- Ten herbicides (atrazine, azafenidin, chlorsulfuron, clopyralid, hexazinone,
imazapic, imazapyr, metsulfuron, pendimethalin, and sulfometuron) were evaluated
for phytotoxicity in first-year seedlings of eight conifer species (Douglas-fir, grand fir,
noble fir, redwood, Sitka spruce, western hemlock, western red cedar, and western
white pine). Six of these herbicides (atrazine, clopyralid, hexazinone, imazapyr,
metsulfuron, and sulfometuron) were also evaluated for vegetation control efficacy.
Azafenedin, chiorsulfuron, imazapic, imazapyr, metsulfuron, pendimethalin, and
sulfometuron were applied as fail site preparation treatments prior to spring conifer
planting. Atrazine, clopyralid, hexazinone, and sulfometuron were applied as spring
release treatments after conifer planting. All herbicides were applied at an operational
rate (lx), as determined by label recommendations and industry standards, and a rate
equal to three times the operational rate (3x). Treatments were replicated at three sites
in western Oregon and Washington. Most of the herbicides tested in this study exhibited high levels of conifer
safety. At the operational rate, hexazinone and sulfometuron were the only herbicides
to cause significant injury to any of the conifer species tested. The hexazinone lx
treatment caused mortality in western red cedar. The spring sulfometuron lx
treatment caused mortality in grand fir and both sulfometuron lx treatments caused
growth reductions in western red cedar. In addition, sulfometuron produced a subtle
but non-significant trend of growth inhibition in five of the remaining six species, and
appears to be a mild growth inhibitor in most conifers. Otherwise, none of the
herbicides caused significant injury at the operational rate.
Hexazinone and sulfometuron afforded the best overall weed control of the
herbicides tested, Sulfometuron provided equal or better weed control as a fall site
preparation treatment than as a spring release treatment. Atrazine provided the best
combination of weed control efficacy and overall conifer safety. Clopyralid provided
good control of weeds in the Asteraceae and Polygonaceae families, but was generally
ineffective against other species. Metsulfuron and imazapyr were generally
ineffective against the herbaceous species present in this study.
Herbicide recommendations for various combinations of conifer crop and
target weed species are provided in convenient reference tables. These tables facilitate
the design of herbicide prescriptions for a variety of reforestation scenarios in the
Pacific Northwest. Few of these herbicides are acutely toxic to any of the conifer
species tested. We specify the conifer/herbicide combinations that should be avoided.
Otherwise, foresters have a great deal of flexibility in developing vegetation
management prescriptions with these chemicals.