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All About Smartweed

Smart and weed are two words that nobody but critters like seeing together. The aquatic smartweed is no doubt a great food source for wildlife. Unfortunately, its ability to regrow from seeds, roots, and nodes makes for a noxious weed.

Read on to learn about a few varieties of this common wildflower and what to do if they’re in your yard.

Water pepper, knotweed or smartweed. Water-pepper weed, Persicaria hydropiper or Polygonum hydropiper, close-up. Wild flower, Polygonaceae family.

Identifying a Smartweed

Its stems’ many swollen nodes mainly identify part of the Polygonaceae (Buckwheat) family with knotweeds, and smartweed. Unlike knotweeds, smartweeds’ flowers are in terminal clusters; they don’t grow from the leaf axils.

There are about 900 species of smartweeds, with 15 in North America. They’re usually in moist, disturbed areas like marshes, ponds, lakeshores, and ditches; other locations include roadsides, forest edges, and waste areas.

Let’s go over four kinds of smartweed you’ll most likely encounter.

Other Species

Creeping smartweed (Persicaria longiseta) flowers. Polygonaceae annual weed.

Pennsylvania Smartweed

Polygonum (Persicaria) pensylvanicum is a summer annual broadleaf weed that’s native to North America. Native Americans used this weed to treat fevers and hemorrhoids, but insufficient medical evidence supports this.

Its red or green stem is tall and spreading, reaching up to 6 feet but commonly 3 feet. Surrounding it are green or brown ocrea (ring-like membranous sheaths). If the stem breaks, it releases a skin-irritating sap.

The leaves are 4–6 inches long and vary in shape. They’re smooth with sparse hairs and reddish joints and occasional dark arrow-shaped spots.

At the end of the branches are 50–80 pink or white stalkless flowers in ¾–1½-long clusters. Each of the five petals is a tepal, for the difficulty distinguishing between a petal and a sepal. Also, it has flattened brown or black seeds that are dry, smooth, shiny, and disc-shaped.

Dotted Smartweed

Polygonum (Persicaria) punctata is native to the Americas and can be either an annual or a perennial smartweed. It is a good weed for bees, but in some places, it has lower-quality honey.

About 3 feet tall, its stem is erect to spreading and also has ocrea surrounding them like the previous smartweed. The leaves are dark green, lance-shaped, and 5½ inches long and over an inch wide.

It has brown or black three-sided seeds that are about ⅛ of an inch long with a smooth, shiny surface. The short-stalked flowers are white or green with dotted glands and are ⅛ of an inch long with five barely-opened tepals.

Mild Waterpepper

Polygonum (Persicaria) hydropiperoides is a semi-erect summer perennial with a simple or branching stem that reaches 2–3 feet. The narrowly to broadly lance-shaped leaves are 2–8 inches long and 1½ inches wide with pointed tips and mostly smooth edges.

It has greenish-white to pinkish flowers with five tepals that are about ⅛ of an inch long. Inside the tepals are eight pink-tipped stamens. And like the previous weed, it also has three-sided, dark brown or black seeds with a smooth and shiny surface.

This smartweed has a history of treating bacterial infections. Researchers found that it could inhibit candida growth and reduce redness and swelling. In addition, it has a good antioxidant capacity.

Lady’s Thumb

Polygonum (Persicaria) maculosa is an annual, rapidly-spreading, non-native weed that should be sprayed. It was first found in the Great Lakes region in 1843 and is widespread from Iceland to Europe and Asia.

Like the first two smartweeds, its seed is brown or black and disc-shaped with a smooth, shiny surface. The erect, forked, and swollen-jointed stem grows up to 3 feet. On each sticky 2–4-inch-long and ⅓–1-inch-wide, lance-shaped leaf is a dark spot resembling a lady’s thumbprint.

Each flower is about ¼ of an inch and varies in color (purple, green, white, or pink). They have four to six tepals clustered together on 2-inch-long spikes.

Spreading Habits

After the flowers go to seed, smartweed becomes invasive. Pulling it up can cause the weed to regrow if any roots are left behind. That also goes for any part of the weed if it has a node on it.

Smartweeds produce about 19,000–20,000 seeds, which are spread by passing through animals that eat them. If the weather is dry, they’ll be dormant for about ten years until wet soil conditions allow them to germinate.

Invasive Issues

Smartweed is competitive and will reduce soybeans, sugar beets, and other crop yields.

Smartweed is still an issue even if moist, disturbed areas aren’t part of your property. When the weed becomes dense, it will impede any boat navigation in lakes and block the drainage on roadsides.

How to Control Smartweeds

Persicaria hydropiper (Polygonum hydropiper) smartweed, water-pepper, water pepper, marshpepper knotweed, plant leaf. Pink wild herb smartweed flower, leaf knotweed nutural medicine pattern background

Frequent Mowing

You cut off the tops by mowing over smartweeds and preventing seed production. Whether or not you use a bag while mowing, ensure no weed debris is left behind.

Flame Weeding

Because it thrives on moist soil, smartweed won’t have a chance against flame weeding. A short second of heat kills the cells and cuts off the water and nutrient flow. For perennial smartweed, give it several treatments at 2–3-week intervals.

Note that this isn’t a good control method in dry weather, and you shouldn’t burn weeds near structures. Be sure to use a weed burner properly and follow its directions.

Organic Herbicides

Apply organic herbicides like acetic acid (vinegar) and citric acid to eliminate young weeds. Make sure to apply them while the weed is still actively growing. Be careful not to overdo the applications or you’ll lower the soil’s pH or harm your garden plants.

Positive Aspects of Smartweeds

Persicaria hydropiper (Polygonum hydropiper) smartweed, water-pepper, water pepper or marshpepper knotweed, plant leaf. Botanical pink wild smartweed flower & leaf knotweed on nutural wooden stump

Food for People and Wildlife

Lady’s thumb is edible when raw or cooked, as are its seeds when the weed is young. Your soups and salads could benefit from a dotted smartweed but moderately since it’s like cayenne pepper. Mild waterpepper also makes a great spice.

Because their habitats are in moist lands, smartweeds are food for waterfowl. Small mammals, songbirds, and an occasional white-tailed deer will also feast on these weeds. They’re also a good food source for insects, especially pollinators like bees and butterflies.

Many species of smartweed are an abundant food supply for animals and insects. But a line should be drawn if they block water flow in certain areas.

Wrapping Up Smartweed

No matter the kind of smartweed, this common wildflower will take over crops or whatever area it grows in. This weed may regrow in three ways, but proper control and removal will mitigate its noxious impact.

If you feel like you need to learn more about these pesky garden tenants, check out our weeds page to learn all about different weed varieties, treatment options, and surprising information.