Site Preparation Weed Control: When Desperate Times Call for Desperate Measures

By W. Carroll Johnson, III, Ph.D

 ike everybody else, professionally I had to “pay my dues.” In 1984, I was the newest and youngest by a substantial margin in my department which meant that I was assigned the duties that nobody else wanted. For the weed scientists, that meant crop assignments on which to conduct weed science educational programs.
One of my early hodgepodge of unrelated crop assignments was weed control in Christmas tree plantings. At that time, weed scientists typically viewed weed control strictly as which herbicide to use and I was no different in that regard. Nobody wanted Christmas tree weed control since there were very few herbicide choices and successful weed control in that system did not rely on a simple approach. That attitude was embarrassing and the commodity deserved more scientific support than it was given. Nationwide, Christmas trees are planted on more than 300,000 acres and more than 17 million trees are harvested annually. In North Carolina alone, Christmas tree production is worth more than $100,000,000.

In many ways, weed control in Christmas tree plantings is very similar to weed control in food plots — complex. Complexity is due to no single approach to weed control will work and successful weed control depends on a broad-based strategy. Weed control in Christmas tree plantings begins months before trees are planted and this is termed site preparation weed control. Considering that Christmas trees are often planted in remote and marginally arable sites, it is easy to see another interesting parallel with weed control in food plots. Food plots are often established in newly cleared sites in forested areas. Food plot sites can be along old logging roads, ramps, or any clearing in the timber large enough for a food plot. Regardless of the previous land use, there will certainly be a proliferation of perennial plants that refuse to surrender and be easily controlled. Common perennial plants that may be encountered include briars, blackberry, vines, trumpet creeper, and dozens of species of deciduous tree saplings sprouting from old rootstock. Of course, there will be the typical perennials like common bermudagrass, quackgrass, johnsongrass, broomsedge, and nutsedges. Weeds in the newly cleared sites defy normal control efforts and require aggressive mowing, tillage, and herbicides to achieve any reasonable level of success.

Tillage and Mowing

Mowing, which includes the heavy-duty ‘mulching’ using tracked vehicles, is a logical first step. Mowing or mulching rough areas will not kill saplings or any other perennial weed. We all know that. Mowing weakens perennial weeds which greatly improves subsequent weed control efforts. Mowing also enhances tillage by shredding the tops of tall plants which improves the operation of any tillage implement. Mowing stimulates succulent re-growth, which is often more susceptible to herbicide uptake than older, tougher foliage. This is an essential step for successful herbicide performance, which is discussed later. Initial tillage of rough non-improved sites is a nasty job and must be carefully conducted to prevent damage to equipment. This is a case where large (heavy-duty) disk harrows are the preferred choice. Harrow size is not necessarily the width of the implement. In this context, harrow size subjectively refers to diameter of harrow blades and robustness of the frame. Site preparation tillage is typically the site’s first tillage in many years. This takes substantial horsepower and a heavy implement to cut through vegetative material and compacted soil. One tillage pass is rarely sufficient for site preparation weed control. I have observed in my recent weed control experiences that tillage during dry periods is far more effective in controlling perennial weeds than tillage during rainy periods. Damaged perennial weeds can survive if tilled when soils are wet or if it rains shortly afterwards. I am a hobbyist food plotter like a lot of Whitetail News readers and fully understand that scheduling conflicts and logistics heavily influence when I work on food plots. Those real-world considerations often trump working around ideal weather conditions. However, this is one of the rare situations when drier conditions are better than wetter conditions.

Herbicides for Site Preparation

This is the best role for non-selective herbicides in food plots and the focal point is certain formulations of glyphosate (Roundup, Accord and others). Glyphosate is well known and highly versatile. For site preparation uses, glyphosate must be applied at very high rates to control perennial weeds and woody saplings. Steps must be taken to ensure maximum efficacy; applying glyphosate at a high rate suitable to control perennial weeds and when environmental conditions are ideal. Use of an appropriate adjuvant is also a wise investment. Refer to the glyphosate label for guidance. In site preparation when woody species are often targeted, glyphosate should be tank mixed with systemic broadleaf herbicides like triclopyr (Garlon 3A) and/or 2,4-D. These chemicals are powerful broadleaf herbicides that are synergistic with glyphosate, with amine salt formulations preferred due to improved mixing qualities. Triclopyr and 2,4-D significantly improve brush control over glyphosate alone, with triclopyr being the superior tank-mix partner in terms of efficacy. In addition, glyphosate controls annual and perennial grasses that are not controlled by triclopyr and 2,4-D. None of these herbicides have appreciable long-term soil activity at common use rates which makes them ideal for site preparation prior to planting food plots. That said, there is always the disclaimer that confuses the matter. Under certain conditions, triclopyr, 2,4-D, and (to a far lesser extent) glyphosate can persist in the soil and injure desirable forages if seeded too soon after application. This is not a certainty, but still warrants caution. A good rule of thumb is to apply combinations of triclopyr, 2,4-D, and/or glyphosate four to six weeks prior to seeding the forage to minimize chances for injury. This is not as restrictive as it may seem on the surface. These herbicides, alone or in combination, need time to control woody species. The delay between treatment and seeding the forage to ensure crop safety basically equals the delay needed for optimum weed control. When everything proceeds according to plan, these herbicide combinations are typically applied late-summer prior to planting fall food plots and will provide outstanding weed control that is noticeable for about 12 months. Re-growth of deciduous saplings and other perennial weeds will soon become evident and retreatment may be needed. Retreatment can be spot sprays or broadcast treatment at lower herbicide rates. Given this likely course of action, initial forage plantings in these sites should be annuals with retreatment between plantings. Avoid planting perennial forages until adequate control of the woody perennial weeds has been achieved. It should be noted and emphasized that direct applications of triclopyr, 2,4-D, and glyphosate will kill forages. These herbicides are used for site preparation weed control for a reason — they are very active on broadleaf plants and unfortunately that includes legume and brassica forages. There is absolutely no ambiguity in that statement. There is little chance of injury when these herbicides are applied four to six weeks before seeding the forage. Site preparation weed control is another example of the need to be proactive. Hunting land often changes hands on a yearly basis and site-preparation weed control may not be feasible in those cases. If land is owned or at least available long-term, site preparation weed control is a worthwhile investment and a significant step towards establishing high quality food plots in rough areas that are often highly desirable for hunting.