Herbicide Application

Apply Herbicide when Flower Tassels Form, Late August -September

Timing is critical so that the herbicide is transported to the roots as plants go into senescence.

You will be provided with 27% glyphosate (Aquamaster or Rodeo), approved for wetland use.  This is appropriate for stem filling only; it must be diluted if used as a foliar spray.

For patches with large canes, at least ¾” diameter: Stem Filling

  1. Cut the knotweed stalks just below a node, about 6" from the ground. Cut only 3-4 stalks at a time; immediately (within 30 seconds) apply 3 ml (about 3/4 teaspoon) 27% glyphosate, to fill the stem. If you cut a large number at a time it is difficult to keep track of which ones you have filled. Also, the filling may not be timely. The Garden Club will lend you the appropriate squeeze bottle.

  2. Bag all plant fragments in heavy-duty contractor bags and incinerate.* Allow 4 to 6 weeks for the herbicide to slowly kill the plants and to be transported to the roots. Do not cut the remaining stalks before 4-6 weeks.

For large, previously cut patches or plants with stems too small to fill, apply as foliar spray: 

  1. Dilute to 3% glyphosate.  Place 8 parts water in a clean plastic bottle. Add 1 part 27% glyphosate. Close bottle and swirl to mix. Be sure to label your bottle "3% glyphosate" and date it. Masking tape makes a good label.

  2. Apply glyphosate in mid August to early September, preferably just before the flowers bloom. The plants should be well leafed out with a lot of surface area to absorb the herbicide. Spray the foliage, to the extent that the solution does not run off the leaves, using 3% glyphosate. Be sure to follow the directions on the label. Protect nearby plants with paper or cardboard.

    If you are adjacent to a stream, use a smaller quantity 9% glyphosate so there will be less chance of run off.

  3. Monitor the patch and reapply glyphosate to any re-growth before senescence. The plants will take 4-6 weeks to die and for the herbicide to be pulled to the roots. They must be left in place for this time.

  4. When dead, about 6 weeks after treatment, cut and bag all plant fragments in heavy-duty contractor bags and incinerate.* The 6 week delay is necessary to allow glyphosate to have maximum impact on the roots.

  5. Be careful to remove and clean your shoes before walking on any desirable vegetation. Just letting them dry is not enough - the glyphosate will remain active.

For knotweed in sensitive areas that cannot be stem filled: Glove Wiping

  1. Put on chemically resistant nitrile gloves followed by a cotton glove. Dip the cotton glove in 3% glyphosate and then use it to wet the leaves and stem of the knotweed while protecting desirable plants with paper or cardboard. Let the glyphosate dry.

  2. After 4 to 6 weeks, cut and bag all plant fragments in heavy-duty contractor bags and incinerate.*

*(The Rockport Transfer station is now allowing large canes to be placed directly in the incinerated trash bins. It is still a good idea to bag the small pieces so they do not escape.)