A few facts worth thinking about while weeding season rolls on:
- University research in Minnesota found anywhere from about 100 to several thousand live weed seeds in a single square foot of soil 6" deep.
- Many weed seeds stay alive for decades; long-term burial studies have found jimsonweed and field bindweed seed still germinating after several decades in the soil.
All of that seed waits in the soil's "weed seed bank" for the right conditions to sprout. Once up, weeds fight crops for water, nutrients, light and space, and often win. That's why the best growers put weed control first. As University of Maine weed ecology research explains, good Weed Management for Sustainable Ag Systems works in two phases: get rid of the existing weeds, then keep new weeds from taking hold and setting seed. Over time the seed bank shrinks and weeding becomes a much smaller job.
On a new farm, field or garden, though, expect weeding to take up a lot of your time. You'll likely face both annual and perennial weeds, which call for different tactics and tools. Here's how to get ahead of each.
Most annual weeds grow from seed that either fell on the soil surface or was brought up by plowing, tilling or other disturbance. Seeds buried more than about 2" (5 cm) deep tend to stay dormant, so in general, disturb the soil as little as you can to avoid bringing new seed up where it can sprout.
Cultivation is your first defense against annuals. The best time is the "thread stage," right after weeds sprout, when they're no thicker than a thread. Then a few quick passes with a wheel hoe or a light tool such as a collinear hoe will finish them off. See our long-handled weeding and cultivating tools guide and the article on sustainable weed management for details.
As weeds grow or multiply, you need more passes and bigger, sturdier tools. Trapezoid and stirrup hoes come in several sizes for tougher weed problems. Read tool descriptions carefully so you have the right hoe for each situation.
Many organic farms use "stale seedbed" planting: till and water a bed so weed seeds sprout, then kill the weeds either before planting the crop or just before the crop comes up. A propane flame weeder is the best tool for this, since it doesn't stir the soil and bring up more seed.
For example: till a bed, sow a crop such as carrots and water it. A few days before the crop should emerge, flame the whole bed or hoe it very lightly. When the crop comes up, it won't have to compete with weeds right away and gets a good start.
To tell when the crop is about to emerge, dig up a few seeds and check whether they're sprouting. Or lay a pane of glass over a short stretch of row; it warms the soil so seeds underneath sprout a few days ahead. When those come up, flame the bed to kill the weeds
You can also smother weeds by covering bare soil with mulchmade from many materials: plastics (some now biodegradable), biodegradable paper, or organic materials such as mulch hay, straw and seaweed. With plastic , biodegradable film or paper, lay the mulch when the ground is weed-free and pull it tight so it touches the soil. Keep planting holes small, just big enough for the mature crop, with no spare room for weeds.
Some growers use landscape fabric on paths and even beds; handled well it can last ten years or more. Heavy black fabric suits long-season crops like tomatoes. Hay and straw take more time and effort to put down, but they feed the soil with nutrients and organic matter as they break down.
You can also get ahead of annual weeds by planting farm seed and cover crops wherever you aren't growing a crop. In a short gap between harvest and the next planting, such as a month or two between spring and fall crops, a fast annual like buckwheat keeps the ground covered so weeds can't move in. Cover crops can also be grown longer, even a whole season or in a sequence, to tackle a weed problem. The trick is picking a cover that sprouts and grows fast enough to outcompete weeds. See Further Readingbelow for more.
Perennial weeds such as bindweed, Canada thistle, yellow nutsedge, pokeweed and dock are much harder to beat, especially for organic growers.
Perennials not only come back every year, they quickly regrow after cultivation from roots, tubers and other pieces left in the soil. Often, cutting off the top growth pushes the underground parts to send up more shoots, and tillage can drag root pieces to new spots. Many perennials also make lots of seed, so they spread several ways during the season.
The rule more weeds, more work holds true. If perennial weeds are few, dig each one out with a shovel and try to get all the roots. If you can't get the roots, keep cutting or mowing to drain the plant's underground reserves, and never let it set seed.
If perennials are everywhere, you may need a summer fallow with no crops. Plow to bring big weed roots to the surface to dry out and die. The perennials will resprout but weaker, and repeated cultivation will kill or set back most of them. Where weeds are bad, a summer fallow every two or three years in rotation with vegetables may be needed.
- Weed management for sustainable farming: a two-part plan for season-long weed control (University of Maine research) • Article
- Long-handled weeding and cultivating tools guide • Article
- Farm seed: uses and benefits • Products
- Cover crop comparison chart • PDF
- Buckwheat: a fast cover crop against weeds • Article
- Planting and managing cover crops for weed control, Virginia Association for Biological Farming • eOrganic article
- Managing Cover Crops Profitably, SARE • Book
