War on weeds

Common purslane

Common Purslane (Portulaca oleracea) is a weedy summer annual found throughout the world. A common weed found in turfgrass, vegetable and ornamental gardens, agricultural fields and open spaces. Unlike most other weeds, common purslane is considered nutritious because of its considerable amounts of vitamins, minerals, antioxidants and omega-3 fatty acids. The leaves are edible and can be eaten alone or in salads or cooked like spinach. Even so, because of its elevated levels of oxalates it should not be consumed excessively by those susceptible to kidney stones. In livestock, oxalacetate can lead to nitrate poisoning.

Common purslane can produce 240,000 seeds a year, and the seeds may last from five to 40 years in the soil. It is a fast-growing weed that can create a thick mat of vegetation that can smother desirable plants. It is drought tolerant and any part of the plant can quickly root and create a new plant.

This low-growing plant has thick, succulent leaves and stems. Stems grow radially from a central rooting point. Its leaves are 0.5-1-inch-long, fleshy, smooth and green with maroon hued margins and undersides. The leaves are sessile, generally oppositely arranged, but may also occur alternately along the pinkish-red stem, particularly near the base. The small (3/8 inch), five notched petaled, yellow flowers are borne singly in leaf axils and only open in sunshine. Although pollinators will visit the flowers, the plants are self-fertile, so almost all plants will produce tiny reddish brown to black, oval seeds about 1/64 to 1/32 inch in diameter.

Integrated pest management (IPM) options:

• Cultural — The best control of common purslane is keeping desirable vegetation healthy and thick. Mulching with landscape fabric or 3 inches or more of organic mulch is effective as it shades young tender plants.
• Mechanical — Hand pulling or digging is not effective as any part of the plant can easily root.
• Chemical — Spot-treat seedlings or young plants with broadleaf-selective herbicides. Use a surfactant according to label directions. Pre-emergent herbicides may be used to manage sites with many seeds. Always read and follow herbicide label directions.

Curlycup gumweed

Curlycup Gumweed (Grendelia squarrosa) is a biennial or short-lived perennial weed common to pastures, rangeland and other disturbed or non-managed sites. Chemical compounds (alkaloids, tannins, resins and glucosides) found in curlycup gumweed make it undesirable as a forage and unpalatable to livestock. In high-selenium soils commonly found in southern Idaho, curlycup gumweed will accumulate toxic levels of selenium. Curlycup gumweed is extremely drought tolerant and will increase its’ population during drought conditions. In overgrazed areas, this opportunistic plant will take over bare ground and become invasive.

Curlycup gumweed begins growth with oblong cotyledons, then develops into a low-growing rosette arising from a stout taproot. In the second growing season, plants reach heights of 1 to 3 feet. Curlycup gumweed produces numerous branching, semi-woody stems with alternate, bright green, clasping leaves. Glands on the leaves secrete a gummy, resin substance, which gives the plant a sticky feel. Leaves are 1 to 4 inches long and have serrated edges. The flowers are 1 inch in diameter, yellow, disk shaped and form at the end of each stem branch. Curved bracts also secrete the sticky resin substance, giving them there “gumweed” name. Flowering occurs in mid-to-late summer and lasts throughout the fall.

Integrated pest management (IPM) options:

• Cultural — Do not transport weed seeds on clothing or equipment; feed weed-free hay; keep ditch banks and roadsides free of seed-producing plants; do not allow young plants to establish. Establish healthy, competitive plant communities that are desirable and adapted to your soil and climate; fertilize pastures to outcompete curlycup gumweed; eliminate overgrazing in infested areas.
• Mechanical — Dig, hoe, pull or till young plants, removing as much of the root as possible; tilling may need to be repeated; plants that are mowed or broken off will regrow. Mature plants are difficult to pull, so it is best to implement this control when the plants are young.
• Chemical — Broadleaf herbicides such as 2,4-D or dicamba are labeled for curlycup gumweed in landscapes, rangelands, pastures and roadsides. Picloram, a restricted use herbicide, and mesulfuron may be used in pasture, rangeland and noncrop sites. These herbicides provide excellent control when applied in the fall or early spring. Late season treatments on flowering plants do not provide satisfactory results. 2,4-D and dicamba are volatile during hot temperatures, so summer applications increase the risk of herbicide drift to nontarget plants. Always read and follow herbicide label directions.

Hairy nightshade

Hairy Nightshade (Solanum physalifolium) is an annual weed found in a variety of habitats including cultivated fields, roadways, overgrazed rangeland, flowerbeds and vegetable gardens. The most common annual nightshade, hairy nightshade is considered poisonous. The toxicity varies based on the environment, plant part, degree of maturity and genetic makeup of the individual plant. The greatest toxicity is present in the seedlings, growing tips and immature berries. Commonly found in potato fields, hairy nightshade is not affected by the desiccant sprays used to kill potato vines and benefit by not having to compete for sun, water and nutrients once the vines are killed. Hairy nightshade is a host for many plant pests such as Colorado potato beetle and the late blight fungus. They may cause insect, disease and nematode problems to increase in potatoes and other rotation crops.

Hairy nightshade has slender stems that are often coated in sticky, spreading hairs and typically reach 1 to 2 feet, but sometimes 3 feet in height. Leaves are egg to lance shaped and between 0.75 and 3 inches in length. They have fine, spreading hairs on the undersides of large veins and sometimes on leaf surfaces. Leaf margins are lightly toothed or untoothed. The flowers are star-shaped with five fused petals and a yellow cone of yellow anthers. Flowers are clustered in groups of four or five. Berries are yellow green to olive when immature and purple-green when mature. They are 0.2-0.4 inches wide, and each contains 10 to 36 seeds.

Integrated pest management (IPM) options:

• Cultural — Crop rotation is necessary in controlling hairy nightshade. Moving out of potatoes allows the use of herbicides that are not possible when growing potatoes. The focus of cultural control should be on reducing the seed bank in the soil.
• Mechanical — Mowing, tillage and hand pulling over several years helps irradicate hairy nightshade if done before seed production.
• Biological — There are no known biological control agents for hairy nightshade.
• Chemical — Several chemical herbicides are available to control hairy nightshade. A complete list of approved chemicals is available on www.pnwhandbooks.org. Type “hairy nightshade” in the search for recommendations. Always read and follow herbicide label directions.

Lambsquarter

Lambsquarters (Chenopodium album) is an aggressive annual weed found in landscapes, lawns, cropland and open space. Producing an average of 72,500 seeds per plant, one out of every three seeds will live in the soil for up to 20 years. It is also very competitive, in corn a loss of 13% has been documented when you find one plant per one and a half foot of row. With sugar beets, a loss of 48% yield has been documented when you find one plant per foot of row. Seed dormancy tends to increase the deeper in the soil it is. Seeds will germinate in the top 1 inch of the soil and tend to break dormancy when exposed to light, high soil nitrates and fluctuating temperatures. They do well in compacted soils. Lambsquarters is a host of a variety of viral diseases that can infect barley and potatoes.

Lambsquarters grows upright and up to 6 feet tall. They have triangular shaped lower leaves with margins that vary from coarsely toothed to shallow lobed. The upper leaves are narrow and linear shaped. Leaves alternate along the stem and have a dull green to grey appearance and may be tinged with purple. Both leaves and stems are hairless. Flowers are small and greenish, occurring in dense clusters on flower spikes at the ends of stems and branches.

Integrated pest management (IPM) options:

• Cultural — The best control for lambsquarters is to maintain healthy, competitive desirable vegetation.
• Mechanical — Dig, hoe or pull young plants. Plants do not usually survive mowing or clipping.
• Biological — There are no known biological control agents for lambsquarters.
• Chemical — Several chemical herbicides are available to control lambsquarters. The best time to apply herbicides is when the plant is young. Both pre-emergent and post-emergent herbicides are available. A complete list of approved chemicals is available on www.pnwhandbooks.org. Type “lambsquarters” in the search for recommendations. Always read and follow herbicide label directions.

Redroot pigweed

Redroot pigweed (Amaranthus retroflexus) is a summer annual found in agricultural fields, home gardens and open spaces. Redroot pigweed is of special importance because of its ability to produce 13,000 to 35,000 seeds per plant that stay viable in the soil for up to 10 years or more. Redroot pigweed is a common weed found throughout Idaho and has been proven to be difficult to control. Its resilience is well documented. In Idaho, adapted strains are resistant to the commonly used herbicide metribuzin. It was named pigweed because pigs liked the taste of it, but at the right stage of life (just before flowering), redroot pigweed can accumulate enough nitrates to be poisonous to livestock.

Redroot pigweed is an upright plant growing between 1 and 6 feet tall with thick stems and a taproot. Its lower stems are often reddish or red striped. Its leaves are oval with a tapering point. Densely clustered green flowers with a pinkish tint appear on a spike at the tip of branches. It germinates in the spring and matures in the summer. Found naturally along riverbanks, redroot pigweed does well in wet soils.

Integrated pest management (IPM) options:

• Cultural — Crop rotations are important in controlling redroot pigweed. Small grains suppress pigweed in rotation. Planting early helps reduce infestations as the crop has a head start and can out compete the pigweed for nutrients, sun and water.
• Mechanical — Hand pulling redroot pigweed early is easy and an effective way to control the population, however, if the plant is mature, it can regrow if it has been cut down. When tillage is used, it is best to till one inch or less, to prevent bringing more seeds to the surface.
• Biological — Redroot pigweed seed is a preferred food source of many seed predators, including the northern field cricket and some species of ground (carabid) beetles. One female northern field cricket can eat more than 200 pigweed seeds a day. Mice and other mammals also eat pigweed seeds.
• Chemical — Post-emergent chemicals are the best bet for controlling redroot pigweed. For more information on different chemicals to use refer to www.pnwhandbooks.org then search “redroot pigweed” in the search. Always read and follow herbicide label directions.

Annual sowthistle

Annual sowthistle (Sonchus oleraceus) is an annual weed found in agricultural fields and home gardens. Annual sowthistle is a well-known host for pests and diseases of crops, in addition, it utilizes valuable water resources, reducing yields for desirable crops. Annual sowthistle can produce up to 8,000 seeds per plant and seeds are dispersed easily by the wind.

Annual sowthistle is a member of the Asteraceae (sunflower) family. It can grow 1 to 4 feet tall and produces an elongated Hershey Kiss shaped yellow flower that opens like a dandelion. Its sessile leaves are deeply lobed with prickly margins. The leaves wrap around the stem and point straight back, as opposed to Prickly sowthistle whose leaves wrap around and down the stem. It has a hollow, spineless stem and when broken open, exudes a white milky substance.

Integrated pest management (IPM) options:

• Cultural — Annual sowthistle’s seeds do not remain viable when on the surface of the ground for more than a year but can remain viable if incorporated into the soil. Shallow tillage, of less than 3 inches will keep seeds from building up in the seed bank. In addition, annual sowthistle germinates well in wet soils, limiting the amount of standing water or saturated soils will reduce germination rates.
• Mechanical — Hand pulling annual sowthistle is easy because of its shallow root system. Shallow tilling or hoeing of annual sowthistle will pull up its roots. Remove the entire root, as re-rooting can occur.
• Biological — Grazing by sheep and cattle suppresses the growth of annual sowthistle making it more susceptible to other forms of control.
• Chemical — Preemergent and post emergent chemicals are both available for use with annual sowthistle. Post-emergency spot spraying has been found to provide excellent control. For more information on different chemicals to use refer to www.pnwhandbooks.org and search “annual sowthistle” in the search. Always read and follow herbicide label directions.

Perennial sowthistle

Perennial sowthistle is a succulent perennial plant considered to be noxious in several states including Idaho. Commonly found in cultivated fields, roadsides, lawns and any disturbed sites, perennial sowthistle outcompetes desirable plants for nutrients, light and space, reducing biodiversity. In addition, perennial sowthistle significantly reduces crop yields, especially in cereal grain crops. Densities of 17 and 32 shoots per yard reduce wheat yields by 15 and 45%, respectively. Perennial sowthistle prefers alkaline to neutral soils and grows best in moist, not dry soils. Perennial sowthistle reproduces through seed and vegetatively through rhizomes. It can produce 4,000 to 13,000 seeds per year, however, less than 40% of the seeds are viable. Seeds can last up to six years in the soil, but most seeds germinate in the first year.

Perennial sowthistle grows up to 6 feet tall, branching in the upper portion. Leaves are deeply toothed to nearly entire with a clasping base and prickly margins. Upper leaves are fewer and much smaller than basal leaves. The flowers are bright yellow to orange-yellow and 1 to 2 inches in diameter. Flower stalks and bulbous bracts are covered with gland-tipped hairs. All parts of the plant exude a milky white juice when broken. The root system grows horizontally and can also reach 10 feet deep.

Integrated pest management (IPM) options:

• Cultural — Crop rotations have been found to be successful in controlling perennial sowthistle in cultivated situations.
• Mechanical — Tillage at the seven to nine leaf rosette stage reduces the reproductive capacity of the root system. The burial of plants 12 inches deep reduced new shoot emergence by 90%. Rotary hoeing and flaming are not effective forms of mechanical control.
• Chemical — Herbicides are available to control perennial sowthistle. The best time to apply herbicides is when the plant is young and in the rosette stage, once the plant bolts, it becomes much more difficult to control with herbicides. A complete list of approved chemicals is available on www.pnwhandbooks.org. Type “perennial sowthistle” in the search for recommendations. Always read and follow herbicide label directions.

Plumeless thistle

Plumeless Thistle (Carduus acanthoides) is a winter annual or biennial that forms dense monocultures, outcompeting other plants for light, nutrients and moisture. Listed as a noxious weed in Idaho, property owners have an obligation to control plumeless thistle on their property. Both wildlife and livestock avoid grazing plumeless thistle, thus allowing it to spread in pastures and rangelands with no natural enemies. Plumeless thistle grows best on overgrazed pastures, rangelands and roadsides or rights-of-way. Growing up to 5 feet tall, plumeless thistle can hybridize with musk thistle causing additional problems.

Plumeless thistle stems are branched and covered with spiney leaves. Rosettes have deeply lobed, narrow and spiny leaves. Rosettes form in late summer to early fall and then bolt in the spring. Bolted stems branch with alternate spiney-lobed leaves that are narrower and wavier than musk thistle and extend onto and around the stem. Leaves are dark green with a lighter green mid-rib. They have hairlike structures on the underside of the leaves. Flowers are small, purple to pink, singular or in clusters, and have needle-like bracts. They bloom mid-June to August and can be bulbous in shape.

Integrated pest management (IPM) options:

• Cultural — Promote grass growth with fertilization, irrigation and reseeding if necessary.
• Mechanical — Prior to flowering, small infestations can be controlled by hand pulling or digging. Make sure to remove the entire root system. Mowing and cutting plants can be effective if done prior to seed production and repeated often.
• Chemical — Herbicides are available to control plumeless thistle. The best time to apply herbicides is when the plant is young and in the rosette stage, once the plant bolts, it becomes much more difficult to control with herbicides. A complete list of approved chemicals is available on www.pnwhandbooks.org. Type “plumeless thistle” in the search for recommendations. Always read and follow herbicide label directions.

Prostrate knotweed

Prostrate knotweed (Polygonum aviculare) is a low-growing summer annual commonly found in lawns, high-traffic areas, waste areas, and along driveways and sidewalks. Prostrate knotweed provides a food source to birds and mammals but is also a known host for many fungi, viruses and nematodes. 

Prostrate knotweed quickly covers bare soil preventing native plant species from becoming established. Toxins from its roots and leaves alter the composition of the soil, making it unsuitable for native species.

Prostrate knotweed is a low-growing plant with wiry stems that radiate out from a central point and grow up to 2 feet in length. Small leaves and tiny white or pink flowers emerge from nodes that are covered in a papery sheath. Flowers have five petals and green centers.

Integrated pest management (IPM) options:

• Cultural — Prostrate knotweed thrives in compacted soils. Avoid compacted soils by aeration. Apply a 3-inch-deep layer of mulch to planting beds to reduce seed germination.
• Mechanical — Hand pulling plants before they go to seed. Rototilling or hoeing will also effectively eliminate plants.
• Chemical — A complete list of approved chemicals is available on www.pnwhandbooks.org. Type “prostrate knotweed” in the search, then select “Annual Broadleaf Weeds in New Stands” for recommendations. Always read and follow herbicide label directions.

Rush skeletonweed

Rush Skeletonweed (Chondrilla juncea) is an aggressive invader that is extremely damaging to production agriculture. Affected areas include cultivated fields, roadsides, rangelands and pastures. Once introduced into a new area, it is extremely difficult to eradicate. It produces dense stands, diminishing plant and wildlife habitat. The plants' wiry stems and the latex it produces can clog and break agricultural harvesting equipment. It thrives in rocky or sandy soils and can flourish in both wet and dry environments.

Rush skeletonweed is a biennial or perennial that can reach up to 4 feet tall. In its first year it grows as a rosette like a common dandelion and chicory. The rosette is 2 to 5 inches in diameter with hairless, reddish-brown leaves and purplish incised margins and deeply pinnate lobes pointing backward toward the leaf base. In the spring of the second year it bolts, growing its green or yellowish, wiry, erect flowering stems. There are a few narrow, linear leaves on the stem. Stems become increasingly tough as they mature. It can be distinguished by its brown, downward-pointing hairs near the base of the flowering stem. Rush skeletonweed produces up to 1,500 small, bright yellow flowers per plant. Flowers are found both terminally (on the ends of branches) and axially (in-between branches). Flowers are attached directly to the branch. It begins flowering in May and continues until the first hard frost producing over 20,000 seeds. Tap roots are 3 to 7 feet deep. Rush skeletonweed can reproduce vegetatively as well through adventitious buds near the taproot and on lateral roots that can become rhizome-like and have root buds.

Integrated pest management (IPM) options:

• Cultural — Prevent the establishment of new infestations by minimizing disturbance and seed dispersal. Healthy plant communities are less susceptible to infestations of Rush skeletonweed.
• Mechanical — Diligent hand-pulling or digging can be effective for small infestations. Any root left behind can begin growing a new plant, so make sure to remove all root parts and check the site often, removing any new growth. Be careful not to scatter any of the seeds if the plant is flowering. Mowing and cultivation are not effective as root fragments regrow.
• Biological — A root moth, a gall midge, a mite and a rust have been introduced in the U.S. as biological control agents. Biological control agents will not eradicate Rush skeletonweed but will reduce seed production.
• Chemical — Herbicides are the most effective control for Rush skeletonweed. Several chemical herbicides are available to control Rush skeletonweed. The best time to apply herbicides is when the plant is young and in the rosette stage. A complete list of approved chemicals is available on www.pnwhandbooks.org. Type “rush skeletonweed” in the search for recommendations. Always read and follow herbicide label directions.

Spotted spurge

Spotted spurge (Euphorbia maculata) is an annual weed in the spurge family. It grows close to the ground and forms a dense mat of vegetation invading gardens, cultivated agricultural areas, cracks in sidewalks and thin lawns. It grows well in poor, compacted soil and loves full sun. Spotted spurge can grow up to 2 feet in diameter, produces seeds within five weeks of germination and can produce two to three generations in a growing season. Spotted spurge is poisonous to sheep and has been shown to kill animals that consumed as little as 0.62% of their body weight within a few hours.

Spotted spurge has small elliptical-shaped, bright green leaves that have an irregular-purple spot in the center of each blade. The leaves are arranged opposite each other on the pink stems that radiate from a center underground taproot. A thick milky sap will exude from any point of the plant that is broken off.

Integrated pest management (IPM) options:

• Cultural — Applying a mulch layer 3 inches thick on planting beds will reduce seed germination. Soil sterilization by covering the soil with sheets of clear plastic for four to six weeks during the summer can effectively reduce the number of seeds in an area where temperatures are above 90°F.
• Mechanical — Hand-pull any plants before seed is set. Remove as much of the root as possible. Make sure to carefully dispose of plant material to ensure seeds are not left behind. Mowing is an ineffective way to control spotted spurge because of its low growth habit. Hand pulling or tillage is the preferred method of mechanical weed control, just be aware that any plant material can vegetatively reproduce and sprout new plants.
• Chemical — Preemergent chemicals can help prevent outbreaks of spotted spurge if you apply them in late winter before weed seeds germinate. Postemergent herbicides are also available. 2,4-D, Dicamba, Fluroxypryr, Flumioxazin and MCPP have all proven to provide good control of spotted spurge, for best control use a combination of one or more of the herbicides listed. As with all chemical pest control, always read and follow herbicide label directions.

White clover

White clover (Trifolium repens) is a perennial cool season legume. White clover is often considered a nuisance in both lawns and pastures. In nitrogen-deficient lawns, it creates unsightly dark green patches that contrast sharply with the surrounding grass. Its creeping growth habit allows it to spread aggressively, often crowding out desirable turf species. In pastures, white clover is generally less desirable than other, more productive legume species. It is commonly found in grazed areas because it tolerates and survives overgrazing better than many other forage plants.

The plant’s leaves are trifoliate, measuring approximately 1/4 to 3/4 inch long. Occasionally a leaf may be found that looks like a “four-leaf clover.” Leaflet edges are slightly serrated and have a small notch at the tip. Leaves have a light green/grey “V” or “U” shaped pattern running through the middle of the leaf. White clover is reproduced by stolons and by seed. As stolons (above ground reproductive stems) elongate along the soil surface, the plant sends out roots at nodes, allowing the plant to spread forming large patches. Seed is produced in globe shaped clusters of small, white flowers that are typically pinkish at the base. Each flower cluster may contain 20-40 individual flowers, each containing a single seed.

Integrated pest management (IPM) options:

• Cultural — Crop rotations are important in controlling redroot pigweed. Small grains suppress pigweed in rotation. Planting early helps reduce infestations as the crop has a head start and can out compete the pigweed for nutrients, sun and water.
• Mechanical — Hand pulling redroot pigweed early is easy and an effective way to control the population, however, if the plant is mature, it can regrow if it has been cut down. When tillage is used, it is best to till one inch or less, to prevent bringing more seeds to the surface.
• Chemical — Can be controlled chemically with several broadleaf herbicides in the spring or fall. A complete list of approved chemicals is available on www.pnwhandbooks.org under the "weed" dropdown menu select "horticultural weed management" and then select "turfgrass. "Always read and follow herbicide label directions.
• For more information, read Integrated pest manage of white clover, BUL 1125.

Wild buckwheat

Wild buckwheat (Polygonum Convolvus) is a summer annual weed found commonly in small grains. In wheat, wild buckwheat can reduce yields by up to 66%. In addition, wild buckwheat climbs up the shoots of plants causing difficulties harvesting and lowering the quality of the crop. In open areas, wild buckwheat can quickly become a problem as it shades out other, more desirable plants.

Wild buckwheat has a shallow root system and can grow up to 40 inches long. Many times, it is mistaken for field bindweed, but its arrow shaped leaves are more pointed. It has long, slender, creeping stems that trail along the ground until they encounter any object, which they immediately begin climbing. Their leaves are alternately arranged on the stem, are heart-or arrowhead-shaped, pointed at the tip and have widely separated lobes at the base. The flowers have no petals but have five green or pinkish sepals. Flowers occur in short-stemmed clusters in the axils of the leaves or at the end of the stem.

Integrated pest management (IPM) options:

• Cultural — practices such as crop rotation and delayed seeding can reduce incidence of wild buckwheat but will not provide effective control.
• Mechanical — Mowing, post-seeding harrowing, post-harvest cultivating can also reduce incidence but will not provide effective control. Tilling can encourage germination, performing another tillage operation after seedlings germinate will kill seedlings. On small infestations hand pulling can be successful if done faithfully. Ensure you pull the whole root to avoid regrowth.
• Chemical — Spraying early for wild buckwheat is the most effective method of control. There are several effective herbicides available. To avoid resistance to herbicides, use a combination of chemical groups and rotate between differing modes of action. For more information visit www.pnwhandbooks.org then search “wild buckwheat” in the weed section. Always read and follow herbicide label directions.