Action Thresholds for Weeds in Peppers

Field experiments were conducted to determine the influence of emergence time and density of redroot pigweed, large crabgrass, and hairy galinsoga on the yield of bell pepper. Weed densities of 1, 2, 4, 8, 16, and 32 plants per 3 feet of row were established for each weed species. Weeds were selected from the natural population and were allowed to establish in a band extending 6′ on each side of the crop row. Unwanted weeds in the row middles were removed by cultivation while those in the row were removed by hoeing or hand pulling. Weed populations were allowed to establish immediately after transplanting in half the experiment. In the other half, all weeds were removed for two weeks, then weeds were allowed to establish.

Both weed density and emergence time relative to pepper transplanting affected the magnitude of yield loss. Yield loss ranged from 25% to 97.7% for early emerged large crabgrass, and 0.5% to 61.4% for late emerged large crabgrass within the range of established seedling densities. Yield loss due to redroot pigweed ranged from 7.4% to 65.9% for early emerged seedlings, and from 0.4% to 41.2% for late emerged seedlings. Early emerged small flower galinsoga caused up to 54.8% of yield loss, while the late emerged seedlings didn’t seem to have much influence on yield loss.

Action thresholds occur when the value of the crop losses due to weed interference exceeds the cost of weed control measures. Therefore, action thresholds are different for different farms and even different fields. Peppers grown for retail sales at your farm stand will have a higher value than those grown for wholesale markets. Peppers intended for early or late wholesale markets may have a higher value then mid-season crops. The yield loss multiplied by the anticipated value of your crop will give you the dollar value of the yield loss, which can then be compared to the cost of additional weed control measures. If the value of crops loss is greater than the cost of additional weed control measures then the weed control measures should be applied. If the value of crop losses is less than the cost of additional weed control measures the use of additional weed control measures is not justified.

Weed emergence prediction experiments were conducted to determine the suitability of using a nonlinear poikiltherm rate equation to describe the relationship between germination and temperature, and a Weibull function to fit the cumulative seed germination for three annual weed species, redroot pigweed, lambsquarters and large crabgrass. Temperatures ranges from 50o F to 94o F were evaluated at 5o intervals. Temperature influenced the duration of seed germination of three weed species, with the relationships of median germination time and constant temperature forming a curve that refutes the degree-day concept. Median times ranged from 14.2 days at 50o F to 3.8 days at 88o F for lambsquarters, and from 9.9 days at 56o to 2.5 days at 94o for crabgrass. No germination was observed at 50o for crabgrass and 94o for lambsquarters.

Coupling both models to a simulation model for weed prediction improves the accuracy because it avoids the drawbacks of the degree-day approach from biophysical and statistical aspects. The output includes information on the day of first emergence, peak emergence, and last emergence for a population. The predicted results can be used to optimize weed control timing, or can be used as input into larger population dynamics models.

By: Richard A. Ashley, Extension Specialist – Vegetables, University of Connecticut, 1376 Storrs Road, Storrs, CT, 06269-4067

Presented: New England Vegetable & Berry Growers Conference. 1999. Sturbridge, MA.

Reviewed by: T. Jude Boucher, IPM, University of Connecticut. 2012

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