Mean Manure & Killer Compost: Grazon after-effects in the Bulkley Valley

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Mean Manure & Killer Compost: Grazon after-effects in the Bulkley Valley

👤Paul Glover 🕔Aug 01, 2013

Cheryl and Les Harmati had been successfully gardening on their property outside Smithers for 25 years. But last year something strange was happening: plants in the garden and greenhouse were curling and wilting. The deformed growth was alarming—and mysterious. What had they done that was different from other years?

The previous summer, their neighbour had his hayfields sprayed with Grazon, a herbicide that selectively kills broadleaf plants without harming grasses. His cows grazed the treated hay later in the summer. That fall, in a gesture of neighbourly generosity, he dumped a couple tractor-loads of manure into their garden. In the spring, the Harmatis spread some on the garden and into the greenhouse soil.

It wasn’t long before their troubles began. “We didn’t know what was going on,” Cheryl recalls. “We finally figured it out by talking with others who had similar experiences and by researching online.” The manure—and now their garden and greenhouse soil—was apparently contaminated with herbicide.

Picloram is the active ingredient in the herbicide Grazon. Because of picloram’s persistence in soil—it breaks down very slowly and continues to prevent weed growth in subsequent years—it is favoured by many in the weed-control business. Here in northwest BC its main use of late has been in hayfields and pasture, prompted most recently by widespread infestations of the non-native and very invasive yellow hawkweed.

Although quite toxic to certain plants, picloram has very low toxicity to mammals, so cows and horses that consume sprayed hay appear to be unharmed. However, picloram is not broken down during digestion, and is excreted unchanged in urine and manure. Even composting does not break down the chemical, and in fact may concentrate it. Picloram is very potent: sensitive plants like potatoes, tomatoes and peppers are affected by concentrations of less than 10 parts per billion.

On a farm not far away...

Bill and Sue, an idealistic young couple, moved to the area with plans of organic farming. They bought a property from an aging farmer who had years ago raised cattle, and were excited to discover some very well-aged manure in the barn; this would be perfect to boost soil fertility in their large greenhouse. But their tomatoes were soon growing cupped leaves and twisted stalks. Bill was puzzled. “I think there’s some soil-nutrient issues,” he theorized.

But neighbours who had had their own experiences with herbicides told him it looked like picloram. “They said the old farmer had used a picloram herbicide on his hay more than 20 years ago, when he still had cattle.” Was it possible the chemical could still be active?

Picloram’s rate of decay is very slow. According to North Carolina State University, hay has been reported to still have herbicide activity after three years’ storage in dry, dark barns. Degradation is particularly slow in piles of manure and compost.

Picloram readily moves through soil, however. According to the US Environmental Protection Agency, “its major route of dissipation appears to be leaching.” In a University of Arkansas study, nearly 100 percent of the chemical leached but none of it degraded over a three-year period.

One might conclude, then, that in dry, dark conditions where no leaching can take place, it could be possible that the chemical can remain potent for years.

Remove and replace

Once picloram is in your garden, it is very difficult to get rid of it. “Once it’s spread on the garden, you’re hooped,” says Bill, who, after three years, finally removed all the soil from their greenhouse and replaced it with new soil—a huge endeavour. They enriched it with compost from their own hay and manure from their cows, pigs and chickens. “This year, the tomatoes are growing beautifully,” he exclaims, visibly relieved.

The Harmatis have removed and replaced all the soil in their greenhouse and in many of their garden beds, and at considerable expense. “Our property here is basically gravel, so we had to buy all new soil,” says Cheryl. Even so, she points to the pea plants growing in their new beds; they are curling and smaller than they should be. “I might not have dug deep enough to get all the old soil out,” she speculates. And the potato plants at the end of the garden where the manure pile once stood are barely growing, and quite deformed.

But another question has crept in. “Now we’re worried about our water,” Cheryl says. “Some of the sprayed fields are on the hillside right above us, where our water comes from.”

Her concerns are not unfounded: picloram’s mobility in the soil can lead to contamination of surface and groundwater. In 2003, residents in the village of Chauvin, Manitoba noticed plants in their gardens wilting and dying. Tests showed that the soil was fine, but when the municipal water was tested it was found to contain picloram.

“We looked at several different options,” municipal administrator Betty Swanson told the

Lloydminster Meridian Booster. “It turned out to be a weed spray and somehow it has gone down through the earth and into our water supply.”

At between 3 and 17 parts per billion, the concentrations in the village’s water were within safe limits for drinking, but high enough to damage and kill plants.

Unfortunately, this is not an isolated incident: the US EPA reports that picloram has been found in the groundwater of at least 10 states.

A growing problem

Here in the Northwest, more stories of picloram-contaminated garden soil are cropping up each year. An organic gardener in Quick used manure from a nearby farmer in her greenhouse. When her tomatoes showed deformed growth, she sent photos to the Ministry of Agriculture in Prince George and took an actual plant to a nursery in Alberta. The response in both cases: “Looks like picloram.”

Elsewhere in the valley, a rancher in Moricetown has sprayed Grazon on his fields on and off for years, and his cattle eat the hay. He has used the resulting manure in his garden and observed strange growth in broadleaf plants, especially potatoes. “For years I thought they were diseased,” he says. But after speaking with a government agrologist he now sees the Grazon connection.

What can you do if picloram finds its way into your garden?

“You have to wait for the process of degradation to take place, through light and microbial action, and this takes a certain amount of time,” says Lisa Jarrett, BC sales representative for Dow AgroSciences, manufacturer of Grazon and other picloram products. “Adding fertilizer, watering—this doesn’t work. It takes time.

“The best thing is to prevent the situation in the first place—do not use manure or compost that contain picloram.”

She goes on to say that Grazon users must carefully read and follow the product label, a legally binding document that lays out the many stringent conditions and restrictions for the herbicide’s use, including handling and distribution of sprayed materials and by-products like manure.

But in spite of the greatest care, accidents still happen, and sometimes in big ways. Several commercial composting facilities across North America have inadvertently found themselves selling soil and compost that contain picloram. Last year, hundreds of customers who bought bagged or bulk compost from Green Mountain Compost in Vermont complained that their garden plants were curling and wilting. Lab tests showed very small but damaging concentrations of picloram.

Everyone who has experienced the deep disappointment of watching a garden mysteriously wilt and curl from contaminated manure or compost will give you the same advice: Make absolutely certain that you know where that manure has come from, and where the hay came from that went into that manure. Did it come into contact with any herbicides? If it brushed elbows with Grazon, or any other picloram-based herbicides, it’s not worth the risk.

Do your own bioassay

This method was developed by Washington State University.


Thoroughly mix 1 part manure or compost to 1 part potting soil. Fill three 4-inch pots with this mixture and label. If testing soil, fill three pots with the soil you want to test.

Fill three 4-inch pots with potting soil and label as “control.”

Place each pot in a separate saucer to keep water from one pot reaching another.

Water the pots and let stand 24 hours.

Plant each pot with three pea or bean seeds. Water as required to keep soil moist.

Observe growth over a four-week period. Symptoms such as cupped leaves, fern-like growth on new shoots or twisted stems may indicate picloram residues in the soil being tested.


Soak hay or straw in water in a clean pail to make a tea-coloured brew.

Fill six pots with potting soil and plant each pot with three pea or bean seeds. Place in separate saucers, as above. Label three as test and three as control.

Use the hay/straw brew to water the three test pots and regular water on the controls.

Observe growth over several weeks, as above.

You can send samples of soil or water to A&L Canada Laboratories in Ontario for testing. The cost is $150 per sample. Call 1-855-837-8347 toll-free, or go to for more information.


Hi! Is it ok to spray Round-Up on a manure pile that has vegetation growing in it?
Kevin Kertz

I hope this comment is not too late in coming.

NEVER use RoundUp! It contains glyphosate which is a PROBABLE CARCINOGEN according to the World Health Organization. Pesticides, herbicides, fungicides are all POISONS which contaminate air, water and soil. Keep all poisons out of your compost and the earth, period. Turn the green vegetation under and let it compost naturally. Work WITH Mother Nature.