Clemson Extension Forestry and Wildlife

Midstory/ Understory Control in Hardwood Stands

Most mature hardwood stands lack an adequate abundance of oak reproduction (seedlings and saplings) on the forest floor. If oaks are to be successfully regenerated into the future stand, silvicultural practices need to begin years prior to the complete removal of the standing timber. One of these practices may include a midstory and understory treatment to control less desirable species occupying these canopy layers. This is especially true when there is a moderate to full midstory canopy layer present.

The density of stem abundance in lesser canopy layers is commonly associated with previous disturbance activity such as partial harvesting, wildfire, or heightened natural mortality from insects or disease. For undisturbed stands, stem density may be limited with few scattered saplings or smaller pole-timber-sized stems distributed across the area. In either instance, midstory and understory stems are more likely to be less desirable, shade-tolerant species such as red maple, American beech, hickory, blackgum, eastern hophornbeam, etc. The presence of these less desirable stems is problematic for land managers who are attempting to promote oak regeneration.

Less desirable stems in the midstory and understory cause two primary complications for oak regeneration efforts. Heavy shading (especially with denser sub-canopy layers) hinders the recruitment and growth of oak seedlings on the forest floor. Research has found that oak seedling growth is promoted when sunlight penetration is between approximately 25 – 50%. Most closed-canopy stands have less than 10% sunlight availability on the forest floor.

Area on the left received midstory chemical control versus untreated on the right.
The area on the left received midstory chemical control versus untreated on the right.

Shade tolerant species on the contrary can continue to advance in height growth as they require less sunlight (photoactive radiation). The larger-sized reproduction has a higher probability of establishing dominance in the future stand over smaller reproduction. This competitive advantage is attributed to the greater mass of rootstock. Should these larger stems be severed during timber harvesting, resprouting will occur and these stems will more rapidly develop in height growth compared to newly established reproduction. Thus, resprout regeneration is most likely to suppress the slower-growing young seedlings.

Herbicide treatment using the hack and squirt method enables the landowner to begin to enhance understory light conditions and gain control over the regeneration of less desirable species. Common herbicides used for stem injection include imazapyr (Arsenal AC, Polaris AC, Imazapyr 4 SL), triclopyr (Garlon 3A, Tricera), and picloram (Tordon RTU). Each of these herbicides has some species that are resistant yielding the chemical ineffective at inducing mortality. Imazapyr does not control leguminous species such as locust, mimosa, etc, and conifer species such as pines and cedar. Triclopyr inadequately deadens sourwood whereas picloram may not control sassafras.

Proper identification and inventory of all target species should be made in order to use a herbicide acceptable to obtain the desired results. Check the herbicide label (attached to the container or available online) to ensure the selected herbicide includes the target species listed as controlled. Some herbicides can be used “as is” and are already pre-mixed such as Tordon RTU (stands for ready to use). Other herbicides such as imazapyr should be diluted in water. Arsenal AC, for example, should be diluted into a 20% solution (20% imazapyr, 80% water). All herbicides that can be used for cut stem treatments (hack and squirt) will depict dilution rates for application on the label. Labels also contain application procedures needed to conduct treatments. Each herbicide will vary for the number and distance between hacks. For example, Arsenal AC can be applied at 1 milliliter of mixed solution at one hack for every 3-inch increase in stem diameter. Refer to the MSDS label for pertinent details prior to treatment conductance.

Expenses for conducting midstory control will likely fall between $120 – $150 per acre pending stem density. For more densely stocked understory and midstory layers, prices may elevate to over $200 per acre. This may especially be true when non-native, invasive plant species are present. Cost-share assistance is available to alleviate the financial burden associated with herbicide treatments. The Natural Resources Conservation Service (NRCS) has a program named the Environmental Quality Incentives Program (EQIP) that may provide funding for this silvicultural practice. The South Carolina Forestry Commission may also be able to provide financial assistance thru the Forest Renewal Program (natural hardwood regeneration). Seek assistance from either your local NRCS district conservationist or SC project forester for additional details on program enrollment.

This article was originally featured in the Fall 2019 Version of CU in The Woods newsletter.


Stephen Peairs, Cooperative Extension, Forestry and Wildlife Specialist

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