Washington is home to a variety of weeds that are definitely a threat to the environment. Most of these weeds grow and spread rapidly, so they should be controlled and removed so that the native vegetation of an area thrives. In this article, we’ve compiled a list of the most common weeds in Washington state, along with their characteristics and effective preventive and control measures.
Table of Contents
Washington Weeds
| Name of Weed | Family |
|---|---|
| Annual Bugloss | Boraginaceae |
| Bighead knapweed | Asteraceae |
| Black henbane | Solanaceae |
| Clary sage | Lamiaceae |
| Dyer’s woad | Brassicaceae |
| Garlic mustard | Brassicaceae |
| Giant hogweed | Apiaceae |
| Japanese Knotweed | Polygonaceae |
| Johnsongrass | Poaceae |
| Kudzu | Fabaceae |
| Purple loosestrife | Lythraceae |
| Shiny Geranium | Geraniaceae |
| Texas Blueweed | Asteraceae |
| Turkish thistle | Asteraceae |
Purple loosestrife
This wood species is an erect plant that grows up to 3-10 feet tall. Its square and sometimes woody stem bears bright purple flowers in long spikes. The purple loosestrife forms a thick stand with dense mats of roots and spreads over large areas. By crowding out the natural vegetation of a place, the weed reduces biodiversity. Perhaps a single plant bears millions of tiny seeds, which are normally dispersed by wildlife, water, and wind.
Type
Perennial
Family
Lythraceae
Control
To treat a small area infested by purple loosestrife, you can consider cutting, hand-pulling, and digging. Spot treatment using approved herbicides is also recommended.
Bighead knapweed
Bighead knapweed is a large herb with yellow, globe-shaped flower heads growing singly on the tops of stems. It grows up to 5.6 feet tall and is perhaps the tallest knapweed in Washington. The tap-rooted perennial grows a rosette in the first year and flowering stems in the following years. This weed species grows well in open grassy areas, including fields and pastures.
Type
perennial
Family
Asteraceae
Control
Small plants can be hand-pulled, whereas it is not possible to pull out large infestations. The flowering stem should be dug out along with the tap root. Otherwise, a new stem will develop from the woody crown, giving rise to another flower head. Repeated mowing is recommended to reduce seed production and eventually weaken the root reserves.
Texas Blueweed
Texas blueweed can form extensive patches with its creeping roots or rhizomes. The leaves and stems are often grey-green or blue-green in colour and grow 40 to 70 cm tall. Each flowerhead has two types of flowers, an outer ray of yellow flowers and an inner ring of reddish to purple-brown flowers. The stems are hairless and often have a whitish, waxy coating. The weed species grow in drainage areas, cultivated fields, dry lakes, and along roadsides.
Type
Perennial
Family
Asteraceae
Control
The best way to control Texas blueweed is to diminish the weed’s energy reserves by pulling and digging it constantly coupled with the use of herbicides and planting of native perennial grass varieties.
Turkish thistle
Turkish thistle is commonly found in canyon grasslands and on rocky slopes. It is an annual thistle that can grow up to 4 feet in height. Its stems are winged, and the purplish flower heads are non-spherical, compressed, and single, or in loose clusters. Turkish thistle spreads readily and infests areas, outcompeting the native vegetation of a place.
Type
Annual
Family
Asteraceae
Control
Hand-pulling and digging are quite effective for small infestations. However, it should be done before seed development. Establishing healthy stands of desired and beneficial vegetation that is capable of competing with weed seedlings is also a good option to control the Turkish thistle invasion. Remember that effective use of grazing management practices encourages the growth of beneficial plants.
Annual Bugloss
This weed species is a small, leafy herb with sky-blue, funnel-shaped flowers. It grows about 1 to 3 feet tall. The leaves are slender and lance-shaped, covered in bristly hairs, crinkled on the edges, and grow alternately up the stem. They get smaller as they go up the stem.
Annual bugloss has become a problem in pastures, fields, and croplands, competing with native crops and plants. Its preferred habitat is pastures, cultivated fields, roadsides, and disturbed areas. The weed prefers sandy, alkaline soil. As its seeds remain dormant for several years, it has become a persistent weed problem.
Type
Annual
Family
Boraginaceae
Control
Small plants can be easily pulled or dug up. However, care should be taken when handling these plants, because the bristly hairs can hurt and may cause serious skin irritation. Spot spraying with an approved herbicide containing glyphosate or 2,4-D plus dicamba is quite effective in controlling the annual bugloss, but it could be difficult due to the bristly hairs on the leaves. The most effective treatment is spot application during the bud stage, before flowering.
Garlic mustard
This weed species is a biennial flowering plant that grows up to 3 feet tall. In early spring, its roots and the new leaves smell like garlic. The kidney-shaped leaves become smaller toward the top of the stem. The small, white flowers have four petals, four sepals, and six stamens.
Garlic mustard is not easy to control once its roots are established. The weed is sun and shade tolerant and outcompetes native vegetation. Moreover, it shows a high seed production rate.
Type
Biennial
Family
Brassicaceae
Control
Hand-pulling proves to be an effective method because even mature plants can be easily pulled out. However, care should be taken to dig out all of the roots. Applying herbicides (spot spraying) in spring and autumn is also known to be effective in controlling garlic mustard invasion.
Clary sage
Clary sage grows extensively in eastern and western Washington. It is a much-branched herb that grows up to 6 feet tall. The plant exhibits a strong odor due to essential oil. The whole plant is hairy. The flower petals fuse to form two ‘lips’ – a purple upper lip and a whitish lower lip.
The weed species thrives on slopes, in well-drained soil, along roadsides, in less well-drained meadow sites, and in other disturbed places.
Type
Biennial or short-lived perennial
Family
Lamiaceae
Control
Digging or hand-pulling can be effective, especially in the case of small infestations. Care should be taken to dispose of the plants properly and ensure all flowerheads are appropriately discarded to prevent new infestations.
Dyer’s woad
Dyer’s woad is an erect herb that normally grows 1-4 feet tall with a taproot that is 3-5 feet long. Its tiny yellow flowers are cross-shaped and appear in clusters on branch tips. The leaves are bluish-green, stalked, and covered with fine hairs.
Dyer’s woad grows in rocky soils, along gravel pits, roadsides, and levees. From there, it quickly spreads by seed to forests, well-vegetated pastures, croplands, forests, waterways, and hayfields.
Type
Biennial or short-lived perennial
Family
Brassicaceae
Control
Hand-pulling before the seeds set can be effective in controlling infestations. It is important to remove the crown so that the plant doesn’t sprout again. Follow-up hand-pulling programs for several years are necessary to prevent re-infestation. Chemical control involves using herbicides containing the selective, translocated active ingredient 2, 4-D.
Kudzu
Kudzu is an extremely aggressive invasive plant that is difficult to control once established. It is so aggressive that it covers and smothers the other plants in its way, by being a solid single weed that eliminates the native plants.
The weed species is a trailing vine, which may sometimes grow up to one foot per day. It has purple or purplish-red flowers in clusters. Flowers have a strong fragrance that is described as grape-like. The weed thrives in well-drained, eroded, or degraded land or in disturbed, deep loam soils in full sun.
Type
perennial
Family
Fabaceae
Control
It can take up to ten years to control well-established kudzu stands. The only appropriate control measure is persistent treatment with patches. While there are chemical methods to control this weed, professional assistance is a must. Kudzu is a food source for livestock. Studies suggest that goats may often provide a practical alternative to chemical control.
Johnsongrass
The wood species is a common weed that can be found in orchards, vineyards, agricultural crop fields, ditches, and along roadsides and fence rows. It is a tall, upright grass that grows 3-10 feet tall with an extensive system of rhizomes. Its stems terminate in a bright red inflorescence.
Johnsongrass grows in different soil types, riparian areas, and disturbed areas like roadsides and old fields. The plant sometimes produces hydrocyanic acid, which can be poisonous to grazing livestock.
Type
Perennial Grass
Family
Poaceae
Control
Since Johnsongrass reproduces by seed and rhizomes, if cultural or mechanical methods are used as control methods, care should be taken to prevent the re-sprout of rhizomes. Herbicides will control the upper part of the plant, but dormant buds on the rhizome may still sprout.
Shiny Geranium
This weed species is a low-growing plant that grows 10 to 12 inches high. The stems are tinged red, giving rise to deeply lobed, shiny green leaves. The leaves turn vivid red in autumn and summer. Its small flowers are pink-colored and grow in pairs.
Shiny geranium can spread rapidly and is not easy to control. The weed prefers shade but can survive in partial to full sun. It tends to form monocultures and replace the native plants of the area.
Type
Annual
Family
Geraniaceae
Control
Hand-pulling is a practical control option for small weed populations. Herbicides should be used only for large populations. Consider spot spraying with an approved herbicide containing glyphosate as its active ingredient. It effectively gets rid of the shiny geranium.
Giant hogweed
As the name implies, the giant hogweed grows up to 14 feet or more. It has white, umbrella-shaped flowers in clusters that are up to 2.5 feet wide! This weed species is a well-known public health hazard. It contains a clear, watery sap that is toxic and can make the skin extremely sensitive to the sun. If the sap gets into the eye, it can result in temporary or even permanent blindness.
The giant hogweed grows well in rich, damp soil and can be found along ditches, roadsides, vacant farmlands, streams, and rivers. It’s in urban areas that 70% of all giant hogweed sites are found.
Type
Perennial
Family
Apiaceae
Control
Performing any manual control to eliminate the giant hogweed is risky. The sap is present in all parts of the plant. Small plants can be dug out or pulled. Following removal or control, mulch is recommended to stop seed germination. Spot spraying with a glyphosate-containing herbicide applied during the budding stage or the active growth stage is highly effective in controlling this giant weed.
Japanese Knotweed
Japanese knotweed can be easily identified by its white flowers emerging from pink buds and reddish-purple shoots. The plant grows through piping, cables, buildings, and foundations, eventually causing extensive damage to the property. The weed species blooms best in late summer and early autumn. Well, the plant is too difficult to control once established.
Type
Perennial
Family
Polygonaceae
Control
Professional guidance is required to control this weed. However, you can use approved herbicides to prevent it from spreading. It normally takes up to three years to completely treat Japanese knotweed.
Black henbane
Black henbane can be identified by its yellowish flowers with purple centers and foliage that is foul-smelling and covered with sticky hairs. It is a highly invasive weed that invades meadows, pastures, and roadsides. It forms dense stands, replacing other native vegetation and decreasing forage production and biodiversity.
Type
Annual/biennial
Family
Solanaceae
Control
Digging and hand-pulling before seed germination is quite effective for controlling black henbane. As the plant is toxic, personal protective gear should be worn at times of mechanical removal. Herbicide treatment also proves to be beneficial. Several herbicides containing the active ingredients dicamba, chlorsulfuron, picloram, glyphosate, and metsulfuron can control black henbane infestations.
