alone in this battle! 

Creeping Charlie is native to Europe — where it is commonly called ground ivy — and western Asia.

It is a member of the mint family that spreads both by seed and stolen. The leaves of creeping Charlie are round and slightly lobed. Plants tend to be entirely dark green with the exception of the flowers which are light purple. 

Anyone who has dealt with this weed knows that pulling a single stem can reveal a network of stolons running several feet in all directions. While the stolons tend to grow along the ground, they can also climb over rocks, stumps, and more in search of moisture.

Creeping Charlie is difficult to control because it resists most herbicides. Hand-pulling is often the best management strategy but is also extremely tedious. To prevent further spreading, I recommend removing all flowering creeping Charlie before it goes to seed.

Pokeweed

Pokeweed forms a large shrub growing from a central taproot. The flowers are white. This weed is also known as an inkberry plant because the dark purple berries were once used to make inks.

Pokeweed is a rare plant that is native to North America. It has also naturalized in parts of Europe and Asia. 

Regardless of its native status, pokeweed is undesirable for two reasons. The first is that it spreads incredibly aggressively. Once pokeweed enters a region, it is near-impossible to control. The second is that it is very poisonous to mammals, including humans, pets, and livestock. Birds, however, love mature berries. Read How to Get Rid of Pokeweed.

Some varieties of pokeweed are cultivated for ornamental use despite their potential hazards. The toxins in pokeweed are also being researched for potential applications — both medical and otherwise.

Black Nightshade

(Solanum nigrum)

Black nightshade is a name given to many members of the genus but Solanum nigrum is the most problematic as a weed. Like other nightshades, this plant is closely related to vegetable crops like eggplant and tomato.

Most black nightshade specimens have white flowers. However, some plants have light purple flowers. This natural variation makes it hard to distinguish it from other nightshade species.

Black nightshade is native to Europe and Asia and has naturalized in many parts of North America. While this weed is rarely a nuisance in residential areas, it does interfere with agricultural operations. 

There is some evidence that black nightshade is becoming resistant to herbicides. It also poses a problem for livestock since the plant is toxic at some growth stages.

Wild Violets

(Viola Odorata)

Since wild violets have no true stems, they grow very close to the ground. The flowers can be almost any shade of purple or, in rare cases, yellow.

Wild violets are native to Europe and Asia but have naturalized in many regions around the world. 

Whether or not you consider this plant to be a weed largely depends on your goals. While wild violets are beautiful, their aggressive growth habit makes them prone to overtaking other plants. 

If you decide to grow wild violets on your property, I suggest choosing an isolated location to prevent unwanted spread. I don’t recommend planting wild violets adjacent to native meadows or forests since the seeds can travel quite far.

Spotted Knapweed

(Centaurea maculosa)

Spotted knapweed is a tall, branching perennial that may be mistaken for a type of thistle. However, this weed does not have any thorns or spines. The flowers are light purple and emerge from a black-specked bract.

Spotted knapweed first appeared in eastern Europe. Today, however, this invasive weed is distributed throughout North America. It primarily impacts native grasslands and livestock pastures.

Spotted knapweed is perfectly suited to taking over wild ecosystems. Not only can a single plant produce up to 40,000 seeds — according to NC State University — but those seeds can travel extremely far via tumbleweeds.

Unfortunately, this weed is resistant to some common herbicides. One of the best ways to control the spread of spotted knapweed is by mowing plants as soon as they flower. Once spotted knapweed is established in an area, it can take several years of maintenance to totally eradicate it.

Forget-Me-Nots

(Myosotis sylvatica)

Woodland forget-me-nots are charming perennials that reproduce a little too freely for their own good. Most plants grow up to a foot tall and produce spring clusters of blue-to-purple flowers.

Like most of the weeds we’ve covered, woodland forget-me-nots are native to Europe and Asia. They have naturalized in much of North America but are only a serious problem in select areas.

Woodland forget-me-nots cause issues when they are allowed to reproduce and spread unmitigated. Even if you live in an area where these flowers aren’t considered invasive, I suggest keeping a very close eye on them. You can minimize self-seeding by cutting spent flowers as soon as they start to die back.