Alfalfa Control in New Tree Planting Sites in Wisconsin

Alfalfa grows to about 3 feet in height and smothers young trees and robs 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.

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.

Alfalfa lives several years and grows a deep, bulky root system which 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 growing season when the tops are actively growing and are large enough to absorb sufficient herbicide--larger than 6 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 in 8 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.

Transline Alfalfa Control: Alfalfa control in background; lack of control in foreground

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 applied when alfalfa is 6 inches in height to as late as early July to uncut alfalfa. A supplemental Transline label allows its use on forest sites in Wisconsin; copies are available from DNR Forest Pest Specialists.

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.

Weeds  ||  Forest Health Protection