Biological information:

• Common name: Pigweed
• Scientific name: Amaranthus retroflexus
• Family or class: Amaranthaceae

Symptoms and/or signs to watch out for:

• Rapidly over topping shorter crops.
• Increasing stem growth and deploying leaves.
• Tall, erect to bushy plants with simple oval to diamond shaped alternate leaves with small greenish flowers.

Life cycle:

• Emerges after spring frost date.
• Grows rapidly.
• Competes vigorously with cash crops.
• Reproduces by seed.
• Dies with fall frost.
• Seeds become dormant .
• Have multiple dormancy mechanisms to germinate at different times the next year.

Mode of reproduction:

• Reproduces asexually by seed.
• Seeds are spread primarily by wind.

Vectors for the pest:

• Pest Nematodes (meloidogynes spp.)

Overwintering habit:

• Plants die after frost, seeds go dormant for the winter.

Alternate hosts:

• Many crop pathogens including fungi
  • early blight in tomatoes and potatoes
  • lettuce drop
  • southern blight
• Viral pathogens
  • cucumber mosaic virus
  • tomato spoiled wilt virus

Horticultural production information:

Preventative strategies:

• Cultivate at cotyledon stage or before 1’’
• Use dust mulch to deter additional pigweed germination
• Avoid recompacting soil

Threshold levels:

• There is no threshold level for pigweed because it is so highly prolific.
• One female plant can produce 150,000 seeds per year.
• If 99% percent were killed, 150 still survive cause the same problem.

Control options:

Physical controls:

• Removing weeds in early fall can reduce pigweed seed rain.
• Mulch – 3-4’’ straw/hay or black plastic after cultivation.
• Use of in row drip irrigation directly on crop.

Biological controls:

• Cover cropping with rye to release allelochemicals, suppressing pigweed.
• Stale seedbeds.
• Use of competitive cover cropping.

Chemical controls:

• Most pre-emergence herbicides for broadleaved weeds.
• Post-emergence application of: 2,4-D, dicamba, mecoprop, bromcynil, or atrazine.

Historical notes from Farm to YoU NH:

Date/circumstances observed (Pictures)

Control method used, efficacy of control method

• manual removal (broadfork, by hand)
• works effectively on small scale

References:

http://articles.extension.org/pages/65208/weed-profile:-pigweeds-amaranthus-spp

http://ipm.ucanr.edu/PMG/WEEDS/pigweeds.html