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Tree Herbicide Application Methods

Controlling Wood Stem Plants

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Controlling woody-stemmed plants that are unwanted in the landscape can become an impossible task. When mowers, chainsaws and axes become useless against unwanted trees and shrubs, herbicides are often the most effective and inexpensive means for their control. Here are application techniques, using readily available herbicides, that can be used to control trees and brush. Understanding that not all methods and chemicals will control every plant species, here are several application methods that may help you in a given situation.

1. Soil Tree Herbicide Application

Photo by Steve Nix

Applying soil herbicides as a total treatment broadcast tool or when spotting compact areas can be quickly applied and cost effective on large acreage. This treatment is useful when treating an area with a high density of small stems needing total control (ex: sweet gum sprouts under loblolly pine) and also to remove individual specimens (undesirable tree sprouts and stems on productive timberland).

This form of timber stand improvement (TSI) uses soil herbicide uptake by a tree's root system to do the job. It demands an area where mechanical equipment can transport and spray or broadcast the chemical effectively such as under lower basal area stands of mature timber or over newly cleared tracts that are heavily populated with a poor tree species.

Only soil active herbicides (imazapyr, hexazinone, tebithiuron) can be used for this type of application. Since this method is subject to rain runoff, surrounding bodies of water and off-site areas should be taken into consideration. Follow label instructions and check state regulations that apply when using the herbicide.

2. Foliar Tree Herbicide Application

Photo by Steve Nix

A foliar application directs the herbicide/water mixture directly onto the leaves of a tree or shrub. This treatment is highly effective on smaller understory plants that can be mechanically sprayed over the entire leaf area. Use a foliar spray to remove undesirable understory plant competition (privet under pines) or as a single species control in patches of undesirable trees and shrubs.

This form of timber stand improvement uses a spray herbicide applied to saturate a tree's canopy and leaves. It also needs an area where mechanical equipment can transport and spray the chemical effectively but can also be done using a backpack sprayer (which can be labor intensive). Complete coverage of the foliage is critical for success but is a great treatment when patches of smaller trees and shrubs are the target species.

Auxin-type herbicides (like triclopyr) are generally most effective early in the growing season as leaves are first appearing. Enzyme-inhibiting herbicides (like imazapyr) are most effective during late summer or fall. Using the ever-popular Roundup (or less expensive generic forms of glyphosate) is most effective in late summer or fall but just before change in leaf color.

3. Bark Tree Herbicide Application

Photo by Steve Nix

A basal bark herbicide application combines a penetrant oil with a herbicide/water mixture and the mix is sprayed directly to the bark of a standing tree. This treatment is most effective on smaller stemmed plants that are less than 6" in diameter (DBH), becoming less and less effective on trees as their diameters increase (not the best control method on large trees like the one in the photo).

Unfortunately, every individual tree target has to be visited and the entire bark surface sprayed to at least one foot up the tree's base. This can be labor intensive where stem counts are high and is usually only done using a backpack sprayer. Basal applications can be made any time of the year, but are most effective during the dormant season when leaves are not present.

Basal applications will not provide rapid control. Herbicide injury is often not observed for several weeks after treatment and total control may require several months. Additionally, basal treatment is not effective on older trees with thick bark. For older trees, other application techniques should be employed.

Pathfinder is a "ready to use" product (basically triclopyr) that can be used at 100 percent strength. Other generic products are used with a basal oil to include imazapyr. This treatment is most effective on trees with smooth bark and thick bark trees may require retreatment.

4. Stump Tree Herbicide Application

The tree stump application method is used after cutting a tree to eliminate, or greatly reduce, resprouts from the stump surface. It is important to apply the herbicide to the stump surface immediately after all sawdust has been removed. A herbicide/water spray is fine but if herbicide treatment cannot be done immediately apply a herbicide/basal oil mixture.

Adding a dye to the herbicide formulation improves application success by showing exact stump coverage. Small stumps should be completely saturated. Stumps greater than 3 inches in diameter can be limited to the outer edge to limit chemical waste and runoff. Remember, the cambial layer around the outer edge is where the action is taking place.

Herbicides using this method can be applied using a backpack sprayer, squirt bottle, or paint brush. Again, no matter how the herbicide is applied, a tracer dye should be included to ensure treatment of all individual stumps. Most of the basic woody-stemmed herbicides can be used including triclopyr, imazapyr and glyphosate.

5. Hack and Squirt Tree Herbicide Application

Photo by Steve Nix

The hack-and-squirt technique is ideal for control of larger trees that limit the use of basal applications. This inexpensive but labor intensive method requires a small ax, machete, or hatchet to cut through the thick bark and into the sapwood. The cuts should create a “cup” to hold the herbicide solution and should ring the entire circumference of the tree.

The addition of a basal oil is not required in this fresh cut. Hack and squirt is best used on trees that are 4 to 5 inches in diameter or larger. Completely severe smaller trees and use the stump cut method. On larger trees you can get by with one cut or frill for every 2 inches of trunk diameter. Do not use this treatment during the spring as upward sap flow in the spring will flush out the herbicide.

Apply herbicides I mentioned (under stump cut) in dilution ratios from one-half to one-quarter strength. Read the product label to determine the appropriate dilution. Roundup (glyphosate) undiluted or half-strength is excellent for hack and squirt applications.

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