Killer compost is still a threat to gardens in the North Thompson Valley, according to Clearwater’s Suzanne Gravelle and Anne Baker.
The two women spoke during the Seedy Saturday event at Clearwater ski hill on April 13 about their experiences with compost that had been made with manure contaminated with a longterm herbicide.
Gravelle wrote about the issue in a letter to the editor last summer.
Since then she has heard of more cases in which garden plants have been negatively affected by contaminated manure.
The situation begins when farmers or ranchers use a long-lasting herbicide to improve their pasture and hay land.
Aminopyralid, which is made by Dow Chemicals, appears to be the main culprit, but there are others.
Gravelle said she could understand why farmers or ranchers would want to use the herbicide, as it selectively kills broad-leafed plants, thereby encouraging grass.
The real problem happens if the farmer or rancher sells the manure from the animals that have grazed on the treated pasture or been fed hay from the treated hayfield.
The herbicide is not broken down by the animals’ digestive systems and, in fact, can retain its activity for several years.
If the manure is used to make compost or added directly to garden soil, it can negatively affect or even kill certain plants.
Aminopyralid particularly affects potatoes, tomatoes and beans, but other plants can be affected as well.
Gravelle said she first became aware of the issue last spring when she re-potted some plants in her greenhouse. Their tops soon curled, cupped and became deformed. Yellow lesions then appeared on the stems.
The person she had bought the contaminated manure from took the piles away, but not before she had spread considerable quantities on her raised beds, making them essentially unusable for three to five years.
Anne Baker had a similar experience more recently. She bought some manure from a farm that had been treated with Grazon (otherwise known as Pilcoram).
When she tried growing tomato plants, some with the manure and some without, those grown with were noticeably less healthy than those grown without.
The best approach is to try to make gardens into closed systems by using homegrown mulches and cover crops, rather than relying on bought manure, Baker said.
Both women felt farmers and ranchers in the Valley should be educated about the hazards that herbicides in animal manure pose to garden plants.