Dealing with weeds
Why take action against weeds?
Weeds compete with crops for sunlight, water and nutrients, and can encourage fungal diseases by reducing airflow. Too-frequent cultivation to remove weeds can leave the soil more prone to erosion. Each tilling or deep hoeing stirs air into the soil and speeds combustion of organic matter. Most weeds respond well to nutrients, especially nitrogen. If you give corn too much nitrogen, even as compost, its productivity will max out and the weeds will use the remaining nutrients.
Remove weeds at their most vulnerable stage, or at the last minute before the seedpods explode —ignore weeds doing little damage.
Types of Weeds
- Annuals and perennials;
- Stationary perennials (docks) and invasive perennials (Bermuda grass);
- Cool-weather and warm-weather types;
- Quick-maturing and slow-maturing types;
- “Big Bang” types (pigweed) versus “Dribblers” (galinsoga).
Sustainable Weed Management
1. Prevent weeds from germinating
- Grow vigorous crops adapted to the locality,
- Switch between spring and summer crops in rotation,
- Mulch or tarp to bury short-lived weed seeds
- Plant promptly after cultivation, so weeds don’t get the head start,
- Transplant rather than direct sowing, giving your crop a head start on the weeds,
- Use close spacings, leaving less space for weeds,
- Use drip irrigation rather than sprinklers, discouraging weed germination between the rows,
- Plant cover crops, including no-till systems,
- Reduce tillage whenever you can, for example, by relay planting, where the new crop is planted while the previous crop is still in place, and prevent new weed seeds coming up to the surface.
2. Reduce weed seeding
- Practice timely cultivation, mowing, flaming, grazing by cattle, chickens, ducks, geese. As Margaret Roach says: “No matter what weed you are facing, if it’s flowering or setting seed now, be sure to behead it: mow it down, harvest the blooms for bouquets, or otherwise prevent a successful sexual reproduction cycle.”
- Reduce weed seed banks to 5% of original levels by preventing weeds from seeding for 5 consecutive years.
- Use post-emergence organic weed killers: corn gluten, vinegar, flaming
3. Reduce weed seed viability
- Reckon that most weed emergence happens within two years of the seeds being shed.
- Encourage seed-eating birds, insects, worms, mice
- Small, short-lived seeds of weeds with no dormancy period, such as galinsoga, will almost all die within a year or two if they are buried a few inches. Till and mulch to bury short-lived weed seeds.
- Longer-lived seeds (pigweed, lambsquarters, velvetleaf) if buried, may remain viable and dormant for years – Leave such weed seeds on the soil surface, rather than tilling them in! Delaying tillage if weeds have already seeded generally reduces the number of seeds added to the long-term seed bank. Seeds lying on or near the soil surface are more likely to deteriorate or become food for seed predators than buried seeds,
- If they do not get eaten, dry out or rot, seeds on top of the soil are more likely to germinate than buried seeds, and you can take prompt action.
- Use stale seed-beds – prepare bed a couple of weeks before planting, water as if you had planted. The day before planting your crop, hoe the surface shallowly to kill new weeds,
- Solarize weedy soil in hot weather to kill weed seeds – mow the weeds, cover the soil tightly with clear plastic, weighted down or dug in round the edges. Bryan O’Hara in No-Till Intensive Vegetable Culture has popularized this technique, which makes a great use for used hoophouse plastic film. Solarizing can produce temperatures of 125˚F (50˚C) whereas temperatures under tarps (see section on perennial weeds) will be more like 110˚F (43˚C). You may need only 1-3 sunny days to kill crop residues with solarization. Cover crops and weeds may take longer to die. The heat will not go deep into the soil in that short time, and so more of the soil life will survive than with tarping.
4. Reduce the strength of perennial weed roots and rhizomes
- Understand apical dominance: when a rhizome grows a green shoot, chemicals from that shoot prevent other nearby nodes on the same rhizome from sending up shoots.
- Act in a timely way – On long rhizomes, after a certain length, the dominance effect is too weak and another node can grow a shoot.
- Reduce the strength of perennial weed roots and rhizomes by frequent tilling or digging out.
- Beware tilling invasive “traveling” perennial weeds once and thinking you’re done – When rhizomes are cut into pieces during tillage, the apical dominance is lost and each piece can grow a shoot of its own.
- Consider tarping: after tarping the plot for two summer weeks, 3-4 weeks in spring and fall, and two months or more in winter, dig out or pull up all the weed roots still alive.
- Next comes a counter-intuitive move (from Jesse Frost ): sow or transplant an intensive valuable crop in the areas with the worst perennial weed pressure. Of course this will motivate you to deal effectively with the weeds!
- Pull out the pieces to dry on the surface – the depleted pieces of root or rhizome may die
- Or cultivate again when the new shoots have reduced the plant’s reserves (in the roots), but before they have grown enough to send energy back to the roots – it’s more effective than going almost daily after every sprig. Removing the shoots whenever the weeds reach the three- to four-leaf stage can be most effective.
- Late summer and fall turn out to be the best time for getting the upper hand over a wide range of common weeds, including Japanese knotweed, ragweed, Ailanthus, bindweed, curly dock and more. See Some weeds are best tackled late summer and fall Margaret Roach in A Way to Garden
Biointensive Integrated Pest Management
The weed strategies above follow the four steps of IPM: prevention, avoidance, monitoring and suppression.
1. Prevention: Focus on restoring and enhancing natural balance and resilience to create healthy plants and soil, better able to withstand attacks. Maintain soil fertility, good drainage and soil structure; plant resistant, pest-tolerant, regionally adapted varieties; grow strong plants; practice good sanitation,
2. Avoidance: The next stage includes actions to reduce the chances of a weeds taking over. These actions are also known as physical controls. Physically remove weeds. Use good crop rotations, remove weed habitat, deter weeds. Provide habitat for weed seed predators.
3. Monitoring: regularly inspect your crops, find out when conditions are right for an outbreak of particular weeds. Be prepared. Identify your weeds and choose good strategies for each type. Decide when it is time to act. How to identify your weeds – online guides
4. Suppression: When the prevention and avoidance strategies have been exhausted, control measures can be used to reduce damage of crops, while minimizing environmental risks. There are four types of sustainable bio-intensive control measures to choose from, starting with the least damaging to the wider environment:
- a) Biological control involves working to boost populations of existing resident weed seed predators. (For a few serious weed pests, like prickly pear, host-specific insect enemies are introduced)
- b) Microbial controls (bioherbicides) are plant-pathogenic fungi, bacteria, and viruses to kill weeds. Not common.
- c) Botanical control uses plant-based products for pest control. Examples include orange, clove and peppermint oils, and phytotoxic plant residues, such as root exudates from winter rye cover crops, and hay from sorghum, which inhibit germination of small seeds.
- d) Biorational controls (aka inorganic, mineral, controls) make use of manufactured products such as herbicidal soaps or strong vinegar.
Critical weed-free period
One important factor is to observe the critical period of weed control for each crop. This is the period when crops are most affected by competition, whether from weeds, sister seedlings or those of an intercrop. Seedlings suffer more than transplants from being out-shaded. Transplants are soon past their critical weed-free period, perhaps half of it before you even set them out. As well as the critical period, take note of the severity of drop in yield for the particular crop. A lot of the information below comes from The Living Soil Handbook by Jesse Frost, which I had the pleasure of reviewing recently.
- Small salad crops like arugula, spinach and baby lettuce mix, really need to be weed-free throughout their growth. Apart from the risk of being smothered and producing poorly, there is the risk of including bits of recognizable weeds in your salads.
- Bulb onions also benefit from being weed-free throughout growth. Like other narrow-leaved plants, they are poor competitors. Carrots also are very poor competitors – for most of us, the over-abundance of carrot seedlings in the row are as much of a threat as the weeds. Parsnips are similar, with the added challenge that they are slow to emerge.
- Peas do best with no competition, although, because they grow vertically, they can do OK with a companion crop such as spinach (or weeds!) a short distance away.
- Potatoes need 1-8 weeks after emergence free from weeds, although small weeds are not a problem and the process of hilling potatoes (needed to provide growing space) effectively deals with weeds.
- Beets need 2-3 weeks after emergence weed-free from direct-sowing. My experience is that beets are their own worst enemy, and the clusters of seedlings that emerge from each seed-ball should be singled as soon as possible. Yields can easily drop 1-5% with small-average weeds. Turnips also need to be competition-free for the first few weeks after emergence.
- Broccoli, cabbage, cauliflower, collards, kale and most Asian greens need 2-3 weeks after transplanting free of weeds.
- Sweet potatoes need 2-6 weeks free of competition after planting. Because it gets hard to wade in and pull weeds later, we try to keep them weed-free.
- Garlic needs 3-7 weeks from emergence free of weeds. If you plant in the fall, start counting in early spring when weeds start to grow again. Like most alliums, the narrow vertical leaves make it a weak competitor.
- Basil, cucumbers, squash, zucchini, lettuce and many other crops need four weeks from transplanting free of weeds. Be careful not to damage squash roots when removing weeds.
- Tomatoes need 5-6 weeks after transplanting free from competition, although they are fairly strong competitors later, and we routinely transplant our hoophouse tomatoes down the center of a bed of salad greens, progressively harvesting the greens over the next month. We have noticed problems only if we leave other crops too close for too long. Always prioritize the well-being of the new crop!
- Peppers need 5-10 weeks after transplanting free from competition, although the drop in yield is small (5%)
- Fava beans need four weeks from emergence free of weeds
- Direct sown kale needs 6 weeks from emergence weed-free.
- Okra requires 6-8 weeks after sowing weed-free. If you transplant okra as we do, half that period will be over by transplanting date.
- Beans are a crop that can generally out-compete weeds (losing only 3% yield from competition), but keeping the rows clean until the beans flower (about 6 weeks from sowing) will maximize yields.
- Corn needs about 7 weeks from seeding free of weeds (until there are 6 leaves).
- Eggplant calls for 8 weeks from transplanting free of competition.
- Leeks, another weakly competitive allium, need 12 weeks post emergence weed-free. If, like us, you transplant leeks at about 10 weeks after sowing, this translates to hoeing the beds of transplanted leeks a couple of weeks of transplanting.
I won’t say more about this here. Click the link to read previous posts.
Mulches are a big asset in weed control. Organic mulches also add biomass to the soil. Remember not to use organic mulches around warm weather crops for their first month, as they need warm soil to grow well, and insulating mulches keep the soil cold.
See our experience with Biodegradable plastic mulch
Read Organic Farming Has A Plastic Problem. One Solution Is Controversial about the controversy surrounding biodegradable plastic in Organic Farming.
Summer cover crops smother emerging weeds, prevent weed seed germination, between a spring food crop and a summer or fall one. Winter cover crops smother emerging winter annual weeds. Good cover crops for this purpose: sorghum-sudangrass, pearl millet, winter rye, wheat, barley, oats, buckwheat, brassicas (beware – rotation, bugs), lupins, red clover, subterranean clover, berseem clover, soybeans, cowpeas.
Ida Gold mustard (Sinapis alba) contains a gluscosinolate, ‘sinalbin’, a non-volatile compound that has shown the ability to inhibit weed seed germination. Tillage radish has a similar effect. The cover crop needs to be mowed and tilled in. Solarization after incorporating mustard is known as biofumigation.
Cover crops also improve the soil for crop production.