Weed control practices

Prevention

Preventing the introduction and spread of weeds can prevent the need for the other methods listed below. Some tactics are more obvious than others.

• Prevent seed spread by cleaning tools, clothes, pets, and other gear between weedy worksites.
• Plant right by checking the species of seeds or live plants that you’re putting into your spaces – not all noxious weeds are banned from sale (this is a separate list – the WSDA’s Prohibited Plants List only includes around half of the state’s listed noxious weeds). Weed your nursery pots before planting.
• Use the right control methods – certain methods can spread certain plants. For instance, digging or cutting plants that reproduce vegetatively (via plant/root fragments) can spread the plant further if not done carefully alongside other methods.
• Proper disposal of weeds is a necessary step in all weed control. It is best to bag and dispose of flowering and seeding plant parts, as the compost doesn’t get hot enough to kill all the seeds. Plants that spread with fragments (for example, bindweed, knotweed) should also go in the trash to prevent their spread. All other plant parts can go into city or home compost, though be careful with toxic weeds in home compost if you use it for edible garden spaces (these weeds have warning banners on our site). These other plant parts can also be left to compost on-site, take care to make sure the plants are not touching the soil as they could re-root.

Learn more about proper weed disposal in our blog post here:

Are your weeds piling up? – Noxious Weeds Blog

Manual control

• Controlling weeds using only hands or non-power tools (shovels, pruners, weed wrench, etc.). The most commonly used weed control method due to ease of access, but on its own it can be ineffective for certain weeds. If trying to remove plants with hardy root systems, it’s best to combine this method with others (i.e. chemical or cultural controls). This method is best used on small or new infestations.
• Digging uses a tool or hands to loosen soil in an area, making it easier to get to the roots of a plant. Take care to get as many root fragments as possible, as many plants can come back from remaining root pieces. This is easier in soft soils; in compact soils, if able, it can be helpful to wet the soil leading up to removal.
• Cutting uses a sharp tool to remove above ground parts of a plant. This can be used to remove plant mass before digging, or sometimes to prevent seed production of a plant for that season by cutting back growth before they can make seeds, or bagging flowers/seeds to prevent their spread. Please note that this method alone leaves the roots, from which most plants can grow back from. Thus, cutting is often only one step of the process and is effective at reducing the impact of a species temporarily by reducing spread, but is not a long-term fix.

Mechanical control

Controlling weeds using power tools (have a motor, electric, or gas). This includes methods like mowing, brush cutting, chain sawing, and tilling. Mechanical methods are often used for suppression, but not total control. They will often just be the first step of many in an integrated pest management plan. Mechanical methods can spread weeds if done at the wrong time without taking precautions. Tips for better results:

• Plant growth timing: the best time to mow/brush weeds is before they can flower. Ideally, mow these plants when they have formed tight green buds (earliest flower stage). Flowering weeds can be mowed, but flowers should be collected (easiest with a bag/collection attachment) as flowers left on the ground often still make seeds. Seeding weeds should not be mowed as this can just further spread and sow these seeds.
• Weather: mowing or brushing weeds is more effective during dry periods as roots have a harder time regenerating. Avoid mowing in moist soil if possible as the heavy machinery can push existing seeds further into the freshly disturbed soil.
• Prevent further spread: any equipment should be thoroughly cleaned following mechanical methods. Weeds that can reproduce from fragments of itself (i.e. bindweed, knotweed) should not be tilled, as tilling can break the plant and roots into fragments from which they can make new plants.

Cultural control

• Changing the local environment of the plant so that target weeds have a harder time, and beneficial vegetation have an easier time. These methods are almost always exclusively used in combination with other methods, as they are not effective on their own.
• Sheet mulching is a common option that uses compostable layers (heavy mulch + papery layer, optional compost) to form a weed barrier following manual or mechanical controls. Prepare the site by making sure plants are controlled to ground level (for example, digging and mowing). Place a papery layer (newspaper or cardboard) followed by a 6 to12-inch layer of heavy mulch (for example, arbor chips). Optionally layer the wood chips with compost to improve soil quality. To be effective, make sure the mulch layer is thick enough to discourage weed growth, if it is too light it may just offer nutrients to the weeds while not preventing new growth. It is best to let this layer break down before planting into it if possible. If you must plant into it, be sure to clear a ring around the planting to not suffocate the plant’s roots.
• Competitive planting is the idea of intentionally seeding or planting things where weeds otherwise grow. This can be used to create a shade layer or take up physical root and ground space. Best done after the initial control methods. If planting following chemical controls, be sure to read the label to learn how long you need to wait to plant into a treated area.
• Tarping/Solarization involves securing a layer of thick plastic or textile over recently cut or removed weeds and leaving the cover in place for months to years. This method can be an alternative to herbicides on harder to control weeds, but often takes years to be truly effective. It should be noted that using this method means the local area cannot maintain plant, animal, or even microbial life – due to a lack of air, water, and nutrient flow. This method is best used on small infestations in areas that don’t need to be used for several years.

Chemical control

Chemical controls include using herbicides to kill weeds. Your species, site, time of year, and availability will all influence your choices for which herbicides to use. You may choose to use non-selective herbicides that will kill all plants, or selective herbicides that will kill broadleaf plants, but not grasses or other monocots. Herbicides are just one tool of many in the IPM toolbox and should be used only as needed. Oregon State University has great resources on safe pesticide use: Solve Pest Problems.

If herbicide seems like the best option for your site, the most important thing you can do is always read the label of any herbicide(s) you use. The label should be able to answer most questions and prepare you for safe use (which protective equipment to wear, appropriate weeds and locations for the product, re-entry times, etc). Be sure to follow state and local regulations, learn more in our regulatory guidelines document.

Depending on your species, additional species-specific guidance may be available in a King County’s Best Management Practices document. If your species does not have a Best Management Practice Guide, look to its webpage (listed A to Z on our homepage) or see if you can find it in the Pacific Northwest Weed Management Handbook.

Common Chemical Control Options for Noxious Weeds

Foliar spray

The most common method, used on a wide variety of species. This uses a low concentration solution of herbicide (usually <1% to 5%) that gets sprayed onto the leaves of a target species. Stores sell pre-mixed/Ready-to-Spray solutions and concentrated solutions you mix yourself. When treating noxious weeds, it’s best to “spot spray”, that is to only spray as necessary directly onto target plants using the lowest effective rate. Most herbicides used on noxious weeds are systemic, meaning the solution hits the leaves and translocates down to the roots, so users can use the minimal amount (typically until leaves are wet but not dripping, and none into the soil). Learn more about foliar spray from the Midwest Invasive Species Network (please note regulations vary locally, see the King County Regulatory Guidelines document linked above for local guidance).

Cut and treat

Used for woody species control. As the name indicates, this method has two parts. Start by cutting the plant low to the ground with a sharp tool, then immediately follow the cut with a stronger herbicide (usually 20% to 80%). It’s important that these steps happen back-to-back so that the plant does not have the chance to heal over its wound before herbicide is applied. Take care when cutting woody species and leave plants that you are not comfortable cutting yourself, as your safety is most important. Learn more about this and other similar methods from the Woody Invasives Group.

EZ-ject

Used for larger woody species (>2 in. diameter). Use a specialized tool to inject capsules of concentrated herbicide into the plants nutrient exchange layer. This method very effective and is a good option for those with large amounts of invasive trees as it takes out the stress of cutting them down before treating. Learn more about the EZ-ject from Arbor Systems.

Hollow stem injection

Used most commonly for invasive knotweed control. King County residents can borrow knotweed injectors for free during the summer months. Check out our knotweed control video series on how to properly use (and borrow) an injector. 

Biological control

Biological control (biocontrols, for short) is a term used when living organisms are used to control weeds. Beetles or weevils are used as biocontrols, but so are moth larvae, goats and even fungi or rusts. Biological controls can control the target weed directly or indirectly, but they will not usually eradicate the plant population from a site. Weeds are directly impacted when the biocontrols destroy plant parts that prevent growth, or prevent seed production. Biocontrols also impact a weed indirectly when the stressed plants are no longer able to out compete native or beneficial plants on a site.

It can take 4 to 5 years for any visible signs of weed control, making this more effective as part of a long-term control plan. Releasing biological control agents might be a good option on sites with large stands of weeds, when immediate control is not possible.

Learn more about biological control of noxious weeds from the Washington State University Extension