Every year, land managers must deal with a set of weedy plants that routinely infest their crops. Most of the weedy plants have plagued the crops for a long time and have evolved to develop traits which enable them to escape control and continue infesting crops and spreading to other areas.

Problematic weeds are plants that can thrive in disturbed habitats which are used in crop production.Several characteristics are associated with weeds that thrive in disturbed habitatsand infest crops.

What are some characteristics of problematic weeds?

• Germinates in a wide range of environmental conditions.
• Long-lived seeds that have dormancy which allows them to control when they germinate.
• Rapid seedling growth and ability to reproduce when young.
• Quick maturation or only a short time in the vegetative stage.
• Ability to reproduce in multiple ways, e.g., by seed and vegetatively.
• Capable of tolerating and growing under a wide range of climatic conditions.
• Produce numerous seeds often more than once during the growing season.
• Great competitive ability for nutrients, light, and water.
• Roots of some weeds can penetrate and emerge from deep in the soil.
• Resistance to control measures including herbicides.

Note that a weed does not need to have all characteristics to thrive. Most weeds have a combination of the listed characteristics.

What are some of the ‘Usual Suspects’?

Only 3% of all known plants are classified as weeds. Of these, some of the most common weeds that infest a wide range of crops include the following, grouped by family.

Pigweed family

• Includes several major weeds of vegetables and other cultivated crops, for example:
  • Smooth pigweed
  • Redroot pigweed
  • Palmer amaranth
  • Powell amaranth
  • Common waterhemp
  • Tall waterhemp
  • Prostrate pigweed
• Well-adapted to high temperatures, are among the most aggressive of summer annuals, and can cause substantial yield losses.
• Palmer amaranth and the waterhemps have become particularly troublesome in the southern and midwestern United States because of their rapid development of resistance to glyphosate.
• A single mature pigweed can shed 100,000 to 1 million viable seeds.

Sunflower family

• Large and diverse plant family that have become serious agricultural weeds, for example:
  • Common ragweed
  • Giant ragweed
  • Common sunflower
  • Canada thistle
  • Musk thistle
  • Common cocklebur
  • Common dandelion
• Crops in this plant family include sunflower, safflower, lettuce, endive, artichoke, zinnia, dahlia, and many other cut flowers and ornamentals.
• This large plant family includes annuals, biennials, and perennials with or without taproots, rhizomes, or tubers.

Mustard family

• Includes a few common weeds of cool-season vegetables and cereal grains, for example:
  • Wild mustard
  • Shepherd’s purse
  • Yellow rocket.
• Garlic mustard is an invasive exotic species of particular concern because its strongly allelopathic properties help it to invade woodlands in the United States.
• Many of our most important vegetable crops also come from the brassica family, including cabbage, broccoli, cauliflower, kale, collards, bok choy, various cultivated mustards, arugula, and other gourmet greens.
• Mustard weeds can be a reservoir of pests and diseases of related vegetable crops, yet flowering brassicas (weedy or cultivated) often attract the natural enemies of the brassica pest complex.

Goosefoot family

• Family of aggressive and competitive weed species, for example:
  • Common lambsquarters
  • Kochia
• Common lambsquarters, whose young foliage is edible and as nutritious as spinach, chard, and beet (also members of the goosefoot family), is one of the world’s worst weeds. It is an aggressive spring or summer annual weed in croplands in temperate regions around the world, and a single large plant can shed 50-100,000 viable seeds.
• Kochia is a summer annual weed infesting annual crops in the northern Great Plains region.

Morning glory family

• The family includes deep-rooted wandering perennials that are recognized among the world’s most damaging weeds and parasitic weeds. For example:
  • Field bindweed
  • Hedge bindweed
  • Annual morning glory
  • Dodder
• Many of these weeds climb any upright support, including crop plants, whose growth and leaf opening can become seriously hindered as a result.
• Weeds in this family thrive in summer heat, as does the sweet potato, one important food crop in this family.

Sedge family

• Family of heat loving plants that are amongst the worst in the world. For example:
  • Purple nutsedge
  • Yellow nutsedge
• Nutsedges are wandering perennials that are highly allelopathic against many crops, and can form underground rhizomes, bulbs, and tubers.
• Sedges are characterized by grass-like leaves deployed in a three-ranked arrangement, and solid stems distinctly triangular in cross section. In contrast, true grasses have rounded and hollow stems.

Mallow family

• The mallow family includes aggressive summer annual and perennial weeds, for example:
  • Velvetleaf
  • Prickly sida
  • Spurred anoda
  • Common mallow
• Velvetleaf is a major weed of corn and other row crops, especially in fields that are plowed annually.
• The mallow family also includes several important crops: cotton, okra, kenaf, hibiscus, and ornamental mallows.

Grass family

• The grass family is the most agriculturally important plant family on Earth. It includes many of the world’s staple food and fodder crops (corn, rice, wheat, millets, sorghums, other annual cereal grains; and many perennial and annual forages).
• Grass family weed species include:
  • Bermuda grass
  • Barnyard grass
  • Goosegrass
  • Johnsongrass
  • Crabgrass (large and smooth)
  • Quackgrass
  • Foxtail (green, yellow, giant, and bristly)
• The most definitive characteristics of grasses are that the single cotyledon remains underground inside the seed after germination.
• The growing point of grasses remains underground for some weeks after emergence unlike broadleaf plants.Thus, the young grass plant is not killed by frost, heat, mowing, or removal of aboveground parts, which makes grasses difficult to control.

Buckwheat family

• The buckwheat family includes some globally important weeds, for example:
  • Knotweeds
  • Smartweeds
  • Curly dock
  • Broadleaf dock
  • Red sorrel
  • Wild buckwheat
  • Mile-a-minute
• Buckwheat is a valuable grain and cover crop.
• Wild buckwheat and mile-a-minute are rampant annual vines that climb, shade, and bind other plants much like bindweeds and morning glories.
• Buckwheat and smartweeds produce plenty of nectar that can attract and support beneficial insects.

Purslane family

• The purslane family consists of one very important weed species:
  • Common purslane
• Common purslane is an edible plant that grows in prostrate mats and can compete aggressively with young or low-growing crops.
• Common purslane is hard to control using cultivation, as broken fragments can easily re-root and grow into new plants.
• Cultivated members of this family include ornamental flowering portulacas as well as some domesticated varieties of purslane grown as a salad green.

Nightshade family

• The nightshade family includes several summer annuals and rhizomatous perennials, some of which are toxic weeds. For example:
  • Jimsonweed
  • Black nightshade
  • Horsenettle
  • Silverleaf nightshade
  • Eastern black nightshade
  • Buffalo bur
  • Groundcherry
• Major food crops in this family include potato, tomato, sweet and hot peppers, and eggplant.
• In addition to competing for nutrients and moisture, nightshade weeds in vegetable fields can be a reservoir of pests or diseases of these crops.

Controlling the Usual Suspects

For most land managers, this is routine. They know what to expect and try to stay a step ahead of the weeds every season with an arsenal of control options. Knowing the weed species is key to effective control, while monitoring crop fields helps determine if there are weeds that escaped control and may be resistant to herbicide. Monitoring field also helps identify new species that may be invasive plants.

Further Reading:

• Pest Biology and The Ecology On Your Land
• Staying a Step ahead of the Soil Seedbank
• Chemical Weed Control
• The Importance and Benefits of Preventative Weed Control
• Pesticide Resistance Management
• Getting Ready to do Battle with the Soil Seedbank

References

• Schonbeck, M. 2019.A Virtual Tour of Major Weed Families. Organic Farming Research Foundation, Virginia Association of Biological Farming [Accessed, November 30, 2022].