Jean R. Natter, OSU Master Gardener
In case you haven’t been outdoors recently, you need to know weeds are growing in spite of the soggy soil from all the recent abundant, rainfall. So, let’s review a few strategic management strategies for successful weed control in gardens and landscapes. As you may know, the PNW Weed Management Handbook (see resource list) has a thorough overview of the subject.
Annual weeds are classified as warm-season or cool-season plants, with each kind genetically destined to germinate (sprout) in the appropriate season, then survive for a year or somewhat less. The same categories are assigned to perennial weeds but these are considerably more long-lived, persisting for 3 or more years.
As a result, gardeners must be vigilant year-round.
Several key principles limit weed populations in gardens and landscapes, among them these:
– Kill weeds when they’re young; ten fingers are always at hand.
– Don’t allow weeds to set seed.
– Annual weeds will die when cut just below the crown. Use a knife, or any one of various hoes – standard garden hoe, diamond hoe, or a scuffle hoe.
– Perennials will re-grow when cut off, even if covered with mulch Vigorous kinds will make it through a lasagna garden; and may lift landscape fabric and/or polyethylene sheeting.
– To kill perennial weeds, you must starve the roots; to do so, remove all green growth every week for as long as it takes, very likely several years.
– Herbicides are rarely a “once-and-done” remedy.
Gardeners can make good use of creating a “stale seedbed” before planting a flower or vegetable bed, or even a lawn. The reason? Hundreds of dormant seeds — termed the “Soil seed bank” – have been deposited in the soil during years and years of poorly managed, or unmanaged, weeds. Whenever soil is disturbed, some of those seeds are brought near the surface where they germinate because they are exposed to light, also appropriate temperatures and moisture.
Start a stale seedbed a month or so prior to the desired planting date, thereby allowing for a cycle or two removing weed seedlings. Do everything needed to prepare the planting bed: dig; remove obstructions and weeds; amend the bed; level the soil; then moisten the soil to settle it and allow for germination. As soon as a good stand of young weeds about inch tall is present, destroy them using your preferred method: hand, hoe, flamer or, if you must, herbicide.
The earlier a stale seedbed is started, the more germination cycles possible, the fewer weeds will be present to steal water and light from the desired crop. Next, seed or transplant your veggies or flowers with minimal disruption of the soil, and deal with any seedling weeds promptly.
With perennial weeds, choose among these destructive techniques:
1. Dig it out; repeat as needed.
2. Cut it off; repeat as needed.
3. Herbicides are effective if the right kind is used according to label directions; repeat as needed for re-growth and/or use #1 and #2, above.
Tools to help limit weeds:
– Herbicides, organic or synthetic: Contact products kill only top growth (fine for annual and seedling weeds); systemic products translocate (move) into other plant parts, sometimes into roots.
– Mulch, with the understanding that new weeds, in the form of seeds, will continue to arrive via clothing; muddy boots; new plants; birds; hitchhikers on mowers; or may be blown in.
Some trees are known to put up root suckers after they are cut down, among them are such broadleaf trees as ash, aspen, cottonwood, flowering cherry and poplar. Conifers won’t re-sprout, even if the roots remain in the ground.
To limit root sprouting after a broadleaf tree is removed:
– Cut down the tree as soon as the new leaves have fully expanded in spring.
(Principle: The tree used most of its reserves for new foliage, thus the stump/roots will put up fewer sprouts than if the tree is removed later in the season.)
– Immediately after the tree is cut off, paint a 2-inch-wide band of the other edge of the cut surface with an herbicide such as triclopyr or 2,4-D, following label directions.
– Remove new shoots as soon as they’re seen:
– The more often new greenery is removed, the better the chances of decreasing the roots’ reserves; once a week is a good plan.
– The longer new growth remains, the more reserves it sends to the roots, thereby extending the battle.
– Be persistent and you will win; pause for a season and you lose.
– Sustainable Gardening (your MG Handbook)
– Weeds and Your Garden: https://ecommons.cornell.edu/bitstream/handle/1813/43859/weeds-in-garden-bro-NYSIPM.pdf;sequence=1
– PNW Weed Management Handbook: Online at http://pnwhandbooks.org/weed/horticultural/home-garden-landscape-management
– PNW Weed image gallery (from the 2010 Weed Handbook): Organized by common name, each weed with 1 to 3 images: seedling, flower and/or mature. http://uspest.org/pnw/weedimages?weeds/id/index.html
– Weeds and other unwanted plants (text and images) http://ipm.ucdavis.edu/PMG/menu.weeds.html