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
- 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.
- 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:
- 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.
- 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.
- 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.
- 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.
- 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
- 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.
- 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.)