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FORESTRY
MIKE NEWTON
Revised November 19, 2008
Undesirable woody and herbaceous plants are problems in commercial forests and on fencelines and rights-of-way. Safe, dependable methods of chemical weed control have been developed for most forestry needs. Reforestation, in particular, may depend heavily on weed control. Depending on local conditions and weed composition, the results sought differ strikingly in degree of control and composition of residual vegetation and, hence, in the choice of method. The Oregon Forest Practices Act mandates
successful
reforestation after logging. It also prescribes certain rules regarding herbicide use to protect plantations and water. Operators should be familiar with these rules and labels whenever herbicides are used.
Forest Land Brush Control
Weed control alone cannot produce a harvest; it must be combined with cultural practices involving either seeding or planting and with practices that bring the crop to maturity. Growing trees is the ultimate objective, not killing brush; brush/weed control is merely a phase of the reforestation procedure. Selective vegetation control also is an efficient method of enhancing certain kinds of wildlife habitat. Much of the early forest vegetation control work with herbicides was directed toward wildlife enhancement, and probably no set of tools is as capable of providing improved wildlife forage and long-term habitat development as a well-thought-out vegetation management program relying primarily on herbicides.
Weed control enhances conifer survival and growth on all potentially productive sites. On harsh sites in southwestern Oregon, almost total weed control is essential even to obtain survival, and it may be necessary to maintain a weed-free site for more than 2 yr. On better sites in the Coast Range, although initially weeding may not be essential for survival, fast-growing herbs and brush will threaten survival in a few years, and treatment to avoid brush problems will enhance productivity greatly. Indeed, weeding pays best on the best sites, even though the
percentage
increase is greatest in poorer sites.
Chemicals
Virtually all brush and weed-tree control in forests is with 2,4
-
D, glyphosate, imazapyr, picloram, or triclopyr; situations determine whether these herbicides are applied as spring or fall dormant or foliage sprays of diluted concentrates. Injections or cut-surface applications rely on high concentrations, including undiluted concentrates. Atrazine, 2,4-D, sulfometuron, and hexazinone are used to control herbaceous weeds in plantations, nearly always in spring. Clopyralid is very versatile in controlling thistles and other composite and leguminous weeds during active growth, when most other herbicides will damage conifers. Fluroxypyr is registered for selective brush control in pine plantations. These materials, singly or in combination, give broad-spectrum activity on forest species, in many cases selectively favoring desirable conifers. Triclopyr and 2,4-D can be used selectively (but avoid high rates; see label) over Douglas-fir seedlings during the dormant seasons but will injure ponderosa pine and noble fir. Glyphosate and imazapyr can be used selectively to favor conifers only in late summer or early fall. Picloram and metsulfuron nearly always harm conifers when applied directly; use them for site preparation only. Satisfactory reforestation brush control can be achieved with the proper formulation of these compounds applied at the proper season and with the right carrier.
Soil-active herbicides that are effective on brush are seldom suited for selective control in this region because of their tendency to injure conifers; they are useful for site preparation and are highly effective when applied before timber harvest to prevent postharvest sprout growth. Glyphosate is highly effective only on deciduous and herbaceous species. In fall, glyphosate will selectively remove brush and herbs from conifers. In midsummer, glyphosate damages conifers and
brush severely. Imazapyr is active on maple, alder, and other brush as a growth inhibitor. At high rates, it also might inhibit conifers. Imazapyr is well adapted for trunk injection or spot treatments. Triclopyr is registered for site preparation and release, and it may be injected as the amine salt. Picloram (when not mixed with other products) is useful only in site preparation and trunk injection. Picloram is a restricted-use product because of its mobility; use it with this in mind. Note restrictions on the label when grazing treated areas.
Two sulfonylurea products, metsulfuron and sulfometuron, are registered for use in forestry. They are used at rates much lower than for most other products. Both are somewhat persistent in soil. Metsulfuron is useful at present only for site preparation. Take care to avoid using sulfometuron at the upper range of registered rates unless you can wait several months between spraying and planting. Until more is known about the residues, metsulfuron should be considered safe only in moderately high rainfall areas and when trees are planted at least 6 mo after treatment. Principal target weeds for metsulfuron are the
Rubus
species and ferns. Sulfometuron is used for grass, fern, and general herb control in reforestation areas. Neither of these sulfonylurea products controls thistles, and so may need to be tank-mixed with hexazinone, clopyralid, or 2,4-D. Both sulfonylurea products are extremely effective on a number of hard-to-kill species, but use them with care to avoid overdosage. Mixing with other chemicals is often recommended to enhance control of thistles and other weeds on which these products may be weak. Some of the above products registered only for site preparation may have future uses for conifer release. Check current labeling before purchasing. A new chemical, aminopyralid, is registered for use to control broadleaf weeds and some brush in forests or on forest roadsides (noncrop) use. The product with aminopyralid alone, Milestone VM, is permitted only on roadsides, whereas the combination of aminopyralid and triclopyr amine is permitted for use in forests for site preparation, directed spray and cut surface applications (see the label for specific uses). Selectivity is not specified, and broadcast application over conifers is not permitted at this time.
Nonchemical Methods
Some weeds lend themselves reasonably well to removal without chemicals. Alder more than 2 inches in diameter can be killed fairly well by cutting at the ground line from mid-June to mid-August. Smaller stems are more likely to sprout at any time. Fungi complement the cutting to kill stumps. Alders cut at other times are likely to sprout vigorously. Most other western Oregon and Washington hardwoods sprout if cut at any time. Applying water-soluble concentrates of 2,4-D amine, triclopyr amine, glyphosate, or imazapyr to freshly cut (less than 20 minutes) stumps keeps sprouting to a minimum unless hardwoods are cut between December and May.
Grazing intensively can in some circumstances remove palatable herbs and many brush species. Great care is required when grazing among conifers to keep animals out of plantations when trees are actively growing. Grazing is among the most difficult methods of control because weed removal must be relatively complete to provide significant benefits, and this degree of control is difficult to achieve without animal damage to trees. However, intensive pasturing of a cut-over hardwood stand for the 2 yr immediately after logging will remove many of the sprouting brush species and may help simplify the weeding job to herb removal only. However, herbs may recover too quickly for this procedure to do the whole job of site preparation. The need to reduce cover to very low levels means the livestock will not gain weight well in programs such as this. If they are not grazed to this intensity, some supplemental weeding will be necessary, either with paper mulch or chemicals. Do not attempt to mulch while grazing, for obvious reasons.
Paper mulch has had success comparable to grass herbicides. In southwestern Oregon and near the Willamette Valley of Oregon, mulch paper should be at least 9 sq ft (3 ft x 3 ft). On coastal areas and in western Washington, 4 sq ft (2 ft x 2 ft) should be adequate. Mulch paper should be fiber-reinforced laminated Kraft paper with an asphalt core for adequate durability. Mulch paper must be well weighted on all four corners or pinned to the ground. Clear plastic will not work. Black plastic will work and is lightweight but difficult to handle.
Mechanical tillage
Using heavy equipment to till forest soil has limited value on cutover land because of stumps, logs, and other obstacles. Recent trials of large subsoil or ripping equipment have shown promise on compacted soil and clayey soils with restricted drainage, leading to easier planting and better root development. The procedure is not a truly effective weed control measure, however, and usually should be preceded or followed by a chemical application to postpone weed development.
Most serious forest weeds are not controlled as well with nonchemical techniques as they are with herbicides. Sprouting is difficult to control without repeated cutting. Most evidence to date clearly points to the need to kill root systems to minimize competition. For growers whose goal is vigorous tree growth and minimum impact on soil and labor force, no alternatives to herbicides have been proven to be as safe or as effective.
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