The session began with introductions and brief descriptions of everyone’s current experiences controlling knotweed without herbicide.

Plastic Sheeting:

There were some questions about the use of plastic sheeting, so Jim Serio was asked to speak about his experiences at the Junction Pool Access in Hancock, NY, where he has been cutting and using plastic sheeting for 5 years. Jim explained: After cutting, the knotweed area is covered with a clear plastic tarp approximately 20 X 50 feet. Clear plastic was found to provide better solarization than black plastic. (Higher temperatures were measured under the clear plastic, and there was a greater impact on the knotweed.) The plastic sheeting is held in place with 10-15 lb rocks on each corner and 3lb rocks along all edges and in the middle to keep the wind from getting underneath and lifting the plastic. [Aaron Schlechter (Creative Habitat Corp) also suggested that trenching along the edge of the plastic and installing landscaping “staples” perpendicular to the wall of the trench, then filling the trench with soil, could also help secure the plastic tarp.] The longer the plastic is left down, the longer the knotweed remains dead. Last year was the first year they have kept it in place all season. Have used in one plot for 2 years in a row. Need to be careful to check plastic since it can fall apart after extended use. Must take it up in the fall and best to use new sheet next spring. When the plastic is removed, the knotweed grows back, but is less dense

After using plastic, lawn seed mix was applied in the areas that are regularly mowed. These areas now have many more small sprouts of knotweed mixed in with the grass. Jim had hoped that the lawn seed would be a transition to native grasses that would need less mowing, but there is too much knotweed to allow for less mowing. Jim is concerned about treating the knotweed without losing the grass. He would also like to have taller grasses which he feels protect better from bank erosion. (Noted after this year’s flooding that more soil was held by tall grasses laying down than by the mown grass.) Jim added that he sees a clear connection between knotweed and erosion and has noticed that they have lost from 1-2 feet of bank every year where the knotweed is. He has seen a widening and warming of the river because of this.

Restoration:

The discussion moved to restoration and revegetation of knotweed infested riparian areas. Heather Jensen (Army Corps of Engineers and former intern at the Hancock site) described her plan of using cocofiber mats, coco-logs, native sedge and grass seeds, and plant plugs to restore the bank in Hancock, and asked if anyone had experience with this method.

Tim Wenskus described his crew’s work with cocofiber mats, coco-logs, seeds, and plants this fall following herbicide treatment of the knotweed at their site on the Bronx River. Heather’s contact at a native plant nursery had stressed the importance of doing the planting in the early spring. Tim will update us on his results this spring. Tim also mentioned that he’d heard of a woman at the Academy of Natural Sciences who was trying roto-tilling.

Jim Bell said that a few years ago, his Trout Unlimited group installed a willow mat for bank stabilization between an area of knotweed and the water. They then buried the knotweed and planted other vegetation. They saw no knotweed infiltration into the willow mat. Keith Mellot (Greene County S&WCD) added that they have used willow mats in several sites and have seen mixed results – sometimes the knotweed returns.

Factors that could be influencing the success/failure of restoration attempts at different sites are can be knotweed’s fast growth, it’s survival in shade, and allelopathy. Also, the presence of viable knotweed seed (remaining viable for up to 2 years) was also considered a risk to successful restoration.

Jonathan Petrillo (Kleinschmidt Associates) was asked about his restoration work. He explained that he set up plots in knotweed patches and planted river birch saplings with root guards. He did not see a difference between trees with shielded roots and those with no protection, and he did have some herbivory. He is still collecting and analyzing the data and should have more to share soon. [NOTE: in a later email, Jon said that this spring he will be trying some re-grading. They will attempt to remove much of the knotweed rhizome and follow with 6 inches of topsoil, 4 inches of mulch, containerized plants and a native seed mix.]

Cutting and other Non-Herbicide Methods

Joanne Steinhart passed around the Japanese Knotweed Manual by Lois Child & Max Wade, Packard Publishing, Chichester, England. She pointed out for those interested the images of knotweed in its native habitat, and also the recommended control methods. Based on greenhouse trials in the U.S., when it comes to cutting to reduce vigor, it was established that in order to reduce below ground biomass, cutting should be carried out at least every four weeks during the growing season. This or greater frequency prevents the replenishment of below ground reserves through photosynthesis. However, Joanne warned that there is concern that cutting can lead to either a) resprouting of many more, smaller stems (as Jim Serio has seen), and also b) lateral sprouting from rhizomes extending out as far as 25 feet from the original plant.

Aaron Schlechter (Creative Habitat Corp) has observed Japanese beetles feeding on knotweed leaves and apparently preventing flowering. He suggested this as a study topic using pheromone lures.

A question was raised about the potential use of salt or salt water to control knotweed. Heather Jensen has read that someone had tried applying salt directly to the cut stems. It only burned the tissue, but didn’t succeed in reducing growth the rest of the season. She offered to find the source and provide a copy.

Preventing the Spread:

Aerial photography was discussed as a means of predicting the spread of knotweed by identifying upstream populations. There is potential for easy identification of many knotweed stands in the winter because the dead canes are distinctive.

Stopping the spread of fragments down river was then discussed. After the recent flood events, it was noted that it was fairly easy to spot knotweed clumps in the river and the flooded areas, and these could be collected and bagged. Catching these fragments and destroying them could help stop the spread.

Conclusions:

The group decided that we should

· emphasize control, not eradication;

· start upstream and work downstream;

· there are benefits and risks to herbicide use; and

restoration is a key component