I had a question about control of mature white pine and hemlock to improve sunlight for enrichment planting of hardwoods. Following is my response, but I would like to know if anyone else has any experiences to share with control of these to conifers.
I would have thought that a 40%+ active ingredient dose of glyphosate would be effective on pine and hemlock, but one forester I know says below he has found otherwise. The Accord XRT label describes the sensitive of pine, conifer and spruce as Christmas trees and nursery stock to glyphosate during a foliar application. I would use this as a drill and fill (= hack and squirt) with 1 ml per 8 – 9 inches of circumference (=1 ml per 2 – 3 inches of diameter). I have never tried an herbicide on hemlock.
I recall a cold-day drill and fill of white pine with nearly full strength Accord XRT with reasonable success. Some of the trees we treated were wolfy pine and had some branches survive, so we likely needed a higher density of holes.
I did a quick skim of the Polaris label (imazapyr) which also suggests sensitivity of conifer foliage to this product, and the effectiveness of injection treatment of pine. The label suggests mixing this product with glyphosate to enhance control of pine. Read the labels, but I suspect about 1:1 Polaris with equal parts of 4:1 accord (herbicide:water).
I’ve had success with girdling pine and hemlock, but death takes a year or more.