War on weeds

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.

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.

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.

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.