Along with fertilizer, weed control is a relatively large expense when it comes to variable costs on a farm. A weed by definition is a plant growing where it is not wanted. Our soils have a huge seed bank waiting for an opportunity to grow. Some are drought tolerant, others like cool conditions, while some need fire to break dormancy. The soil wants to be covered in all situations and plants have adapted to be ready.
If we supply enough diversity and have plans to keep the soil covered, most of the seeds in the soil seed bank will stay dormant. In problems areas like saline spots, plants like kochia and foxtail barley have adapted to grow and dominate these areas. We can manage these areas like the rest of the field and allow the weeds to flourish, or we can create a zone to manage. By adding species like safflower, sugar beet, radish, sweet clover, Japanese millet, sorghum sudan, and sunflowers, we can start to reclaim these areas. The area may not get reclaimed in one year, but the reclamation process has started.
Knowing what weeds require to be targeted is crucial. When and where they grow, competitive nature, and when they are susceptible to disturbance or competition. In most cases, using fast growing plants will smother out weeds if the cover crop can get ahead of the weeds, and the cover crop has enough plant density to cover the ground quickly.
Crops like radish, turnip, collards, and buckwheat grow quickly in the fall and will cover the soil in a hurry, leaving little room for other weeds. If weeds do start coming through the cover crop canopy, the weeds should be clipped, making them regrow while the cover crop is allowed to continue growing.
During the summer, using crops like Japanese millet, winter triticale, fall rye, and sorghum sudan grass all grow in the heat of summer. They are all able to be cut to manage weeds and top growth. They all will regrow quickly once established. Fall rye needs to be managed more carefully because it has a very good chance it will over winter.
When we replaced canola with cover crops, one of the first things we noticed was improved emergence of our winter triticale in the fall, and in the spring, less weeds. The regrowth in our cover crop created a nice green mat that the triticale grew through, but when the cover crop winter killed and was laying on the soil surface, there were few weeds that grew through it. By June, the residue disappeared, exposing the earthworm burrows in the soil surface.
When plants start to decay, they release organic acids. This acid, when at high enough concentration, will kill any seedlings attempting to grow through it. Established plants are not affected. Some plants, like fall rye, give off allelopathic chemicals through their roots to prevent other seedling to establish. Dandelion is another plant that has chemicals in its roots to prevent other plants to establish around it. That is why it is important to control dandelions before they get well established.
One of the concerns of allelopathic residues is some species are more susceptible to the chemistry than others. Under dry years, the chemistry may break down slower, allowing weeds to sneak through. Normally, the smaller the seed the more susceptible to allelopathic residuals.
Organic acids on the other hand are just released as the rotting process proceeds. Using a high rate of radish in the fall, getting a competitive stand in the fall to smother weeds, then getting the rotting leaves to control weeds in the spring acts as two weed control passes. If minimum till seeded, we have seen effective weed control from rotting leaves until middle of June.