Weed Control

Summer 2007
Three MOFGA growers – Matt Williams, Dave Colson and Rob Johanson – told a large, enthusiastic audience about their organic weed control methods at a MOFGA-sponsored talk at the Maine Agricultural Trades Show in Augusta in January.

Winter Grains and Summer Fallow

Matt Williams grows oats and wheat at his Aurora Mills & Farm in Linneus, in Aroostook County, processing the grains in his mill for human consumption. He noted that growing winter cereals is the best way for him to control weeds.

“When you start to grow a crop, you start to select a weed [that goes along with that crop],” said Williams. For him, perennial grasses such as quackgrass compete very successfully with grains. One way he controls quackgrass is to plow the field during a summer fallow season – usually in July – as shallowly as possible, to bring up the quackgrass rhizomes and let them dry out. When the persistent weed starts to grow again, Williams disks or harrows again, a couple of times during the course of a month, to deplete the plants’ energy. Then, when he’s ready to sow his grains, he plows 6 or 7 inches deep, burying the weakened quackgrass so deep that it doesn’t have the reserves to surface again.

Williams uses the stale seedbed technique to fight other weeds. He says to have the bed ready at least 10 days before you’re ready to sow a crop, and do some kind of pass to control weeds just before planting – but without disturbing the soil below 1 inch.  Then, plant into a firm seedbed, rolling the soil. The most important thing he does, says Williams, is to “plant twice at slightly less than the recommended rate, in a manner that the rows intersect each other so that the spacing between rows ends up one-half of the grain drill’s spacing. Then you will have 150% to 200% of the normal seeding rate with narrow rows, allowing the canopy to close out the potential for weeds to grow. You want the canopy to close in as fast as possible.” He goes over the field with a finger weeder right after planting.

“Then check the soil,” he recommends. “Rub the soil surface. When you see small white threads of emerging weeds,” cultivate again. “It’s best to control weeds before the cotyledon stage.”

Williams also uses a rotary hoe, only post-emergence, in conjunction with a Lely cultivator if the soil has set up hard. The rotary hoe breaks up compaction; following it with the Lely hoe does a better job, but the Lely hoe alone doesn’t work so well.

After harvest, Williams cleans his crop and burns residual weed seeds and undersized grains in a corn burner, further reducing the weed seed bank in his fields.

Cover Crops, Rotations, Weeding and Mulching

Dave Colson of New Leaf Farm in Durham, Maine, says that weed control “really starts with getting the weed seed bank under control before planting vegetables. Mechanical cultivation may get 70% or so, but if you have a heavy weed bank, that leaves a lot.”

Colson uses short- and long-term rotations on his farm to control weeds. Rotating with grains, however, didn’t work well for him. “When we tried to go into grain production, it ruined everything. It got too weedy.” They were jeopardizing the $20,000 to $30,000 worth of vegetables they could grow on their 3 to 5 acres for $400 to $500 worth of wheat. When growing grains and potatoes, which take larger-scale field crop work, you can get away with controlling a lower percentage of the weed population, Colson explains; but if you’re following wheat with carrots and you’ve built up the weed seed bank, you’ll have problems.

Now, cover crops, rotations, timely weeding and mulches control weeds. Because an intensive vegetable production system can be so busy, it’s easy to lose track of time and not get in the field to weed. Now, when Colson puts a planting date on his calendar, he adds a weeding date two weeks later to remind himself to get into the field.

Colson divides his crops according to the four ways he works with them:
1. transplanted crops that grow for a long season, such as solanaceous crops and cucurbits;
2. direct-seeded or transplanted crops that grow for a long season, such as carrots (direct seeded) and onions (transplanted);
3. direct-seeded crops that grow for a short period, such as salad greens and radishes;
4. transplanted crops that grow for a short season, such as head lettuce and broccoli.

He is using black plastic to control weeds in a lot of crops, including all of his crucifers, tomatoes, cucurbits, peppers, basil and more. “The economics of paying a crew to do ‘heroic weeding’ in broccoli…we would have to charge $5/lb. for the broccoli.” His broccoli is a single-harvest variety that comes out of the field after harvest. Drip irrigation is placed under the plastic for greenhouse-grown crops. “Mulch layers are not that expensive,” says Colson; “it’s much cheaper than paying people to weed.”

Outdoors, Colson uses 1.5 mil black plastic, embossed mulch with hay mulch between the rows of plastic. He hasn’t seen any advantage to using the more expensive IRT mulch on his farm. The hay mulch, from hay grown on the farm and cut before it goes to seed, raises organic matter in his soil more than using cover crops would. When weeds start coming through or from the mulch, he runs a lawnmower over it to keep them from going to seed.

Winter squash is planted so that the space between beds is the same width as his disk harrow, so he can harrow it before the squash spreads. His crew then weeds the edges of the plastic, and “then we’re done with weed control – squash fills the area.”

As an aside, Colson notes that he plants cole crops under floating row covers that are held up with wire hoops, since he had too much damage to the crops without the hoops. Cucurbits, however, seem to be able to hold up row covers without being damaged.

He also notes that he’s been transplanting about 1/4 acre of sweet corn for the past few years because the weather has been too wet for direct seeding. Now he’s transplanting that corn into black plastic mulch.

Salad greens bring more than 50% of the income to New Leaf Farm. They’re seeded in double rows in beds and clipped with scissors. As Matt Williams, does, Colson uses the stale seedbed method of weed control: He prepares his beds for greens 10 to 14 days before seeding them, then goes over the beds with a flame weeder just before sowing greens seeds. He covers the bed with fabric row covers, and 10 to 14 days later pulls the fabric back and does a quick hoeing and hand weeding, installs wire hoops and puts the row cover over the hoops to minimize abrasion on the plants when wind blows the cover.

Colson hills potatoes, which helps control weeds. Spinach and green beans are cultivated with basket cultivators early in the season, and early in the day so that the weeds dry out.

The late summer planting of broccoli and cauliflower at New Leaf Farm is also grown on black plastic. Colson had had problems with Alternaria on this crop, but the combination of black plastic and hay mulch reduced the problem significantly. He now gets as much cauliflower from a 50-foot bed covered with plastic as he did from a 200-foot bed without plastic.

Colson likes to keep some ground out of vegetable production and unturned for a couple of years to maintain good soil health. He likes red clover and finds that when he rotates two years of clover with two years of vegetables, he gets good weed control; but two years of clover and one year of vegetables enables weeds such as quackgrass to become established. He does use shorter-term smother crops, such as buckwheat, after an early season cultivation. Tilling the buckwheat in provides another chance for weed control through cultivation.

Minimizing Deposits to the Weed Seed Bank

Rob Johanson and Jan Goranson of Goranson Farm in Dresden grow mixed vegetables for farmers’ markets, for their farm store and for a CSA. Their weed control strategy is to reduce the weed seed bank and to control weeds seeds during the growing season.

They take land out of vegetable production for two years to reduce the weed seed bank and add nutrients. Hairy vetch and oats or vetch and winter rye (depending on the rotation) are planted around mid-August, and Japanese millet or sorghum-sudangrass are grown in the summer to add organic matter to their fine sandy loam.

Johanson cannot use the stale seedbed method to control weeds in his early crops, because the two-week delay would cut into production for the early farmers’ markets.

For potatoes Johanson recommends cultivars such as Green Mountain and Kennebec, which have vigorous top growth that shades out weeds.

Black plastic, for weed control and earliness, covers about 1 1/2 acres of the farm’s most valuable crops, such as tomatoes, green peppers, eggplant, summer squash, zucchini and cut flowers.

For mechanical weed control, Johanson uses a belly mounted blade that skims the soil surface very shallowly; basket weeders in small seeded crops; and Lilliston rolling cultivators if more aggressive weeding is needed. “We try not to get to that point,” says Johanson. Hilling corn and potatoes also helps with weed control. Lely tine weeders are used primarily on potatoes when they’re 6 to 8 inches tall.

Flame weeding is done on some stale seedbeds (in later-season crops) and just before some crops emerge. A piece glass is placed over a small section of an area seeded to, for example, carrots. The seed will germinate and emerge under the glass a day or two before the rest of the crop breaks through the soil. This is the time for a last flame weeding. Goranson plans to flame weed sweet corn and potatoes this year.

Zone tillage is used to grow winter squash and pumpkins. Vetch and oats are planted in late summer; the oats winterkill, and in the spring, Johanson cultivates 8-inch-wide strips 6 feet apart in the vetch. He transplants winter squash and pumpkins into the cultivated strips; he cultivates the plot and walks through it once to hand hoe it; then the crop expands and shades out the vetch, so that the vetch doesn’t even have to be mow-killed. “I didn’t turn all that soil and lose carbon to the atmosphere,” says Johanson. “I’m going to do more of that.”

Johanson transplants sweet corn, an $11,000/acre crop at Goranson farm. In addition to earlier production, transplanting uses a lot less seed, and fewer plants are lost. (Gardener Adam Tomash, who was in the audience, noted that transplanted sweet corn pollinates and yields better because the crop has germinated uniformly and is, therefore, at a more uniform stage of growth than direct-seeded corn, with all plants shedding pollen at about the same time.) Johanson grows only varieties that produce one ear per stalk. Early varieties are set 6 1/2 inches apart; midseason – 7 1/2; and main season, 8 1/2 inches. (Colson plants his corn 8 inches apart in double rows.)

 – Jean English

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