The information below is intended to help manage some specific weeds in new tree planting sites and in forest sites. There is also information about general weed management in new tree plantations.
Alfalfa grows to about 3 feet in height and can smother young trees and rob them of moisture and nutrients, which stunts their growth and often kills them. If you are planting trees in an old hay field, you may need to control the alfalfa. But alfalfa also provides valuable wildlife cover and protects the soil from erosion, so you may want to keep some of it.
The best time to control alfalfa in the planting site
If you need to completely eliminate the alfalfa from your planting site, it is best to do it before planting. Once your trees are in the ground, your control options are greatly reduced and control can be expensive, difficult and can damage your young trees.
Physical control options
Alfalfa lives several years and grows a deep, bulky root system that makes it difficult to control with herbicides. Moldboard plowing or rototilling are effective methods for controlling alfalfa. Chisel plowing, disking or field cultivating usually require more treatments and persistence.
To control alfalfa with herbicides, you need to know a few things about its life cycle.
Alfalfa over winters with large amounts of nutrients stored in the roots. These nutrients are used to produce rapidly growing tops in the spring. The rapidly growing green tops in turn produce carbohydrates that move down to sustain the roots. In early June, the alfalfa begins to flower and is normally harvested. If it is not harvested, it completes flowering and produces seed. During and after seed set, transport of food reserves to the roots is greatly reduced. During a normal growing season of repeated harvest and regrowth, the alfalfa stores most food reserves in the roots from late August to mid October.
In order to control alfalfa with herbicide, you must kill the roots. In order to kill the roots, the herbicide must be applied to the tops when nutrients are moving to the roots. The herbicide can be successfully applied anytime during the growing season when the tops are actively growing and are large enough to absorb sufficient herbicide (larger than six inches). Herbicide applications are most likely to be successful if applied during the storage period in late summer. The effectiveness of herbicides is greatest when applied in the middle of this period--mid to late September in southern Wisconsin when alfalfa is eight to 12 inches tall and in the bud to early flower stage.
Alfalfa that has set seed will translocate reduced levels of nutrients to the roots and be more difficult to control. Alfalfa that is growing slowly because of drought or other stress will also be difficult to control. Herbicides might "burn" the tops but would do little or no damage to the roots which would produce new shoot growth in the spring.
Herbicides that work best
Several herbicides have proven effective in controlling alfalfa: clopyralid, glyphosate, dicamba and 2,4D. Clopyralid (Transline) can be applied over the top of most of the commonly planted forest tree species during the growing season. It is effective when alfalfa is 6 inches in height as late as early July to uncut alfalfa. A supplemental Transline label allows its use on forest sites in Wisconsin; copies are available from forest health staff.
Glyphosate (Accord), dicamba (Banvel) and 2, 4-D should be applied before trees are planted or as hand directed sprays during the growing season. They should not be applied over the top of actively growing tree seedlings. Glyphosate controls grasses as well as broadleaf weeds; 2, 4-D and Banvel kill only broadleaf weeds. With Banvel and 2,4-D, fall treatments are highly preferable to spring treatments; these growth hormone products prevent the plants from becoming cold hardy and even if the herbicide fails to kill all the plants by first snowfall, the cold winter temperatures kill remaining plants.
Reed canary grass
Reed canary grass is a tall, coarse, cool-season perennial grass that invades disturbed sites and many wet sites and interferes with tree planting operations. There is a detailed description of the species in the Department of Natural Resource's invasive species control manual. The information below only includes control options.
Three well-known forestry herbicides have been effective in controlling reed canary grass: Arsenal AC, Accord and Oust. Reed canary grass is listed on the Arsenal AC and the Oust labels; while reed canary grass is not listed on the Accord label, it has been effective in field tests (it can be used because of its registration for forestry site prep and release). Reed canary grass does appear on the Roundup Pro label which has the same active ingredient (glyphosate) as Accord.
Arsenal is registered for site preparation and release of red pine, white pine, jack pine, white spruce and black spruce. Apply at least six months before planting (if you apply it before planting other species, you should apply it at least a year before planting). The label rate for listed species is 12-32 ounces/acre. A late summer or fall (before first frost) is recommended.
Make site prep applications of Accord in late summer to early fall before the first frost at a rate of three quarts per acre. The maximum conifer release rate of two quarts/acre is not likely to control reed canary grass.
Because reed canary grass greens up early in the spring, Accord may be effective at two percent when used to selectively remove it where it has invaded later developing grasses. Because of its rapid height growth, wiper applications of six percent Accord may be feasible for selective application where it is invading shorter grasses.
The Oust label recommends three to five ounces per acre to control reed canary grass (in noncrop sites), which is at or above the maximum for site preparation and release of most species normally planted. If three or more ounces are applied, wait a year before planting.
Arsenal and Accord tank mix
Apply eight ounces per acre of Arsenal and three quarts per acre of Accord in late summer or fall before first frost. Expect 80-85 percent control of reed canary grass. In spring, wait for green up; if reed canary grass survival is excessive, treat with Accord before planting the species listed on the label. This scenario will be more effective if you burn dead grass before application.
Weeds are some of the worst enemies of young tree plantations. Unchecked, weed growth will weaken young trees, slow their growth and often kill them.
Until recently, most growers of forest plantations relied on herbicides to keep their plantations weed-free. However, restrictions on the use of herbicides and worries about their environmental effects are causing tree planters to adopt a new approach to weed control called "integrated vegetation management."
Integrated vegetation management
Integrated vegetation management integrates several practices in combination to minimize weed competition. It relies on tilling, cover crops and mowing to control most of the weed growth, while herbicides are used selectively around the tree seedlings themselves. Experiments are also underway to test the value of mulches and protective tubes.
Let's look at an example. In late summer, a grower plows down an old alfalfa/brome grass hay field and disks, drags and seeds the entire field to a perennial grass cover crop. The following spring, the farmer plants hardwood and conifer seedlings in rows and treats a two-foot band of soil in each row with a pre-emergent herbicide. The weed-free band is maintained and the grass between the rows is mowed once or twice per year until the crowns start to touch each other.
How to control weeds
There are numerous methods used to control weeds.
Tilling can often be used over an entire planting area on level ground, or in contour bands in erodible terrain. A single moldboard plowing will kill alfalfa and most perennial grasses (quack grass may require a year of summer fallowing).
Disking between the tree rows can be an effective means of controlling weeds after planting. If the trees were originally planted in a square grid, disking can be done in both directions.
Cover crops protect the soil and prevent invasion by weeds.
Timothy is inexpensive to plant, stays reasonably short and can be mowed once a year, after it forms a seed head.
Perennial ryegrass stays reasonably short and can be mowed often. However, it is expensive to plant, is subject to winter kill and low, frequent mowing often allows invasion by dandelions.
Orchard grass is inexpensive to plant, stays relatively short and is moderately shade tolerant.
Tall fescue and red fescue are often used as cover crops but may be allelopathic to black walnut roots.
Mowing is a well-known weed control method and is effective between rows. However, because it does not kill the weeds, the grower must monitor weed growth regularly and mow often.
Mulches may be applied with varying success. Plastic mulches are difficult to keep in place and are not very long lasting. Shingles are long lasting but can be blown around by strong winds. Wood chips and sawdust are sometimes effective for a season or two if perennial weeds have been removed first. Protective tree tubes are in the demonstration stage but are expensive.
Herbicide restrictions change frequently. For example, the forestry section has been removed from several simazine labels. Oust is now registered for preplanting and post-planting application in over the top of several tree species. Keep abreast of new products and new pesticide restrictions by checking with your local DNR forester or a private forester.
Using integrated vegetation management, a grower can achieve effective and environmentally-safe weed control. It requires careful planning but your efforts will pay off with a well-established, healthy tree plantation.
Of special note
Perennial grasses are among the worst enemies of young walnut trees through the sapling stage. Heavy brome and quack grasses overtop and smother newly planted trees and heavy bluegrass sod often causes stagnation of sapling walnut plantations.
Alfalfa is difficult to kill after trees are planted. Once the trees are planted, the only control method available may be hand application of herbicides.