Controlling Perennial Weeds When They Emerge From Seeds

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Perennial weeds often produce seeds that are adapted to being moved by the wind (hemp dogbane, milkweed, or Canada thistle) or produce large seeds or berries that are eaten by birds and animals and spread around (pokeweed). We conducted a greenhouse study (funded by DE Soybean Board) to examine which commonly used soil-applied herbicides are effective in controlling these plants when they originate from seeds. Perennials seedlings can produce a perennial root system after only 3 – 4 weeks. Being able to select the correct herbicide when you know seeds are coming into your fields can help prevent headaches and frustration in years to come. If at all possible, keeping the perennials mowed along the ditches and field edges will reduce (or eliminate) seed production. Prevention is the best approach.

Johnsongrass, bermudagrass, Canada thistle, hemp dogbane, common milkweed, common pokeweed, and horsenettle were planted in the greenhouse and sprayed with common soil-applied herbicides (Dual, Prowl, Command, Lorox, Sencor, Lexone, atrazine, Scepter, and Canopy). Next to each weed are the herbicides that provided the best level of control (over 90% control). This study was conducted a few years ago and neither Lumax nor Canopy XL was available at that time for testing.

Bermudagrass – Dual, Prowl, Command, and Sencor or Lexone
Johnsongrass – Command
Canada thistle – Command, Sencor or Lexone, atrazine, and Canopy
Hemp dogbane – Command, Canopy, Sencor, and atrazine
Common milkweed – Sencor or Lexone, and Canopy
Common pokeweed – Canopy, and Sencor or Lexone
Horsenettle – Sencor or Lexone, atrazine, and Canopy

This was control of plants emerging from seeds. This study did not examine control of plants emerging from rootstocks. If you are concerned about perennial weeds establishing in your fields due to seeds be blown in, there are options for controlling them before they become established as seedlings. As the list indicates, one herbicide will not control all the different perennial weeds. Be sure to match your herbicide with the species.

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