CORVALLIS, Ore. — They float in the wind, get shaken off pets and wildlife, cling to clothes and luggage, and travel the globe by plane, ship, train, truck and car.
“With some of these weeds, you have to fight them forever. Many times, it’s more a process of controlling them rather than eradicating them."
Invasive weeds can enter your yard in multiple ways — and once they do, they often cause serious problems.
“With some of these weeds, you have to fight them forever,” said Ed Peachey, a retired weed specialist with the Oregon State University Extension Service. “Many times, it’s more a process of controlling them rather than eradicating them.”
According to Peachey, the first line of defense is identifying what’s growing. Whether weeds are annual or perennial determines the best control strategy.
Annual weeds spread by seed and die when the weather turns cold, but their seeds can remain viable in the soil for years. Common examples include sharp point fluvellin, velvetleaf, puncturevine, horseweed, western bittercress and oxalis.
Perennial weeds return year after year, often with aggressive root systems. In addition to seed dispersal, many perennials spread through creeping roots. These include blackberry, Scotch broom, bindweed — also known as invasive morning glory — horsetail, English ivy, poison oak and old man’s beard — also called traveler’s joy, a type of invasive clematis.
Peachey’s advice: if you don’t recognize a plant, pull it and get it identified. Invasive weeds spread quickly and are easier to manage before they become established. Just look at natural areas like Forest Park in Portland, where English ivy has overtaken native vegetation.
Need help identifying a weed?
- Post a photo to Ask Extension, OSU Extension’s online question-and-answer platform.
- Bring in a photo or fresh sample in to your local Extension office.
- Extension experts can help you identify the weed and recommend control strategies.
Annuals: a matter of timing and persistence
The best hope for managing annual weeds is to pull them before they go to seed.
“Get them out when they are small,” Peachey said.
Since seeds can persist in the soil for years, vigilance is key. If you keep pulling seedlings as they appear, the population will eventually decline and weeding will become easier.
Perennials: a tougher challenge
Annual weeds may be frustrating, but perennials require more patience — and more persistence. Anyone who has battled blackberry knows the struggle. Dig out as many roots as possible, then keep pulling new shoots. Over time, this can starve the plant of carbohydrates and eventually kill it. The process can take years.
For more tips, Peachey and Chip Bubl, an OSU Extension horticulturist, offer the following strategies:
Organic mulches
Organic mulches like bark dust, wood chips, leaves, straw and grass clippings can help suppress weeds and improve soil health as they break down.
- Apply 2–4 inches of mulch around your plants.
- Avoid using leaves from black walnut (Juglans nigra) or tree of heaven (Ailanthus altissima), as they can inhibit plant growth.
- Don’t use lawn clippings if the lawn was mowed while weeds were in seed.
- To suppress perennial weeds, lay down landscape fabric before applying mulch.
Compost
Compost can be a major source of weed seeds. Ask friends or neighbors for compost supplier recommendations, or ask compost vendors how they treat their compost to reduce weed contamination.
Bird seed
Bird seed is a common culprit in weed outbreaks. Choose black oil sunflower seeds, which many birds prefer, or place a tray under feeders to catch stray seeds.
Hay vs. straw
Hay often contains weed seeds and herbicide residue. Instead, use straw, which has fewer seeds and is less likely to cause problems.
Plastic sheeting
Black plastic sheeting blocks light and prevents weeds from growing. For vegetable gardens, install drip irrigation and fertilizer before laying the plastic.
- Cut slits to plant crops, and remove weeds that appear in the openings.
- In non-planting areas, cover weedy soil with black plastic for six weeks. The weeds will die off due to lack of sunlight.
Irrigation methods
Sprinklers water large areas and encourage weed growth. Drip irrigation is more targeted — it waters only where needed, reducing weedy areas.
Hand pulling
Hand pulling is effective for small spaces and raised beds.
- Pull weeds when soil is moist, but not wet.
- Remove annuals before they go to seed.
- For perennials, pulling new shoots consistently can weaken and kill the plant over time — a method called carbohydrate starvation. This takes daily attention, but it works.
Hoes
Hoes are traditional and effective tools for weeding.
- A scuffle hoe is good for large areas.
- The hula hoe (a lightweight scuffle hoe) uses a push-pull motion to dislodge seedlings.
- The Warren hoe, with its heart-shaped blade, is useful between plants.
Small hand cultivators
Small hand tools work well in tight spaces.
- A dandelion digger (also called a weeder or asparagus knife) is ideal for deep taproots.
- The Hori Hori knife is a popular all-purpose option for hand weeding.
Herbicides
Herbicides should be used carefully and according to label instructions.
- Choose products registered for the weed you’re targeting.
- Avoid spray drift, which can harm nearby plants.
- Roundup is more effective on annuals than perennials.
- Crossbow can work better for woody shrubs like blackberry and Scotch broom.
Proper disposal
If weeds are in seed, don’t compost them. Bag them and put them in the garbage bin to prevent further spread.
Previously titled Roll up your sleeves: Use multiple strategies to control invasive weeds
