Undesirable vegetation can drastically reduce commercial production in forests. Pine stands containing 20–30 percent hardwood basal area can suffer up to a 50 percent reduction in pine volume growth, or be so thick that sunlight does not reach the forest floor at levels sufficient to maintain vegetation needed by wildlife. Conversely, hardwood stands may contain species undesirable for landowner objectives. In either case, if undesirable stems become large enough, they can no longer be easily controlled through prescribed burning or herbicide applications appropriate for low-growing vegetation. Millions of acres of Mississippi’s commercial forestland could benefit from control of these larger undesirable stems (Figure 1).
One of the more appropriate methods for controlling large unwanted stems is individual stem treatment with herbicides. Three commonly encountered modes of individual stem treatment exist (e.g., basal bark, injection, and hack-and-squirt). While “injection” technically describes a treatment method involving more specialized equipment (described below), injection and hack-and-squirt are often referred to collectively as “injection.” Due to susceptibility of most hardwoods to a wide array of forest herbicides, injection may be the only herbicide application capable of controlling these stems without unintended damage to crop trees in hardwood stands. This method can be particularly attractive to private landowners since the technique is easy or can be easily contracted and supervised as they see fit. If needed, it can be performed throughout most of the year, but the greatest levels of efficacy are achieved during fall and winter months. Additionally, if landowners complete the work themselves, it can be performed as time permits with very low investment in equipment. Landowners can treat even large acreages if timing is not a limiting factor.
Tree injection involves depositing herbicide into undesirable tree stems using cuts spaced around the trunk of the tree with an ax, hatchet, machete, or tree injector. Non-overlapping horizontal cuts that penetrate into the sapwood are cut around the tree.
A small quantity of herbicide is then deposited in each cut. The precise amount of herbicide used is specific to the herbicide label, but per-acre rates are typically very small compared to broadcast rates. Tree injection is more effective and less expensive than mechanical girdling with or without herbicide. This is primarily a function of less labor involvement (see MSU Extension Publication 3673 Tree Injection with Reduced Labor Requirements).
Injection is applicable in both pine and hardwood systems. However, in hardwood stands, you must consider possible nontarget impact. This is rarely a problem if you follow application procedures and properly complete the method.
Where Is Injection Used?
Historically, injection was widely used during several stages of forest management. Injection was commonly used in combination with other herbicide applications or with prescribed burning. For example, a common practice after timber harvest was to burn logging slash, thus removing planting obstacles and controlling smaller hardwoods. Larger stems could then be treated. The method was used to control undesirable trees during site preparation, release operations, and timber stand improvement (TSI).
Due to improvements in chemical site preparation in pine silviculture, injection has become relatively uncommon in these systems. However, the technique is widely used in hardwood TSI work and natural regeneration efforts. Injection is typically reserved for use on stems larger than 1 inch diameter at breast height (DBH) and in areas with up to 350–400 undesirable tree stems per acre. While the method is effective on smaller stems and greater numbers of stems per acre, labor cost becomes prohibitive when smaller/more stems are treated.
Benefits
As with any herbicide treatment, perfect—or even good—control is not automatic. Control can range from 20 to 100 percent, depending on a variety of factors. Inadequate control of targeted stems can result from using an herbicide not effective on the species being controlled, applying herbicides without regard for proper timing (for example, injecting imazapyr during the spring when fluids are traveling to the crown), or injecting herbicides while trees are under severe drought stress. However, if all factors are accounted for, you can expect excellent control. For example, in a recent North Mississippi injection study, over 72,000 stems were injected across 90 acres (Alkire et al. 2012). Injected stems included American hornbeam, blackgum, deciduous holly, green ash, several hickory species, paw-paw, red maple, sweetgum, winged elm, and red buckeye. These stems, along with those of several other species occurring in minor quantities, exhibited first-year control of 96.8 percent. These results mirror those observed in many commercial injection efforts.
