Weed Management

Fall 2008
Pigweed and Lambsquarters
Don’t become lax in the fall and let summer annual weeds, such as pigweed (left) and lambsquarters go to seed. Learn about the life cycles of different weeds so that you know when and how to control them. English photo.

by Eric Sideman, Ph.D.

The most important distinction between organic and conventional growing is that organic growers have replaced inputs, whether organic or conventional, with management skills. This is easy to see with insect and disease management, but it is also important with fertility and weed management. Once you become a serious manager of your fields, you recognize next season’s problem while harvesting late summer and fall storage crops.

Weed seeds falling, and perennial weeds storing food for the winter – just like you, are common late summer and fall sights and are easily ignored as you convince yourself that you will do better next year; but getting a handle on weed control begins with halting their growth and reproduction in late summer.

The key to keeping weeds from getting ahead of you in the spring is to know which ones are getting the upper hand in the fall; and figuring out the best practice for fall care depends on knowing the biology and ecology of your farm or garden community. Who is there and what are they doing?

To Till or Not?

Managing fertility also begins in the fall. Fall cover crops can fix nitrogen to be used by crops next spring or can catch nutrients that are already in the soil and hold them until next season. (See MOFGA’s Fact Sheet #10, “Using Green Manures.”) Managing insects and diseases may begin in the fall, depending on the species of pest, how that pest overwinters, and other details. (See Resource Guide for Organic Insect & Disease Management.) Both the Resource Guide and the Fact Sheet are available at www.mofga.org.

Managing weeds begins in the fall, too. Preventing weeds from going to seed in your field is probably the most important practice. Many summer annual weeds, such as lambsquarters and redroot pigweed, produce millions of seeds. You may have done a great job of weed control during the growing season but become lackadaisical late in the season, thinking, “What harm can that weed do now?” But letting just one of those plants go to seed can ruin your vacation next summer, so think about hundreds of them going to seed. Lambsquarters and pigweed seeds can survive a decade or two without germinating and then germinate suddenly, when conditions are right (if they are brought to the soil surface when it is warm and moist). So, relaxing in the fall can create a big headache way down the road.

If your garden is loaded with summer annual weeds that have not gone to seed yet, and it is early enough to plant a cover crop, then I suggest tilling and planting a cover crop. You can do this in bits and pieces of your garden where no crops remain, and do bits more as the crops are harvested.

Perennial weeds are different. Quackgrass, for example, spends the summer pumping food down to a rhizome, an underground stem that serves as a storage organ. Other perennials, such as curly dock or Canada thistle, store food in a large taproot. In either case, seeds play a very small role in the weed problem, as most reproduction is by spreading rhizomes or resprouting taproots.

If your garden is loaded with perennial weeds, then tilling once and planting a cover crop at the end of the season will do little good. This is a great example of management skills. You have to know which weeds you have and how they live. For example, quackgrass will thrive if you till your garden in early September and plant something like oats or winter rye. The quackgrass will send up new shoots from all of the tilled, chopped quackgrass rhizomes and will grow along with your cover crop.

If your garden is loaded with quackgrass, I recommend tilling in late summer and again every time the quackgrass gets about 2 inches tall. Don’t let it get taller than 2 inches: You want the plant to use up the food that it stored for the winter – which it does each time it grows to 2 inches. If you let it get much larger, it will be big enough to replenish the food that it used from the rhizome. If you repeatedly till and let it grow again only a little, you force the plant to use a lot of food and send the plant into winter with insufficient stored food. When you think the ground will freeze in three or four weeks, plant a cover crop at double the seeding rate to make up for the short fall it will have to grow. You won’t have much of a cover crop, but the quackgrass will be very weak and easier to control in the spring.

Winter annual weeds, such as shepherd’s purse and chickweed, are yet another story. They germinate in the cool soil of fall, go into a resting period for the winter and then grow again and set seed in the spring. In fact, both these weeds are quite shade tolerant and even grow happily in the shade of a cover crop. So if you till in late August or early September and plant a cover crop, you actually favor these weeds and, after years of that practice, you may end up with lots of them. Shepherd’s purse seeds germinate in soil that is below 60 degrees, so they lie dormant all summer and germinate in the fall, even after you tilled and planted the cover crop. Then they live happily all fall. In late fall they form a rosette that passes the winter and sprouts a stalk of flowers in spring.

If winter annual weeds are your problem, then till your garden very late in the fall, when it is, in fact, too late to plant a cover crop. This will kill the overwintering rosette of winter annuals. To protect your soil from pounding winter and early spring rains, cover it with a mulch of straw or leaves.

Sometimes fall tilling is not recommended at all. If you have enough weed growth or crop residue to protect the soil, and it is too late in the fall to establish a good cover crop, then you probably are better off leaving the weeds alone so that you don’t have bare or nearly bare soil going into winter.

So the question “To till or not” has no simple answer. Organic growers want to protect the soil and use management skills to avoid pests. Weeds are opportunists that survive best when we give them an opening. Good garden managers control weeds by learning about their weaknesses.

Eric Sideman is MOFGA’s organic crops specialist. You can contact him at 568-4142 or [email protected].

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