White Sweet Clover

3 – Control Practices

Integrated Vegetation Management

This section will discuss concepts of integrated vegetation management, and then discuss control options for white sweetclover.

Integrated vegetation management (IVM) is a common sense approach to managing vegetation that achieves the desired outcome using the plant’s biology to identify effective, safe, and efficient methods of control. IVM uses five general methods of control: prevention, physical, biological, cultural, and chemical. Below is a description of each of these methods.

Prevention: Includes approaches such as using certified weed free materials, and cleaning equipment to prevent movement of seeds of weeds. Prevention measures should always be part of an IVM plan.

Physical: Includes mowing, and hand pulling.

Biological: Using biological organisms such as insects, grazers, and pathogens to control a weed population. Grazers may be appropriate on some sites, but may be problematic in areas where vehicular traffic is present. Insects and pathogens can be effective, however they require large populations of the weed and extensive preparation that can take many years to be approved. In Alaska, there are no weeds with both a high enough population and an approved insect or disease biocontrol agent. Biological control agents are not known for white sweetclover.

Cultural: Actions that promote desirable vegetation are considered cultural methods for controlling weeds. Examples of cultural methods include revegetation of bare ground with plants that will out compete weeds, and activities such as mowing that promote the growth of desirable grasses while eliminating the weeds. Revegetation is key to effective cultural control.  For strategies to revegetate refer to the Alaska Division of Agriculture, Plant Materials Center publication “A Revegetation Manual for Alaska”.

Chemical: Herbicides are used to control a variety of weeds, and are sometimes the most efficient method of control when eliminating a weed infestation is the desired outcome. In Alaska, all applications to land you do not own or manage must be made by a certified pesticide applicator. Become familiar with additional times one needs to be certified to use pesticides by reading the Department of Environmental Conservation Fact Sheet. In some cases a pesticide use permit may be necessary, and in others an integrated pest management plan is needed without a permit. Read the information on when you need a permit or IPM plan at the DEC website, and contact the Alaska Department of Environmental Conservation for information on when a permit is necessary if you are still unsure. The University of Alaska Fairbanks, Cooperative Extension Service provides courses and proctors tests for certified pesticide applicators in Alaska. This course identifies herbicides that will work with white sweetclover, though we do not present an exhaustive list of chemicals or products. Many products may work just as well, particularly different brand names with the same active ingredient.

Timing chemical applications is critical because the chemical used may only be effective at certain life stages of the plant. Life stages of plants that are pertinent to chemical applications include early growth and full flower.


Early Growth White Sweetclover
The plant growth stage of white sweetclover is critical to effective control. Early growth (image above) when white sweetclover is in the bud to flower stage it is most susceptible to systemic herbicides. As white sweetclover matures (image below) it is less susceptible to herbicides.

Mature White Sweetclover

Product labels provide detailed instructions on how to apply the chemical, and what is the maximum amount you can apply per year. Labels list a range of application rates. Generally speaking, if you and others in the area do not have experience controlling the plant we recommend using the higher rate, and if you desire to use a lower rate do so on a small portion of the infestation so that you can monitor the effectiveness. Treatment methods covered include broadcast and spot treatments. After treatment with a chemical the plants absorb the toxin which is distributed throughout the plant. The distribution of these chemicals throughout the plant is critical to success of the control effort. Treated plants must remain undisturbed for up to two weeks following treatment to be successful.

When weed seed is suspected to contaminate soil in the surrounding area of the targeted infestation it may be prudent to treat the soil with an herbicide that has residual soil activity. The residual soil activity of the chemical will kill seedlings that sprout later in the season, and in some cases in future years. Active ingredients aminopyralid and clopyralid are examples of herbicides that have residual soil activity. Care should be taken with the use of persistent chemicals with residual activity. Plans for future revegetation should be considered to avoid the effects of soil persistent chemicals on sensitive species. If fill material is moved from the site it may contain the herbicide, and disrupt revegetation efforts in the area it is moved to.

After treatment of weeds with herbicides it is imperative to monitor for effective control, and to avoid developing herbicide resistance in a species. If some of the plants are not affected by an herbicide they may be resistant to the chemical mode of action chosen. If those resistant plants produce seed their offspring may also be resistant. Control those unaffected plants with another method or choose a new herbicide with a different mode of action. Planning ahead for resistance management by mixing two chemicals with different modes of action, or budgeting the time to implement mechanical or other control measures, will help with overall control effectiveness.

All herbicides have detailed instructions on how to apply the chemical to various sites. Labels are considered legal documents for the use of these chemicals. Always refer to the label for the appropriate way to apply that product.

a. Prevention – address pathways, what commodities need to be certified, inspected, etc.

Preventing the spread of sweetclover to new areas during construction, landscaping, regular maintenance, and recreational activities is the most effective method of control. Common ways invasive species biologists believe that sweetclover is spread include when it contaminates equipment, topsoil, fill and gravel. To ensure that machinery is not contaminated clean regularly, particularly in areas around wheel wells, blades on mowers, and other areas that tend to collect dirt. Also, if travel or work through areas known to be infested with sweetclover is unavoidable ensure you clean equipment after leaving the infested area, and try to work or travel in infested areas last.

Gravel, topsoil, and fill material should be inspected and certified as weed free whenever possible. The weed free gravel certification program is hosted by the Alaska Division of Agriculture, who organizes trainings for inspectors and maintains data on which pits were inspected and whether or not they passed.

Clean vehicles and equipment regularly

Vehicles and equipment collect dirt and debris when in use. With the dirt and debris comes seeds and other parts of plants (roots and stem fragments) that are capable of spreading to a new area. Scientists have sampled dirt stuck to vehicles and found seeds in areas such as wheel wells, behind license plates, tire treads, and other areas that collect dirt and debris. One Alaska community on Annette Island uses a vehicle wash station at the ferry terminal. They sent the dirt and debris to the Alaska Plant Materials Center for analysis, which showed they captured viable seed from washed vehicles. Weed species such as spotted knapweed (Centaurea stoebe) have been found primarily in places that serve as vehicle pullouts or potential staging areas for equipment or recreational vehicles. Regular cleaning of vehicles will help prevent the spread of weeds.


Below are the ideal steps you may take to wash vehicles and equipment:

  • Clean vehicles and equipment using a pressure washer or compressed air.  Focus on wheel wells, tire treads, bumpers, and other areas that can collect dirt or debris.
  • Ideally clean vehicles prior to leaving a weedy location. Doing this ensures that the weeds are left at the site instead of dislodging from the vehicle en route to where the equipment is staged until further use. Limited access to water or compressed air may leave you unable to clean vehicles or equipment before leaving a weedy area. When it is not possible to clean the vehicles and equipment before leaving a weedy area, clean vehicles and equipment in a designated area such as the maintenance yard.
  • When returning to the maintenance station or equipment staging area clean vehicles.These are good areas to clean vehicles because cleaning equipment is regularly accessible, and monitoring for and management of weeds that start to grow because they were washed from vehicles or equipment is easier to manage in areas that are regularly occupied by employees.



Cleaning Equipment With A Pressure Washer
Clean equipment with a pressure washer or compressed air. Pay special attention to areas that are likely to collect seeds, plant parts and dirt. Photo courtesy USFS, Technology and Development Center, from video “Dangerous Travelers”.



Avoid infested areas

Weeds established in a particular area are easily spread if vehicles and equipment park in or travel through infested areas. If work is scheduled to be done in weedy areas then other best management practices will be used. Avoiding infestations is about choosing equipment staging areas and travel routes that are free of weeds.


The ideal steps to avoid infestations:

  • Do not travel through infested areas
  • Do not park or stage equipment in infested areas.
  • If an infested area is used, control weeds to prevent additional seed development.
  • If an infested area is used, wash vehicles and equipment when leaving the area.



white sweetclover in an equipment staging area
Shown here is white sweetclover in an equipment staging area on the Dalton highway. Vehicles and equipment staged in an area infested with weeds will spread weeds. If it isn’t possible to stage equipment elsewhere, control weeds in the area, and wash vehicles and equipment before leaving the area. Photo courtesy Alaska Division of Agriculture.

Work from uninfested to infested areas

Ripe seed or seed and other propagative parts of plants (root and stem fragments for example) are spread from one area to another when mowers, graders, and other work is completed in infested areas. If work in infested areas is unavoidable, and cleaning equipment during the field day is not possible, then one strategy is to work in those infested areas last to prevent moving weeds to new areas. For example, if you are mowing brush in a utility corridor, and the north end has no weeds while the south end has an abundance of white sweetclover, then starting work at the north end of the utility corridor will help prevent spreading the white sweetclover from south to north. In many cases areas furthest from towns are the least infested areas.



Dalton Highway Sowthistle
Mowing and grading are excellent examples of ways to spread weeds in whatever direction the work is headed. Shown here is perennial sowthistle that was recently graded on the Dalton highway. Notice it appears to be happily growing despite the disturbance. While it is not in seed, sowthistle can be spread by its’ roots. Any seed that happened to be remaining in the soil could also propagate down the road.

Use certified weed free materials

Materials such as gravel, and straw used to make erosion control materials can be available as certified weed free products under a program managed by the Alaska Division of Agriculture. National standards are set by the North American Invasive Species Management Association, and individual states are allowed to add to the lists of weeds not allowed in such a product. Originally this started as a weed free forage program, that later was adapted to straw, and gravel. Information about the Alaska programs are found at http://plants.alaska.gov/invasives/weed-free.htm. A field or pit is inspected during the growing season. Gravel pits are inspected twice, and hay or straw are inspected within 7-10 days of harvest. A field or gravel pit cannot have listed weeds present to pass inspection, however, inspectors may flag off part of a field that can be harvested. If certain listed weeds are present but not in seed, a management plan can be implemented to allow for the commodity to pass certification. Storage areas for the products must also be inspected. Take some time to view the inspection standards and list of weeds for weed free forage/straw, and gravel.


Sweetclover and other weeds growing in a gravel pit a
Sweetclover and other weeds growing in a gravel pit are spread when the gravel is used in other areas. Using certified weed free gravel, and implementing weed control plans for existing pits, will help prevent spreading weeds to sensitive areas.


Seed should also be weed free. By regulation (11 AAC 34.010 – 34.070) in order for seed to be sold in Alaska it must be inspected, although with a different list of weeds than weed free forage and gravel. Seed is required to abide by the State of Alaska Noxious Weed list (11 AAC 34.020) which includes prohibited weeds that cannot be present in the seed, and restricted weeds which have allowable tolerances for contamination of seed. The weed free forage/straw, and gravel certification programs uses a more stringent list that includes additional species of concern. When purchasing seed, gravel, topsoil, and erosion control materials made from straw it is generally preferable to buy locally produced materials. Many weeds that are abundant in other parts of North America are not present in Alaska, or are currently in very low numbers. Purchasing locally produced materials lessens the risk that those weeds are repeatedly introduced to Alaska.



b. Mechanical/manual

Mechanical and manual control options for sweetclover include mowing or pulling. If mowed to within one inch of the ground, and mowed regularly, sweetclover may not come back or at least will not produce seed. Approximately two cuttings per summer are necessary to prevent seed production with both cuttings occurring when plants begin to flower. The month that flowering occurs may vary depending on the area, but typically flowering will occur once in the middle of summer and again in late summer into fall. Manual control of sweetclover is a viable option when infestations are light, and/or an adequate labor force is available. Care must be taken to remove entire root crown to prevent individual plants from resprouting.

c. Weed barriers

The long seed longevity of sweetclover is not conducive to using weed barrier fabric. In order to ensure eradication weed barriers would be left in place for 70+ years, which is likely beyond the lifespan of any fabric on the market considering the traffic and disturbance that occurs in areas sweetclover infests. Sweetclover also grows in places (roadsides and floodplains) where weed barriers can be difficult to maintain.

d. Herbicide

Herbicides can be highly effective at managing white sweetclover. Herbicides are sometimes more effective at certain times of the plant’s growth phase. For white sweetclover, make herbicide applications in spring or summer to seedlings or plants in early bud to flower stage.

Here we will discuss some herbicides that control white sweetclover, some of their label restrictions, and maximizing benefit while minimizing impacts of each herbicide. Note that we discuss the active ingredient, and there are many brand name herbicides associated with each active ingredient. Brand name herbicides, not the active ingredient, are registered for use on specific sites. It may be legal to use one brand name on residential lands, while another with the same active ingredient cannot be used in that setting. Always read the label to appropriately use the product. The label is the law.

Chlorsulfuron provides the most thorough control. However, plants controlled with chlorsulfuron are known to develop resistance with repeated applications. To avoid selection for resistant populations, rotate repeated applications with different modes of action or consider tank mixing products with two modes of action. Chlorsulfuron is absorbed through the foliage or roots of plants, and can be applied to foliage or soil. Chlorsulfuron kills a variety of broadleaf plants. Chlorsulfuron is active on woody vegetation providing some suppression and may be beneficial to use if woody vegetation control is desired in areas where sweetclover grows such as roadsides.

Triclopyr works well on white sweetclover. Triclopyr is an active ingredient often found in tree and brush killers, is also effective on a variety of broadleaf plants, and does not affect established grasses. Triclopyr works through absorption through leaves, stems, and roots. Some products containing triclopyr are registered for use in residential and other landscapes in addition to a variety of other sites. Triclopyr is not a good herbicide choice for work around desirable trees and shrubs. Triclopyr persists for a short time in the soil where it is available for root uptake so replanting an area where sweetclover was removed using triclopyr may require waiting until herbicides are no longer present. This active ingredient is an excellent choice for use in areas where woody vegetation management is desired in addition to the removal of sweetclover.

2,4-D works well on white sweetclover controlling most of the treated population. 2,4-D is an active ingredient often found in products used on lawns, and is incorporated in many products that are available for use on almost any site. 2,4-D is a selective herbicide that is only active on broadleaf plants. The herbicide works through absorption through leaves, stems, and roots.

Aminopyralid works very well on white sweetclover. Aminopyralid is an active ingredient found in a few products that are used to control a variety of broadleaf weeds in many settings, but does not damage established grasses. The herbicides are active on plants through the leaves and stems, as well as uptake from the roots. Aminopyralid can damage trees and shrubs from root uptake after control of adjacent white sweetclover, but damage does not appear on all species of trees and shrubs. The herbicide is known to persist in soil for some time after application and remain available for plant root uptake. Persistence is desirable when you need to control a seedbank, but can be undesirable if quick transition to susceptible species is desired. For example, if these products are used to clear a field of sweetclover with the goal to establish potato, peony, peas, or another susceptible broadleaf plant you will have to wait an unknown time for the herbicides to degrade. However, if aminopyralid is used to remove sweetclover from a roadside where grasses are desired, it will provide lasting control of the sweetclover. If transition to a susceptible species is desired after application of aminopyralid, a bioassay may be necessary to ensure herbicide is no longer bioavailable for plant uptake. If aminopyralid is used to remove sweetclover from pastures and hay fields restrictions on movement of hay and manure exist because the herbicides will accumulate in grasses and manure causing damage to susceptible species if used as a soil amendment such as compost.


Milestone Label
The label of Milestone (active ingredient aminopyralid) clearly shows restrictions of movement of treated hay and manure to prevent damage to non-target plants.


 e.Revegetation after control measures

Establishing vegetation after disturbance helps to control which plants will grow there. If left fallow, native vegetation may move in, but weeds or generally undesirable vegetation may move in. Vigorous non-invasive perennial grasses can prevent the establishment of weeds, and in many cases native species are commercially available. Information on revegetation in Alaska is found through the Alaska Plant Materials Center. The Plant Materials Center has manuals on revegetation and erosion control that detail species that will work and their general availability in the state. Specialists on staff at the Plant Materials Center are able to help navigate their manual and provide assistance for specific revegetation issues.


Fescue along the Parks Highway
Planting vigorous perennial grasses can help prevent re-invasion by invasive species. Shown here is red fescue planted on the Parks Highway in an area where sweetclover was manually removed. Photo courtesy Alaska Association of Conservation Districts.


Below are the ideal steps for revegetation:

  • Revegetate after any disturbance
  • Use native or non-invasive seed
  • Use species that grow quickly and are adapted to the amount of disturbance in the right-of-way.
  • Aggressive, but native or non-invasive species can prevent establishment of invasive species.
  • Contact the Alaska Plant Materials Center or an environmental analyst with the Department of Environmental Transportation for recommendations on seed mix.