Don't Get Swept Away by Scotch Broom: Use the Best Management Options
Photo: Eric Coombs, Oregon Dept. of Agriculture
By Wendy Descamp, Washington State Noxious Weed Control Board
Originally introduced as an ornamental plant, Scotch broom, Cytisus scoparius, is now widespread in western Washington and Oregon and is a noxious weed in both states along with Idaho and Montana. This invasive plant is a nitrogen-fixing shrub in the legume family (Fabaceae) that is native to much of Europe and the Canary Islands.
Mature plants range in size from three to 10 feet in height. Young stems are dark green with ridges, five-angled or star-shaped in cross-section, with upright growth that can resemble a broom. Older stems turn a tan-brown color and become woody and smooth. Leaves toward stem tips are simple and undivided, while lower leaves can be summer if the plant is under stress. Plants begin to flower when they are three years old, but younger plants may flower under the right light and moisture conditions. Flowers are pea-like and yellow, sometimes having red coloring, and bloom in the spring. They occur singly or in pairs in leaf mills and are around one inch in size. Flowers develop seedpods that are up to two inches long, brown-to-black divided into three leaflets. Leaves are deciduous and may drop early in the summer if the plant is under stress.
Plants begin to flower when they are three years old, but younger plants may flower under the right light and moisture conditions. Flowers are pea-like and yellow, sometimes having red coloring, and bloom in the spring. They occur singly or in pairs in leaf mills and are around one inch in size. Flowers develop seedpods that are up to two inches long, brown-to-black color when mature and have hairy margins. The pods contain three to 12 seeds and, when ripe, the pod's two halves audibly split apart. wrapping in alternate directions and catapulting the seeds a short distance. Scotch broom reproduces by seed with mature plants able to produce more
than 8,000 seeds per year. Seeds can germinate over a prolonged period and in a broad temperature range. They have a hard coat and can be viable for many years in the soil, with estimates ranging from five to 30 years or even longer under ideal conditions. This large seedbank of long-lived seeds makes Scotch broom a difficult species to control once it is established.
Though Scotch broom is infamous west of the Cascade Mountains for lining the highways with bright yellow flowers in the spring. it can also be found growing and spreading east of the mountains. Scotch broom grows along forest edges. prairies and grasslands, oak woodlands, coastal areas and disturbed sites. Including cleared forestland, burned areas and roadsides. They are more common in open areas, but plants can also survive in low light conditions, such as in a forest understory.
Scotch broom forms aggressive stands that shade out native plants and reduce forage for livestock and deer. It is particularly problematic in our native grassland and open forest habitats. Although seldom grazed, Scotch broom is toxic, especially to horses. The woody biomass increases the risk and intensity of fires. Its seeds can sprout for years after removal and cut stumps can regrow. Scotch broom can change the soil chemistry, making it less suitable for native plants even years after it is removed. Scotch broom also impacts the timber industry, as it rapidly colonizes newly cleared timberland and aggressively competes with tree seedlings.
Control methods
Taking steps to prevent Scotch broom invasions is an important part of a management plan. Monitor areas that may be vulnerable to invasion and control any seedlings and young plants. Areas that experience disturbance, such as from cutting trees or clearing brush, create disturbed conditions that Scotch broom loves to invade given a chance. Do not move soil, gravel or other materials with Scotch broom seeds to new locations.
Because of Scotch broom's high rate of seed production and the longevity of its seedbank, a long-term integrated pest management plan that includes monitoring will be needed for invaded sites. Keep in mind when working with Scotch broom and other invasive species that it is important to control seedlings and young plants before they produce seeds if possible. Controlling Scotch broom plants in the fall and winter months could be advantageous, as it would prevent nonnative grasses and forbs from receiving possible nitrogen inputs from dying plants.
When isolated plants or a small infestation of Scotch broom are present, manual or mechanical control can be effective, followed by seeding or planting desirable species and continuing to monitor and control Scotch broom seedlings as needed. For large infestations, a combination of methods will be needed, targeting outlier plants with mechanical methods and main infestations with biological control, fire, mowing, herbicide use or a combination of these methods. Soil disturbance will result in seed germination from the seedbank, so minimize on-site disturbance. Planting or seeding native plants in areas under restoration is important to provide competition and diversity, as invaded sites can be native seed-limited. Also, establish
trees and shrubs to create shade where Scotch broom plants do not prefer growing.
Manual/mechanical
Small- to medium-sized Scotch broom plants can be hand-pulled, making sure to remove the roots. Use a tool like a Weed Wrench, Extractigator or Uprooter to lever plants, along with their roots, out of the ground. When possible, time the removal when soils are moist, as roots will be easier to remove. Handpulling small plants will create less soil disturbance than removing the root system of large plants.
Scotch broom plants can resprout after cutting alone. If only cutting or mowing is possible, maximize damage and possible death to the plants by cutting when they are drought stressed-typically late summer to early fall-cutting plants back to the ground to minimize soil disturbance. Avoid cutting plants during the rainy season when the rate of resprouting will be the highest. This drought-stress cutting method will need to be repeated, ideally before plants produce seed. Mowing or the cutting back of plants may also need to be done multiple times throughout the growing season if it is not timed for when plants are drought-stressed. Even plants that are just a few months old may have developed large enough roots to recover from one mowing, so repeat mowing or cutting will be needed.
Cultural
Controlled burns can be an excellent method to manage large Scotch broom infestations. Check for state and local restrictions, burn bans or permit requirements before burning. Though plants can resprout after a fire, most fires are hot enough to destroy Scotch broom's cambium, thus killing the plant. One burn alone will not be enough to control the infestation, as new plants will grow. It will take several cycles of prescribed burning to reduce invasions or other control methods will be needed. In the South Puget Sound area, where fire is used to control Scotch broom, prairies are burned every two to three years, as waiting four years between burns will allow plants to go to seed.
To prevent fires from burning too hot and killing native plants, dense stands of Scotch broom may need to be mowed prior to burning. Landowners may not have the option of conducting controlled burns on their land, but mowing or cutting back the Scotch broom and then using a weed torch could be an effective option. Following this approach, resprouting Scotch broom plants or seedlings could be controlled with an herbicide treatment, and then the area could be replanted. After burning larger infestations, areas may also be heavily seeded with annual grasses or other fast-growing, non-invasive species. The fast-growing plants can effectively crowd out many Scotch broom sprouts. Pull the Scotch broom that does sprout by hand, or use a broadleaf herbicide that will control the Scotch broom but not desired grasses.
For smaller areas that will be landscaped, mulching, including sheet mulching, or covering the soil after control work, can help reduce or suppress seed germination. Replant the area to provide competition and create shade while continuing to monitor for and remove any seedlings.
Chemical
In general, use herbicides in combination with other control methods to reduce usage. Wherever possible, treat Scotch broom plants before they develop seeds. Herbicide application methods used for Scotch broom include foliar spray, basal bark treatment and cut stump treatment. If using a foliar spray, treat plants when pollinators are not present or are the least active to avoid harm. Cut stump herbicide treatments, where the stem is cut off near its base and the fresh cut is treated, can provide effective control. Basal bark treatment is where the herbicide is applied directly to the bark at the base of the plant. This method, like cut stump treatments, allows targeted herbicide treatments of Scotch broom, with little or no damage to surrounding plants. Herbicide rates for foliar sprays and contact treatments differ, so make sure to check the label for directions.
Depending on the treatment, it may take more than one herbicide application to kill Scotch broom plants. Examples of selective herbicides that can be used to control Scotch broom include those with the active ingredient triclopyr (e.g., Vastlan or Garlon 4 Ultra) or triclopyr combined with aminopyralid (e.g., Capstone). Herbicides with triclopyr are to be used as foliar sprays when plants are rapidly growing. Cut stump and basal bark treatments using these herbicides can occur whenever the ground is not frozen. These selective herbicides typically kill broadleaf plants but do not harm grasses, though there may be exceptions so make sure to check the label. Non-selective herbicides that contain the active ingredients glyphosate or imazapyr can be used to control Scotch broom but will also kill any other plants they come in contact with. Application timing varies depending on treatments. Glyphosate should be used as a foliar spray in late summer or early fall, or used in a cut stump treatment in late summer, early fall or during the dormant season.
Always read and follow the herbicide label directions carefully for proper dilution rate, surfactant information, application timing and proper sites. For more detailed information on timing and application, contact your county noxious weed board or program. state department of agriculture, extension service or conservation district. Also, check the Pacific Northwest Weed Management Handbook for additional herbicide information available online. (The label is the law!)
Biological
Biological control agents are available for Scotch broom and can be a valuable management tool for large infestations. Larvae of the Scotch broom seed weevil, Exapion fuscirostre, feed on seeds of Scotch broom in developing seedpods. The adults also feed on flowers and stem tips, though their damage is not significant. The Scotch broom bruchid, Bruchidius villosus, larvae feed on developing seeds and impact the plant's reproduction. While these biological control agents will not kill the Scotch broom plants, they will reduce the number of seeds produced. Use them in the center of large infestations while working to contain infestations and control other isolated plants and populations. Do not release biological control agents in populations that will soon be burned or mowed as they will not have an opportunity to establish on the plants and impact seed production. Check with the Oregon Department of Agriculture's Weed Biological Control Program and Washington State University's Integrated Weed Control Project for information about these biological control agents. Goats can also help control resprouts after the cutting or burning of Scotch broom. Plant parts are toxic to other livestock though, so only let goats do the grazing. Make sure to protect desired plants from goats as they will be grazed too.
After Treatment
There may be an increase of nitrogen-loving, non-native grasses and forbs that grow after Scotch broom removal due to the possible increase of available nitrogen in the soil. Planting and seeding non-invasive plants to outcompete weeds will help reduce the number of new invaders. Newly planted tree seedlings may need some help, as non-native grasses and other weeds may have an initial growth advantage: they are better adapted to the altered soil nutrients. Native species are adapted to nitrogen-poor soil, so they may not grow as quickly as non-native invaders. Make sure to watch for and manage new weeds as necessary.
Disposal
Dead or pulled Scotch broom plants may be left in place to decompose, but they could increase soil nitrogen. Plants can be piled to decompose or burn, though again make sure to check on local restrictions, burn bans or permit requirements before burning. Plants without seeds may also be chipped. Local transfer stations or landfills may also be an option for removed plants.
If you have any questions regarding Scotch broom control, contact your local experts such as a noxious weed board or program, state department of agriculture, extension service or conservation district.
Wendy Descamp is the Education Specialist for the Washington State Noxious Weed Control Board. She can be reached at 360-725-5764 or wdescamp@agr. wa.gov. Excerpted by permission from an article originally appearing in Northwest Woodlands, Winter 2018.
Originally introduced as an ornamental plant, Scotch broom, Cytisus scoparius, is now widespread in western Washington and Oregon and is a noxious weed in both states along with Idaho and Montana. This invasive plant is a nitrogen-fixing shrub in the legume family (Fabaceae) that is native to much of Europe and the Canary Islands.
Mature plants range in size from three to 10 feet in height. Young stems are dark green with ridges, five-angled or star-shaped in cross-section, with upright growth that can resemble a broom. Older stems turn a tan-brown color and become woody and smooth. Leaves toward stem tips are simple and undivided, while lower leaves can be summer if the plant is under stress. Plants begin to flower when they are three years old, but younger plants may flower under the right light and moisture conditions. Flowers are pea-like and yellow, sometimes having red coloring, and bloom in the spring. They occur singly or in pairs in leaf mills and are around one inch in size. Flowers develop seedpods that are up to two inches long, brown-to-black divided into three leaflets. Leaves are deciduous and may drop early in the summer if the plant is under stress.
Plants begin to flower when they are three years old, but younger plants may flower under the right light and moisture conditions. Flowers are pea-like and yellow, sometimes having red coloring, and bloom in the spring. They occur singly or in pairs in leaf mills and are around one inch in size. Flowers develop seedpods that are up to two inches long, brown-to-black color when mature and have hairy margins. The pods contain three to 12 seeds and, when ripe, the pod's two halves audibly split apart. wrapping in alternate directions and catapulting the seeds a short distance. Scotch broom reproduces by seed with mature plants able to produce more
than 8,000 seeds per year. Seeds can germinate over a prolonged period and in a broad temperature range. They have a hard coat and can be viable for many years in the soil, with estimates ranging from five to 30 years or even longer under ideal conditions. This large seedbank of long-lived seeds makes Scotch broom a difficult species to control once it is established.
Though Scotch broom is infamous west of the Cascade Mountains for lining the highways with bright yellow flowers in the spring. it can also be found growing and spreading east of the mountains. Scotch broom grows along forest edges. prairies and grasslands, oak woodlands, coastal areas and disturbed sites. Including cleared forestland, burned areas and roadsides. They are more common in open areas, but plants can also survive in low light conditions, such as in a forest understory.
Scotch broom forms aggressive stands that shade out native plants and reduce forage for livestock and deer. It is particularly problematic in our native grassland and open forest habitats. Although seldom grazed, Scotch broom is toxic, especially to horses. The woody biomass increases the risk and intensity of fires. Its seeds can sprout for years after removal and cut stumps can regrow. Scotch broom can change the soil chemistry, making it less suitable for native plants even years after it is removed. Scotch broom also impacts the timber industry, as it rapidly colonizes newly cleared timberland and aggressively competes with tree seedlings.
Control methods
Taking steps to prevent Scotch broom invasions is an important part of a management plan. Monitor areas that may be vulnerable to invasion and control any seedlings and young plants. Areas that experience disturbance, such as from cutting trees or clearing brush, create disturbed conditions that Scotch broom loves to invade given a chance. Do not move soil, gravel or other materials with Scotch broom seeds to new locations.
Because of Scotch broom's high rate of seed production and the longevity of its seedbank, a long-term integrated pest management plan that includes monitoring will be needed for invaded sites. Keep in mind when working with Scotch broom and other invasive species that it is important to control seedlings and young plants before they produce seeds if possible. Controlling Scotch broom plants in the fall and winter months could be advantageous, as it would prevent nonnative grasses and forbs from receiving possible nitrogen inputs from dying plants.
When isolated plants or a small infestation of Scotch broom are present, manual or mechanical control can be effective, followed by seeding or planting desirable species and continuing to monitor and control Scotch broom seedlings as needed. For large infestations, a combination of methods will be needed, targeting outlier plants with mechanical methods and main infestations with biological control, fire, mowing, herbicide use or a combination of these methods. Soil disturbance will result in seed germination from the seedbank, so minimize on-site disturbance. Planting or seeding native plants in areas under restoration is important to provide competition and diversity, as invaded sites can be native seed-limited. Also, establish
trees and shrubs to create shade where Scotch broom plants do not prefer growing.
Manual/mechanical
Small- to medium-sized Scotch broom plants can be hand-pulled, making sure to remove the roots. Use a tool like a Weed Wrench, Extractigator or Uprooter to lever plants, along with their roots, out of the ground. When possible, time the removal when soils are moist, as roots will be easier to remove. Handpulling small plants will create less soil disturbance than removing the root system of large plants.
Scotch broom plants can resprout after cutting alone. If only cutting or mowing is possible, maximize damage and possible death to the plants by cutting when they are drought stressed-typically late summer to early fall-cutting plants back to the ground to minimize soil disturbance. Avoid cutting plants during the rainy season when the rate of resprouting will be the highest. This drought-stress cutting method will need to be repeated, ideally before plants produce seed. Mowing or the cutting back of plants may also need to be done multiple times throughout the growing season if it is not timed for when plants are drought-stressed. Even plants that are just a few months old may have developed large enough roots to recover from one mowing, so repeat mowing or cutting will be needed.
Cultural
Controlled burns can be an excellent method to manage large Scotch broom infestations. Check for state and local restrictions, burn bans or permit requirements before burning. Though plants can resprout after a fire, most fires are hot enough to destroy Scotch broom's cambium, thus killing the plant. One burn alone will not be enough to control the infestation, as new plants will grow. It will take several cycles of prescribed burning to reduce invasions or other control methods will be needed. In the South Puget Sound area, where fire is used to control Scotch broom, prairies are burned every two to three years, as waiting four years between burns will allow plants to go to seed.
To prevent fires from burning too hot and killing native plants, dense stands of Scotch broom may need to be mowed prior to burning. Landowners may not have the option of conducting controlled burns on their land, but mowing or cutting back the Scotch broom and then using a weed torch could be an effective option. Following this approach, resprouting Scotch broom plants or seedlings could be controlled with an herbicide treatment, and then the area could be replanted. After burning larger infestations, areas may also be heavily seeded with annual grasses or other fast-growing, non-invasive species. The fast-growing plants can effectively crowd out many Scotch broom sprouts. Pull the Scotch broom that does sprout by hand, or use a broadleaf herbicide that will control the Scotch broom but not desired grasses.
For smaller areas that will be landscaped, mulching, including sheet mulching, or covering the soil after control work, can help reduce or suppress seed germination. Replant the area to provide competition and create shade while continuing to monitor for and remove any seedlings.
Chemical
In general, use herbicides in combination with other control methods to reduce usage. Wherever possible, treat Scotch broom plants before they develop seeds. Herbicide application methods used for Scotch broom include foliar spray, basal bark treatment and cut stump treatment. If using a foliar spray, treat plants when pollinators are not present or are the least active to avoid harm. Cut stump herbicide treatments, where the stem is cut off near its base and the fresh cut is treated, can provide effective control. Basal bark treatment is where the herbicide is applied directly to the bark at the base of the plant. This method, like cut stump treatments, allows targeted herbicide treatments of Scotch broom, with little or no damage to surrounding plants. Herbicide rates for foliar sprays and contact treatments differ, so make sure to check the label for directions.
Depending on the treatment, it may take more than one herbicide application to kill Scotch broom plants. Examples of selective herbicides that can be used to control Scotch broom include those with the active ingredient triclopyr (e.g., Vastlan or Garlon 4 Ultra) or triclopyr combined with aminopyralid (e.g., Capstone). Herbicides with triclopyr are to be used as foliar sprays when plants are rapidly growing. Cut stump and basal bark treatments using these herbicides can occur whenever the ground is not frozen. These selective herbicides typically kill broadleaf plants but do not harm grasses, though there may be exceptions so make sure to check the label. Non-selective herbicides that contain the active ingredients glyphosate or imazapyr can be used to control Scotch broom but will also kill any other plants they come in contact with. Application timing varies depending on treatments. Glyphosate should be used as a foliar spray in late summer or early fall, or used in a cut stump treatment in late summer, early fall or during the dormant season.
Always read and follow the herbicide label directions carefully for proper dilution rate, surfactant information, application timing and proper sites. For more detailed information on timing and application, contact your county noxious weed board or program. state department of agriculture, extension service or conservation district. Also, check the Pacific Northwest Weed Management Handbook for additional herbicide information available online. (The label is the law!)
Biological
Biological control agents are available for Scotch broom and can be a valuable management tool for large infestations. Larvae of the Scotch broom seed weevil, Exapion fuscirostre, feed on seeds of Scotch broom in developing seedpods. The adults also feed on flowers and stem tips, though their damage is not significant. The Scotch broom bruchid, Bruchidius villosus, larvae feed on developing seeds and impact the plant's reproduction. While these biological control agents will not kill the Scotch broom plants, they will reduce the number of seeds produced. Use them in the center of large infestations while working to contain infestations and control other isolated plants and populations. Do not release biological control agents in populations that will soon be burned or mowed as they will not have an opportunity to establish on the plants and impact seed production. Check with the Oregon Department of Agriculture's Weed Biological Control Program and Washington State University's Integrated Weed Control Project for information about these biological control agents. Goats can also help control resprouts after the cutting or burning of Scotch broom. Plant parts are toxic to other livestock though, so only let goats do the grazing. Make sure to protect desired plants from goats as they will be grazed too.
After Treatment
There may be an increase of nitrogen-loving, non-native grasses and forbs that grow after Scotch broom removal due to the possible increase of available nitrogen in the soil. Planting and seeding non-invasive plants to outcompete weeds will help reduce the number of new invaders. Newly planted tree seedlings may need some help, as non-native grasses and other weeds may have an initial growth advantage: they are better adapted to the altered soil nutrients. Native species are adapted to nitrogen-poor soil, so they may not grow as quickly as non-native invaders. Make sure to watch for and manage new weeds as necessary.
Disposal
Dead or pulled Scotch broom plants may be left in place to decompose, but they could increase soil nitrogen. Plants can be piled to decompose or burn, though again make sure to check on local restrictions, burn bans or permit requirements before burning. Plants without seeds may also be chipped. Local transfer stations or landfills may also be an option for removed plants.
If you have any questions regarding Scotch broom control, contact your local experts such as a noxious weed board or program, state department of agriculture, extension service or conservation district.
Wendy Descamp is the Education Specialist for the Washington State Noxious Weed Control Board. She can be reached at 360-725-5764 or wdescamp@agr. wa.gov. Excerpted by permission from an article originally appearing in Northwest Woodlands, Winter 2018.