Q: My wife and I are very experienced gardeners. This past winter I picked up a load of horse manure from a stable and let it compost under a tarp. In spring I top dressed my flower beds and mixed the compost with soil when I renovated my raspberries. Now the flowers are showing diminished vigor and the raspberries have simply not grown. I used the compost when I planted my tomatoes and they all have died. Could the manure be to blame?

A: The manure could well be the culprit. My bet is that the horses were fed hay from a pasture treated with picloram (Grazon, etc). Picloram is a common pasture herbicide but it has a long half-life in organic matter: 60 to 300 days, depending on environmental conditions.

I’d spread the compost pile on your lawn, where it may help you control broadleaf weeds.

Next time you get manure, make certain you know how the horse pasture has been maintained. You can also do a simple bio-assay by mixing the manure 1:1 with potting soil and planting a tomato in it. If the tomato shows no harm, the manure is safe.

Testing Manure for Herbicides

tomato herbicide damage

tomato herbicide damage

tomato herbicide damage