Weed management in organic systems

by Sally Colby

Organic growers list weed management as their top production challenge, but having a plan and planning ahead can help. Sam Hitchcock Tilton, horticulture instructor of Lakeshore Technical College in Wisconsin, advises growers to look at the big picture and manage weeds at each stage of growth throughout the season.

A key factor in organic weed control is the size difference between the crop and the weed. Growers should consider weed and crop root depth, top height, weed seed population, germination rate and available tools when making decisions about when and how to manage weeds.

Effective weed control begins with a low weed seed bank. Tilton said if a perfectly aligned cultivator kills 90% of 10,000 weeds in an area, 1,000 weeds still remain. Most weeds are anchored in the ground with a certain amount of force, with some having stronger root anchorage than others, and weeds may be taller or smaller than average. A uniform crop of plants with similar anchorage force via the root system should result in a greater gap between the weed and the crop, resulting in more weeds killed and minimal crop loss.

Tilton described the false seedbed as an important weed control technique. “Tillage is a wonderful way to stimulate germination with fresh, oxygenated soil,” he said. “Once we till, we’re stimulating weed seed germination, but we wait to plant. We want [weed] seeds to germinate, then we’ll either flame weed or till shallowly one more time to kill them.” By the time the crop is planted, fewer weed seeds will remain at the top.

Working depth is important for both the false seedbed technique and when tools are used later. Tilton explained that most weeds germinate from the top 1.5 inches of soil, but if soil requires deeper tillage and tools are not precise, weed seeds from deeper in the soil are brought to the surface and cultivation results in reseeding the seed bank.

Flame weeders are often used to control weeds between rows, and Tilton said the technique is valuable in a false seedbed. “When a flame weeder is used, we call it a stale seedbed,” he said. “A flame weeder doesn’t disturb the soil.” Newly germinated weeds are killed by the flame weeder, but the crop is undisturbed. Without tillage, no new weed seeds are brought to the surface.

A uniform seedbed contributes to both a uniform crop and lower weed population.

“Seeds want oxygen and moisture to germinate, so the better tilth soil has, the more even conditions seed will have,” said Tilton. “Clods of soil cause the planter to jump around, which decreases precision later on. Seedbed preparation is the last chance to do any intensive tillage. Any clods that are left are sort of like a coral reef for weeds – weeds to grow inside the clod, and the clod shields them.”

In organic systems, seeds should be planted at the most uniform depth possible so they emerge at the same time and are rooted with the same force. Tilton said since most population recommendations are for conventional growers, organic growers make rate adjustments. “We’re applying steel to those plants,” he said. “Increase conventional seeding rates for row crops by 5% to 10%, knowing we’re going to take some of that crop out with weeding tools.”

Another consideration for weed management is row spacing. Closer row spacing allows faster closing of the crop canopy, resulting in fewer weeds. Tilton said organic farmers insist on half-inch precision for best weed control.

“If they’re planning 30-inch rows, they want seeds at exactly 30 inches,” said Tilton. “Maybe 30 and one-half inches, but no more – all the mistakes add up.” Tilton added that it’s a good idea to check row spacing measurements prior to each planting because spacing can quickly become inaccurate if the planter hits a rut or a rock. He recommends measuring between the seed chutes since that’s where the seed will drop to the ground.

In discussing improvements in cultivators, Tilton explained that crop shields both protect a crop and manage the amount of soil being thrown in the row. Shields can be set so soil is being tucked in underneath, preserving the crop. Coulters act as a rudder and help lock the cultivator in place. Rigid shanks with a trip spring will move if the implement hits a rock.

When adjusting the sway bars, decide whether the bottom of the sway bar should be tight or loose. “For cultivation, we generally want a few inches of play in the sway bar,” said Tilton. “As the tractor jumps around, the cultivator stays on target.”

Knives can play an important role in weed removal if they’re sharp and set at a shallow angle to the ground. Tilton said European farmers maintain extremely sharp edges on knives, which allows a defined, shallow cut that moves less soil. Properly sharpened and set knives can be used when the crop is very young with minimal soil disturbance.

Tilton urges farmers to consider using finger weeders in conjunction with knives. The knives cut a sharp line in the soil, followed by plastic finger weeders that crumble the soil. Rather than flicking weeds out of the row, fingers work at a depth of about one inch, breaking capillarity. The result is less moisture available to weeds and sufficient moisture retention for crop roots deeper in the soil.

Tilton said finger weeders work best when mounted on a spring-loaded arm, and suggests choosing fingers that flex up and down to adjust to terrain level and soil moisture while maintaining consistent pressure in the soil. Tilton added that while finger weeders can solve problems, it’s important to pay attention while working because they can also grab stones, pull them along and remove a crop.

Guidance for cultivators has improved greatly with tools such as Real-Time Kinematic (RTK) positioning, a satellite navigation technique that enhances precision positioning data from satellite-based positioning systems. “Or we can have camera guidance on the cultivator,” said Tilton. “Instead of RTK telling where things were planted, it’s reacting to where things actually are.”

Another option is mirrors that allow the tractor driver to view what’s happening on the ground. Some farmers opt for a GoPro camera to cell phone via Bluetooth, which Tilton said helps growers learn how to fine-tune their cultivator. A more basic tool is a stick – a piece of rebar attached to the body of the tractor and used as a sight guide to check the position of a rear-mounted cultivator.

Tilton reminded growers that organic cultivation includes multiple steps, and for best results, each step should build on the previous. “We might start with a false seedbed or a stale seedbed if we have a flamer before planting,” he said. “After planting, we might use a pre-emergence flex tine or rotary hoe. After emergence, we might use a flex tine or rotary hoe again. Later, finger weeders, and for the last cultivation, we throw soil into the row in anticipation of row closure.”

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