The Weeds Can be Won - The Southeast PA Organic Crop Producers Network
Posted: January 20, 2011
A summary of the December 15, 2010 study circle.
Know your weeds and use their weaknesses against them. Scientists call it ecological weed management. But, it comes down to learning as much as you can about weed biology in order to avoid the conditions weeds love, and time your management practices to effectively utilize their weaknesses.
One piece of knowledge that we can use against weeds is their germination requirements. Some weeds like common ragweed, lambsquarter and smartweed come up early in the spring. Other weeds like redroot pigweed germinate more frequently in mid June into July. It turns out “weeds are programmed to germinate under specific soil temperature, moisture and even light conditions,” Penn State Weed Scientist Dave Mortensen explained to us. Take for example a yellow foxtail seed. Yellow foxtail starts to germinate when the soil reaches 50 to 59 degrees. Pigweed on the other hand likes soil temperatures around 75 degrees. Most weed seeds also have moisture requirements. That means there might be only a short period of time when both the soil temperature, moisture and even light conditions are perfect for that seed to germinate. That is why the number of foxtail or other weeds is different every year. You might have the right moisture and temperature conditions at the same time and get a huge flush of weeds, or you might not have enough moisture until the soil temperature is warmer than what the seed really likes. Unlike crop plants weed seed germination can be “turned off” once the germination temperature range is exceeded.
Soybeans on left were planted into a field plowed and disked 12 days after soybeans on the right. Where beans were planted later weeds had already germinated and were killed by primary tillage. Photo by Dave Mortensen
The very time we work the field and plant will determine what weeds we will have there. For example, the longer we wait in the spring the more likely that most of the common ragweed, common lambsquarter, smartweed and foxtail have already germinated. Then when we do our primary tillage we will knock them out and they won’t be a problem in the next crop. For example in an experiment at Rodale a number of years ago soy beans were planted into ground tilled on May 28th or June 10th. Where beans were planted later they were almost weed free. In ground tilled in May ragweed, redroot pigweed, and giant foxtail took over.
The trick is to change up the types of management that you do. Then you won’t get the same weeds each year and build up a huge weed seedbank. “Diversity in crop rotation and tactics is what works in weed management,” Dave Mortensen told us.
Rotation is #1 to manage weeds in organic agronomic crops. Growers shared their rotations with each other during the study circle. Every grower has a different rotation but growers happy with their rotation had a few key things in common - crop diversity and hay. For example Wade Esbenshade’s rotation is hay (3yrs) > corn > soy > spelt > clover >corn> soy > spelt > hay. Bob Keller’s rotation is alfalfa (3yrs) > corn > soybeans > wheat > hairy vetch > corn > soy > wheat. Compared to non-organic growers who may only grow row crops of corn and soy, the organic rotations are diverse. The row crops harbor the most weeds, growers said. “Soybeans are our biggest challenge,” Wade said to the nod of many other heads. In a diverse rotation you always keep the weeds guessing and don’t give any one weed a chance to build up. Perennial hay crops act to smother weeds and repeated cutting starves the root system on problem perennials like thistles.
Weed Control in Soybeans is a challenge in organic systems but growers identified a number of things that work well for them.
Get ‘em up quick – later planted soybeans emerge quickly when soils are warm. These vigorous plants quickly start to form a canopy and compete with weeds. “Planting into good conditions where soybeans will emerge within 7 days seems to be almost essential,” Wade Esbenshade told us. Focusing on soil moisture, soil temperature and planting depth can help you achieve this quick, even emergence.
Focus on timing – “Mechanical cultivation is your #1 priority for the 1st two weeks after emergence,” said Bob Keller. One grower goes in 3-4 days after planting soybeans with a flex -tine cultivator. Ideally the next day the beans are up. He tine weeds again when beans are at the 2nd trifoliate. Later he will do two or three passes with a row cultivator. Often the first two row cultivations are one after another with about three days in between to get any skippers. It is important to do that first cultivation before the weeds are even up. They call it the white thread stage because when you disturb the soil you will see a small, white, threadlike weed seedling that has not yet broken through the surface. These tiny weeds die easily when the hooked metal tine of a tine weeder flicks them out onto the soil surface.
Tine Weeders vs Rotary Hoes
Both tine weeders and rotary hoes are blind cultivation tools. They disturb 100 percent of the soil without regard to where the crop rows are. These are important tools to get the weeds in the row.
Tine Weeder – Photo by Klaas Martens
Tine weeders, or flexible harrows, use rows of fine teeth to bury weeds or hook their roots, dragging them out of the ground. Most tine harrows are either drawn by a toolbar and suspended from chains or attached to ‘U’ shaped pieces, or “wishbones,” that can self-level laterally and be leveled from front to rear with a hydraulic top link that tips the toolbar back and forth. Tine weeders come in many shapes in forms. In general tine weeders allow us to knock out weeds without a large amount of soil disturbance. The less we disturb the soil the fewer weeds we bring up to the surface.
A rotary hoe has rows of fingers with small cups that rotate and pull weeds out of the ground. But they do not flex and bounce around so they tend to create more skips. Growers agreed that tine weeders are generally more effective. The exception is when there is a hard crust on the soil, which a rotary hoe can break up more effectively.
Does Primary Tillage Affect my Weed Control Later?
The answer from this group was a resounding yes. With a show of hands we saw that almost all the producers use a moldboard plow as shallow as possible. Tim Bock tried chisel plowing in a research study and it was a dismal failure. An alternative to inversion with a moldboard plow might be European style plows which turn the soil on its side instead of completely inverting.
Wade Esbenshade just recently got a “Kvernland” European style plow that allows shallow tillage. Instead of completely inverting the soil, it turns it on its side – integrating the cover crop but not burying it where it might not get enough oxygen to break down.
Roller-crimper rolling down vetch. The vetch will be no-till planted to corn. Photo by Dave Wilson.
Rolled Down Covers for Weed Suppression
Growers plant a fall cover crop mixture of rye and vetch or other appropriate cover crop and roll it down in the spring when covers have reached sufficient maturity to ensure cover crop kill. Then field crops are no-till drilled into the resulting thick mulch. The system that Tim Bock uses is rolled down rye for soybeans. He moldboard plows in late summer, manures and plants rye in September after oats, wheat or hay. Then he rolls down the rye in mid May to plant beans. “I have practically no weeds with this system,” Tim told us. He does not do any cultivated beans now.
The system that Owen uses at Rodale is hairy vetch for corn. They usually plant the vetch in mid Aug – Sept after wheat at 30 lb per acre. The wheat was plowed under for the vetch. They roll down the vetch in the spring when the vetch is in full flower and plant corn in late May/June. When asked how often this system works Owen said “about 1 out of 3.” They have not worked all the kinks out. Some years there will be no weeds. Others the weed pressure is high.
This is the first of three study sessions for organic crop producers. Look for next month’s session on soil management.