Herbicides can best be classified based on whether the chemical is soil- or foliar-applied.
Soil-Applied Herbicides - As the name implies, soil applied herbicides are applied to the soil, although some require incorporation into the soil. They are carried into the soil by water and absorbed by the roots of weeds. These herbicides effectively control weeds for a few weeks to several months, with the exact time depending on the particular herbicide, rate and time of application, weather and type of soil. Because of this long-term residual effectiveness, soil-applied herbicides are sometimes referred to as residual herbicides. Examples of soil-applied herbicides currently used by Christmas tree growers include simazine (Princep), atrazine (Aatrex), pronamide (Kerb) and hexazinone (VelDar).a
a. Reference to a herbicide in thi sarticle, either by trade or common name in no way represents a recommendation or endorsement of that chemical.
Soil-applied herbicides are the "backbone" of chemical weed control in Christmas tree plantation management and are the chemicals used year-in and yearout for maintenance of relatively weed-free areas around the base of young trees. Depending on the particular herbicide and management system, these herbicides are applied either in late fall or early spring.
Many of the residual herbicides such as simazine can normally be applied directly over trees at any time with no expected damage to the foliage. With some, such as atrazine, care must be taken to be sure the trees are not actively growing or some damage will occur. Check the herbicide label for recommended time of application.
Foliar-Applied Herbicides - Foliar-applied herbicides are applied directly to the weeds and kill them primarily by being absorbed into the foliage. For this reason, they are often referred to as contact herbicides. In addition, some of the contact herbicides have a relatively short-lived soil activity. Historically, the contact herbicide most commonly used by Christmas tree growers was Amitrol-T. Glyphosate (Roundup) is a relatively new contact herbicide gaining wide acceptance among growers.
In Christmas tree plantations, contact herbicides are of more limited and special use than residual herbicides. Some growers use them in site preparation before planting, as described earlier. They may also be useful in salvaging plantations in which weeds have gotten out of control or in eliminating difficult to control deep rooted perennials. Soil-applied herbicides, at recommended rates, usually do not "knock down" tall, well-established weeds. A foliar herbicide is needed to weaken or kill such weeds and allow the soil- applied herbicide to gain control. Generally, a soil-applied herbicide is applied with or after the contact herbicide.
When foliar-applied herbicides are sprayed in established plantations, trees should usually be protected (read the label). This can be done by rigging shields on the sprayer, placing stovepipe or a similar shield around each tree as it is hand sprayed or carefully keeping the spray off trees on a very calm day.
Hand or tractor drawn rope-wick applicators are an alternative method of applying contact herbicides. With these applicators, the herbicide is applied by rubbing the weeds with a rope wetted with the chemical. A reservoir continually supplies the rope with herbicide. The rope applicator can be drawn down either side of a row of trees, or if the weeds are taller than the trees, the rope applicator can be passed over the row of trees, applying the herbicide to the taller weeds but not the trees.
Some foliar herbicides can be applied over Christmas trees in the fall while the weeds are active but after trees have hardened off sufficiently. Such applications are useful for controlling deep- rooted perennial weeds not easily controlled with residual herbicides. In central Ohio, such an application is usually made between the end of the first and third weeks of September, depending on weather conditions in that particular year. Extreme care should be taken when using this technique because the condition of the trees is critical, and dates for safe application vary from year to year. Growers considering using this technique should test the technique on a few trees to absolutely identify when it is safe to apply herbicide to each species of tree. If the chemical is applied too early, trees may be injured or die, and nothing can be done to save them.
The label of a foliar-applied herbicide may recommend the use of a wetting agent or sticker-spreader. This is a compound that reduces the surface tension of the herbicide-water mix and allows the drops to spread out, resulting in better coverage. Sticker-spreaders are very effective for some foliar-applied herbicides but are not recommended for others because they reduce the effectiveness of the herbicide. Check the label.