Smartweed Identification – How To Control Smartweed Plants

Smartweed Identification – How To Control Smartweed Plants

By: Jackie Carroll

Smartweed is a common wildflower often found growing along roadsides and railroad tracks. This wild grain is an important food source for wildlife, but it becomes a noxious weed when it gets into garden plots and lawns.

What is Smartweed?

Smartweed (Polygonum pensylvanicum) is an annual broadleaf. As an annual, it reproduces through seeds that drop near the parent plant to produce new plants. The most effective control methods focus on preventing the plants from producing seeds.

Before we discuss how to control smartweed, let’s take a look at a few key physical features that can help with smartweed identification. One of the first things you might notice is that the stems are divided into segments. The swollen areas that separate the segments are called “knees,” and they are covered with pale green sheaths. Smartweed leaves are shaped like lancets and may have purple blotches. The leaves have smooth edges and sparse hairs on the surface.

Getting Rid of Smartweed Plants

Getting rid of smartweed begins with good cultural practices. Weeds have a hard time gaining a foothold in a healthy, well-kept lawn. Water the lawn as necessary and apply lawn fertilizer on a regular schedule. Frequent mowing helps keep the grass healthy, and it removes the tops of weeds, such as smartweed, before they have a chance to produce seeds. Rake up and bag debris that may contain seed heads.

Smartweeds have shallow taproots that make it easy to pull them up when you only have a few. Some organic herbicides, such as acetic acid and citric acid, are effective at killing young smartweed plants, but they might also harm garden plants unless applied very carefully.

Flamers can also help you take control of smartweed in your lawn or garden. It only takes one-tenth of a second of heat from a gas torch to kill smartweed, and once killed with flame, the weed won’t return. Flamers are most useful in a vegetable garden where you have long, straight rows.

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Department of Plant, Soil and Microbial Sciences Weeds

Pennsylvania Smartweed and Ladythumb (Polygonum pensylvanicum L. and P. persicaria L.)

Summer annuals. Emerge in the spring and set seed in late summer/fall and die. There is also a perennial smartweed, swamp smartweed (P. amphimbium L.). The rhizomes are white and thin hence this weed is often called "shoestring".

Pennsylvania smartweed and ladysthumb (smartweeds) emerge in early in the spring. Ten percent emerge by 150 GDD (base 48 F).

Smartweeds emerge from soil depths of less than 1".

Production Average : Each smartweed plant produces an average of 19,500 seeds.

Dispersal Mechanisms: None.

Longevity: Smartwed seed exhibits moderate persistance. It takes 4 years to reduce the seedbank by 50% and about 26 years to reduce it by 99%. In one study, 3% of the smartweed seed still germinated after 10 years. In another study 6% still germinated after 16 years.

Dormancy: Smartweed seed is less dormant when the seed is locate in the upper inch of the soil profile. As burial depth increases, seed dormancy increases. Smartweeds require wet soil conditions to germinate hot, dry soil conditions cause seed dormancy. Fresh seeds are always dormant and require a cold period (winter) before being able to germinate the following spring.


Smartweeds are somewhat competitive, but they are low growing weeds. Soybean yield was reduced 13% by 8 smartweed plants in 30ft^2 of field (about one plant per 4 feet of row). Smartweeds are more competitive in sugar beet.

Preferred Soil / Field Conditions:

Smartweeds prefer wet, poorly drained soils and those high in nitrogen and phophorus. They do not tolerate drought conditions.

Tillage: Tilling at night (dark tillage) will reduce smartweed emergence by 30-50%. If perennial smartweed is present in the field, clean tillage equipment before moving to another field- the rhizomes will wrap and hang on the tillage equipment.

Rotary hoeing: Hoe smartweeds before they exceed 1/4" in height.

Flaming: Flaming is effective on small smartweeds (less than 1" in height).

Crop rotation: Smartweeds are not a weed of small grains and forages.

Planting date: Tilling in the spring triggers smartweed to germinate. Therefore, tilling in early May and planting later (mid-May) will reduce smartweed infestations because many smartweed plants will have emerged and been controlled by tillage.

Application timing and effectiveness: Herbicides are available to control smartweeds. Control can be difficult in sugar beets.

Wild Violets

Wild violets (Viola papilionacea) are lovely when they bloom in spring, but then these perennials turn into persistent weeds. Common in the eastern half of North America, the weedy species has heart-shaped leaves. Wild violet colonies usually start in shady spots and spread into sunnier areas. Control by digging, and make sure you get the whole root, which includes seed-bearing capsules right at the soil's surface. In lawns, use a sharp knife to sever roots before pulling up the plants.

Weed Control Techniques

Digging. Weeds that regrow from persistent roots must be dug. Use a spade or digging fork to dig spreading perennials, such as bindweed, Canada thistle, and quackgrass. Start digging a foot away from the plant's center to loosen the soil. Then lift the weed from beneath, which reduces how many root pieces are likely to break off and regrow. Dandelion, dock, and other weeds that grow from persistent taproots can be dug the same way, or you can use a special fork-like tool called a dandelion weeder to pry them up. Dig very large taproots that are difficult to pry loose. In lawns and other places where digging dandelions is not practical, use a sharp knife to slice off the leaves and the top inch or two of taproot at a diagonal angle. Some weeds that are easily pulled when the soil is moist must be dug from dry soil.

Pulling. Most young weeds can be pulled from the soil. They will slide out most easily if you pull them when the soil is wet. Getting the root up is crucial, so think of the main stem as the root's handle, and grasp it as close to the soil line as you can. If you find that the weeds are breaking off at the crown as you pull, slip a kitchen fork, dandelion weeder, or similar tool under the weed, and pry and twist as you pull it up. Weeds that have taproots, such as dandelion and plantain, usually must be pried out. A flexible pair of waterproof gloves will keep your hands comfortable as you weed, and it's good to have a nice sitting pad, too. Let pulled weeds bake in the sun for a day or so before composting them. If pulled weeds are holding mature seeds, compost them separately in a hot, moist pile before using this compost in the garden.

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Systemic herbicides, such as Shoreline Defense ® , will work to kill the root of the plant.

Anytime you use chemicals treat weeds and algae, please keep in mind the following:

  • Treat your pond in sections. Treat only half the pond's surface at a time. During hot weather or when treating heavy growth, it is important to treat no more than 1/4 of your pond at a time and wait the full 14 days before re-applying. This helps lower the risk of fish loss during hot weather or when treating heavy growth.
  • Once the weeds have browned & died, use a weed cutter & rake to remove as much dead material as possible. This prevents an accumulation of dead plant material and muck.
  • Take a proactive approach to pond management. Use PondClear ™ , MuckAway ™ and Pond Dye to keep your pond looking great. For more information, see our article on the Airmax ® Ecosystem ™