Weeds can compete with tree plantings, choking out young tree seedlings or robbing water and nutrients from growing and mature trees. Chemical herbicides are one option for weed competition control in tree plantings, but application of chemical herbicides requires significant care to avoid damage to the trees, especially at the seedling stage. Pre-emergent herbicides are most often used in tree plantation site preparation, while post-emergent and residual herbicides are recommended after planting.
Pre-emergent herbicides must be incorporated into the soil and activated by rain. They work to preclude germination of weed seeds. The Purdue University Extension Service recommends pre-emergent herbicides containing the chemicals simazine or sulfometuron as broad-spectrum weed killers that deter broadleaf weeds and a variety of grasses. However, higher rates of application of these chemical herbicides can interfere with growth in young tree seedlings. Be attentive to the label recommendations for application rates around trees. Purdue Extension recommends applying pre-emergent chemicals to the soil around tree plantings in late winter to early spring, to allow seasonal rains to activate them.
Post-emergent herbicides are applied directly to green, growing weed foliage. These herbicides then work their way through the plant to its roots to kill it. Glysophate-containing herbicides are the most common post-emergent weed killers used around tree plantings. Glysophate is non-selective, so the planted trees must be carefully protected from any drift or overspray. The North Dakota State University Extension Service recommends putting a pail upside down over the nearest tree seedlings while spraying with glysophate-based herbicides. Once the herbicide is on the seeds, the bucket or other shielding can be removed from the tree seedlings; the herbicide will stay within the weed plants and not do further damage to the young trees.
The North Dakota Extension recommends applying a granular, residual herbicide in the fall after trees have gone dormant, or in early spring before the trees leaf out. Residual herbicides like those containing the chemical picloram may be applied at the post-emergent stage, but they also remain in the soil to inhibit future weed growth. Like other post-emergent herbicides, picloram acts by entering the weed plants through their broad leaves, so applying it when the trees do not have green leaves on them will diminish any potential harm the chemicals might cause to the growing trees. Because residual herbicides remain in the ground to act as pre-emergent herbicides the following spring, take the same care in applying at rates recommended on the label as you would with ordinary pre-emergent chemical herbicides.