Weed Management in Organic Corn and Edamame Soybeans

Weed Management in Organic Sweet Corn and Edamame Soybean


Organic growers need reliable weed management techniques. Organic standards prohibit the use of synthetic herbicides, but allow a range of tools (physical controls), and natural herbicides (chemical control), as elements of an integrated organic weed management program. These are usually used in combination with cultural controls, such as crop rotation and cover cropping.

Corn Gluten Meal

Corn gluten mealCorn gluten meal, sold as a herbicide for organic growers, was  one of the materials used for weed management in this field-scale study. Click here to see results of our complementary studies evaluating its effectiveness.

The success of a particular weed management technique may depend on other elements of the organic system. Primary tillage, for example, affects not just soil structure and quality, but also the distribution of weed seeds through the soil profile. Spading machines, used throughout Europe, are becoming popular among US organic growers as a replacement for the moldboard plow and roto-tiller. By thrusting blades in and out of soil, they incorporate cover crop residues and loosen soil without inverting the plow layer.

In 2004 we established replicated research plots to test the effectiveness of six organic weed management strategies used over four growing seasons in combination with either conventional tillage or spading. Our main crop were sweet corn (Zea mays L.) and edamame, a type of soybean (Glycine max [L.] Merr.) that is boiled and eaten green as a vegetable.


    1. Compare the abilities of three cultivation methods, two flame weeding methods, and a natural herbicide to control weeds in organic vegetable row crops.

    2. Compare the effects of conventional tillage and spader tillage on weed control in organic vegetable production, and test for interactions between the effectiveness of primary tillage and weed management techniques.


Three replicate blocks of twelve plots (5 x 10 m) were established using a randomized complete block design on land in transition to organic production at the Kentucky State University Research Farm (Fig. 1). Each treatment was a unique combination of one of two primary tillage tactics (Table 1) and one of six organic weed management strategies (Table 2).

Experiment site

Figure 1. Experiment site, 2005.

Table 1. Primary tillage treatments.
Treatment Description Frequency Picture
1. Conventional (moldboard plow + roto-tiller) Moldboard plow inverts soil and roto-tiller breaks clods. Single pass with plow, double pass with roto-tiller Moldboard plow
2. Conservation (spading machine) Spading machine loosens soil without inverting. Double pass Spading machine

Table 2. Weed management treatments.
Treatment Description Frequency Picture
1. Control Hand hoeing Weekly (weather permitting) Hand hoeing
2. Rolling cultivator Tined wheels disturb soil between rows and throw it into rows. See video from this study below, or from Four Corners Farm, Newbury, VT. Weekly (weather permitting) between crop emergence and row closure Rolling cultivator
3. Spring-tine weeder Tines comb soil between rows. See video from this study below, or from Edgewater Farm, Plainfield, NH. Spring-tine weeder (Williams toolbar)
4. Flame weeder Propane torches heat weeds between rows, rupturing cells. See video from this study below, or from Killdeer Farm, Norwich, VT. Flame weeder
5. Stale seedbed Full coverage with propane torches Once, two weeks after primary tillage and just before crop emergence Stale seedbed
6. Corn gluten meal Soil amendment with a herbicidal byproduct of corn processing Single treatment (100 g/m2) at crop emergence Corn gluten meal

The main crop alternated between sweet corn (2004 and 2006) and edamame soybean (2005 and 2007), with winter cover crops of rye and vetch. Each plot contained six crop rows, spaced 90 cm apart.

Data collection


Crop yield and weed pressure

Yields fell with increasing weed pressure. The rolling cultivator was the only weed management treatment that controlled weeds in the crop rows. It gave yields that did not differ significantly from the weed-free control plots.The stale seedbed and corn gluten meal treatments suffered the worst weed pressure and gave the lowest yields. The spring-tine weeder and flame weeder gave similar moderate levels of weed control and crop yield.

Corn and soybean yield and weed pressure

Edamame soybean was more sensitive to weed pressure than sweet corn, and produced lower yields overall. The relative effectiveness of the various weed control treatments was similar for sweet corn and edamame soybean.

Primary tillage with a spading machine did not affect weed pressure or crop yield relative to conventional tillage with a moldboard plow and roto-tiller.

Weed diversity declined over the four years. Smooth pigweed (Amaranthus hybridus) accounted for an increasing proportion of weeds with successive seasons.

Analysis of soil quality data continues.


The rolling cultivator offered the best weed control among the alternatives tested. It is also less expensive than most of the other treatments tested. The spring-tine weeder was similar to the flame weeder in terms of efficacy, but was less expensive, lighter, could be operated at faster tractor speeds, and did not require propane for each treatment. Treatment with corn gluten meal was the most expensive and least effective option. Sourcing corn gluten meal that was not made from genetically modified corn became increasingly difficult over the course of the study, and added to the cost of this treatment. The use of corn gluten meal made from genetically modified corn could render land ineligible for organic certification for a period of three years.

Most organic growers do not rely on a single weed management tactic, but integrate a range of tactics. Using a single tactic on a piece of land, as in this study, emphasizes the differences in effectiveness between treatments but is not sufficient for an integrated organic weed management program. A combination of treatments will be more effective.

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Updated April 6, 2009

College of Agriculture, Food Science and Sustainable Systems