Pigweeds: Redroot pigweed, smooth pigweed, and Powell amaranth
Redroot and smooth pigweeds (Amaranthus retroflexus L. and A. hybridus L.)
Powell amaranth (Amaranthus powellii)
Summer annuals. Emerge in the spring set seed in late summer/fall and dies.
The emergence of these pigweed species occurs after common lambsquarters and the ragweeds. Ten percent emergence occurs between 150-300 GDD (base 48 F).
Pigweeds emerge from soil depths of less than 1".
Production Average: Pigweeds produce 13,000 to 35,000 seeds per plant.
Dispersal Mechanisms: None.
Longevity: Pigweed seed is moderately persistent. It takes 3 years for the seedbank to be reduced by 50% and about 20 years to deplete the seedbank by 99%.
Dormancy: The pigweeds are less dormant when seed is located in the upper inch of the soil profile. As burial depth increases, seed dormancy increases.
Pigweeds are less competitive than velvetleaf and common lambsquarters.
Preferred Soil / Field Conditions:
These weeds prefer rich soils, fertile in N, P, K. They grow well on compacted soils. Growth is reduced on low pH soils.
Predation: Pigweed seed lying in the soil surface is a preferred food source of many insect seed predators, including the northern field cricket and some species of ground (carabid) beetles. One female northern field cricket can eat more than 200 pigweed seeds a day. Mice and other mammals also eat pigweed seeds.
Tillage: Tilling at night (dark tillage) will reduce pigweed emergence by 50-75%.
Rotary hoeing: Pigweeds are easily controlled with a rotary hoe when less than 1/4" in height.
Flaming: Flaming is very effective on small pigweed plants (less than 1.5" in height).
Crop rotation: Small grains suppress redroot pigweed in the rotation.
Planting date: Planting early in the spring (early May) will reduce pigweed infestations becuase pigweed will emerge after the crop has emerged and will be less competitive with the crop.
Application timing and effectiveness: Because redroot pigweed is a later emerging weed it is usually not the driver weed that has to be watched so it doesn't exceed the maximum size for effective postemergence control.