Asked March 1, 2017, 10:18 AM EST

I have a few acres of replant fir trees that are now 8-10 years old. We use this area for horse turn out. Tansy appeared last year, I imagine due to over grazing. There is also a lot of foxglove and what i think is wild carrot. Is there a spray that I could use to kill these weeds that is also safe to use around the fir trees? Or, what is the best way to deal with these weeds?

Columbia County Oregon

1 Response

Tansy ragwort, wild carrot, and foxglove are indeed more abundant if there is less grass cover. In terms of an herbicide, glyphosate (the active ingredient in Roundup and other products) should probably control all three species. It is inactivated when it contacts soil and can't be root absorbed. So if you spray very carefully and don't get the solution on the green portions of the tree, you should be ok. Technically, I can't tell if this is a pasture with trees or a forest with a little pasture. There are different labeled glyphosate products for both situations. But if you keep it off the trees, it won't make much difference. You will kill grass and anything else green that is growing that the spray touches. But if you remove the horses for a bit, you could seed grass back into the area and cover some of the bare ground. If you want to avoid killing any more grass, you would need a broadleaf weed product. But those can potentially do more harm to the fir trees. There are some formulations of 2,4-D that might fit a forestry situation but use them carefully to avoid plant injury. Not sure of effectiveness on foxglove, good on tansy this time of year, ok but weaker on wild carrot. Avoid ester forms of 2,4-D and make sure he product has a forestry label (if you consider the site a forest ragther than a pasture. Remove livestock from the treated areas until the weeds are dried because herbicide treatment sometimes makes the weeds more attractive to eat. Hope this helps. You can call me at 503 397-3462 or email me a chip.bubl@oregonstate.edu to discuss this further. Chip

Chip Bubl
OSU Extension/Columbia County