Chemical Release Treatments for Pine Regeneration

The importance of sustaining South Carolina’s southern yellow pine resource is essential both from economical and restoration perspectives. In 2015, softwood (pine; 660.9 million cubic feet of roundwood and 316 million cubic feet of pulpwood) dominated South Carolina’s total timber production (764.1 million cubic feet of roundwood and 381.3 million cubic feet of pulpwood).¹ Pure stands of loblolly/ shortleaf (43%) and longleaf pine (5%) comprise just under half of the forestland in South Carolina. Pine plantations make up 25% of the forestland in South Carolina, which remain plantations following harvest, barring conversion to non-forest.² The time after harvest is the point in forest management that the future pine stand will either be established by natural or artificial regeneration techniques, pending the land manager’s preference. Prescribed fire, herbicides, and harvesting methodology to encourage natural regeneration are potential land management tools selected to achieve a specific outcome. Such examples might include relying on natural regeneration in longleaf pine stands by implementing a shelterwood harvest with multiple prescribed fires.³ Alternatively, for loblolly or shortleaf pine, landowners typically choose to employ clearcutting followed by a chemical site preparation treatment and then artificially regenerate these stands by planting.

Competing Vegetation – The Necessity of Chemicals

Land managers typically use prescribed fire when naturally regenerating longleaf pine. Standard management recommendations may call for a two-year or longer delay before burning to reduce fire intensity by allowing logging slash and foliage debris to decay.³ In some instances, competing vegetation could outcompete the longleaf reproduction cohort before a prescribed fire can be implemented; this may especially be true in the presence of invasive species. Invasive species can make management endeavors more difficult as they may have heightened recruitment rates and quickly sprout from stump sprouts, root suckers, or rhizomes on these altered seedbeds. Invasive plants include tree species such as the tree of heaven (Ailanthus altissima),⁴ royal paulownia (Paulownia tomentosa), callery pear (Pyrus calleryana), mimosa (Albizzia julibrissin), Chinese tallowtree (Triadica sebifera); grass species including Japanese stiltgrass (Microstegium vimineum)⁵, cogongrass (Imperata cylindrica), bamboo (Phyllostachys sp.), and giant reed (Arundo donax); and vines or woody shrubs like multiflora rose (Rosa multiflora),⁵ Japanese climbing fern (Lygodium japonicum), Japanese honeysuckle (Lonicera japonica), Chinese wisteria (Wisteria sinensis), and bicolor lespedeza (Lespedeza bicolor). Whether native or non-native, fast colonizing species such as bracken fern (Pteridium aquilinum),⁶ can also be problematic due to canopy disturbance (increased sunlight conditions) and prescribed fire.⁷ Each physiographic region in South Carolina hosts most of these invasive plant species. Chemical control of invasive species is necessary and will achieve greater control than prescribed fire.

Harvest of some stands involves silvicultural clearcutting that removes (fells) all standing trees to liquidate the standing timber. Clearcut stands rely on artificial regeneration (typically planting of genetically improved loblolly pine) to establish the next stand. Some form of site preparation is conducted after clearcutting and before implementing artificial regeneration techniques. Mechanical (e.g., rolling drum chopping, bedding, shearing, etc.) or chemical treatments are applied to control competing vegetation that emerges the following growing season after harvest. Most site preparation activities are completed in late summer or early fall before seedling planting when soil conditions tend to be drier and before woody plants enter dormancy. Competing herbaceous vegetation will often re-establish during the first growing season after planting. Competing vegetation may be problematic for planted seedling growth and may reduce first-year survival, especially if drought occurs during the first few months after planting in the spring. Competition for water, nutrients, and growing space is intense depending on the types of plants that respond post-treatment. Problematic plants likely were present in the seed bank, root systems remained after harvest or were dispersed on-site from an adjacent area, or a combination of these. Some of the vegetation could even be allelopathic. Allelopathic plants release chemicals from leaf debris or roots in areas adjacent to the plant, suppressing the growth of other plant species and further reducing the pine stock potential. Competition that reduced stock potential may be observed on sites with established invasive plants or if the site preparation treatment was inadequate. Inadequate vegetation control may occur due to application error. Possible application errors include improper herbicide selection to control the vegetation community on-site, inadequate amount of active ingredient applied, insufficient spray coverage of the area, improper equipment calibration, and applications made during less desirable weather conditions, among others. If the biomass of competing vegetation is excessive, the land manager may justify using a chemical seedling release on the competing vegetation to minimize losses attributed to mortality and reduced seedling growth. This chemical seedling release application would be either a herbaceous release or a woody release, depending on the abundance of live woody or herbaceous species within the target zone.

Herbicide Selection

Multiple forestry-approved herbicides are available for use in pine seedling release. Soil characteristics, the plant species targeted, and the desired pine species dictate herbicide selection (single and tank mixes) and application rates. Soil characteristics, including texture (dictated by percent sand, silt, and clay), pH, and drainage class, should be assessed before or during the herbicide selection process. Prior to herbicide application, identify the soil type to ensure adequate control of competitive species. Generate a soil assessment using the Natural Resources Conservation Service’s web soil survey website. A landowner may also seek free soil identification assistance from their local South Carolina Forestry Commission project forester, Clemson Cooperative Extension Agent, or the local NRCS District Conservationist.

Soil Characteristics

Figure 1. Product label for Arsenal AC detailing pine release rates.

Application rates depend on soil type. Coarser soil types and soil with lower percent organic matter, typically require a lower per acre application rate. Coarse soils and soils low in organic matter likely also experience a shorter weed control duration (soil persistence) with most soil active herbicides used for forestry applications. For pine seedling release, soil persistent active ingredients include aminopyralid (Milestone^®), metsulfuron-methyl (Escort XP^®, MSM 60^®, and others), sulfometuron-methyl (Oust XP^®, SFM 75^®, and others), hexazinone (Velpar^® products, Velossa^®), and imazapyr (Arsenal AC^®, Polaris AC^®, and others).

Soils with extreme pH conditions, whether alkaline (high pH) or strongly acidic (low pH), can also inhibit the effectiveness of certain herbicides. Extreme pH conditions can either be naturally occurring or influenced by human activities. For example, when agricultural fields are converted from row crops to pine forestland as part of the Conservation Reserve Program (CRP), soils within those fields could have more alkaline pH from lime additions to enhance nutrient availability. Poorly drained soils exhibiting prolonged standing water conditions can also reduce the probability of successful plant control. Thus, site conditions at a given locality influence the herbicide selection process. The herbicide product label directs individual herbicide application rates associated with various soil factors (figure 1). Managers should always reference potential herbicide labels during the herbicide selection process.

Sensitivity of Pine Species

The tolerance or sensitivity of individual pine species to a given herbicide varies. Heightened sensitivity may require a reduced application rate to avoid damage to a particular pine species. The applicator must discern the difference between the rates used for herbaceous and woody release around planted seedlings and those used for site preparation treatments. Broadcast sprays, treating the entire area, applied for release (herbaceous or woody) that mistakenly use site preparation rates will result in some form of seedling damage, whether reduced potential growth or possible mortality.

Herbicide selection and use options for post-plant, release treatments are more limited to avoid injury to planted pine seedlings. Proper herbicides selection and application rate are particularly important when managing vegetation within longleaf and slash pine plantations. Note the footnote at the bottom of figure 1 taken from the label of Arsenal AC^®. The application rate range for seedling release for both longleaf and slash pine are lower than for loblolly pine. Also, note the time restriction for spraying over the top of longleaf and slash as both season (late summer only) and stand age are critical to ensure application effectiveness. Herbicides applied in pine stands for hardwood control should only be applied after pine seedling root growth has begun. Root growth initiation usually begins one month after planting for loblolly pine and two months after planting for slash and longleaf. The label also indicates the need to avoid surfactants and the use of a lower rate in sandy soils.

Herbicide Application Timing

Late winter and early spring are typically the prime time for performing herbaceous weed control applications (sixty days or more after planting in most cases). A number of potential herbicides are available that provide adequate vegetation control in young planted pine stands. It is imperative that the applicator thoroughly read the label and adhere to all restrictions and guidelines regarding herbicide use. If off-label application occurs, time, money, and potentially the pine seedlings themselves, could be lost to inappropriate application and environmental harm (e.g., herbicide entering water sources) is likely. The applicator must remember that the label is the law. Online resources such as the Greenbook website provide quick access to product labels prior to purchase.

Weather is also a critical factor when planning herbicide spray operations. Avoid conducting herbicide applications during high temperature, high humidity, rainfall (permit adequate time for herbicide absorption into the plant after application before rain begins), inversion layers, wind (over 2 to 3 mph), and freezing temperatures as these conditions reduce herbicide effectiveness. Avoid hardwood control applications to seedlings when stressors like drought, excessive rainfall, insect damage (e.g., pine tip moth) prevail. The product label also addresses potential issues related to weather conditions, enabling the user to understand potential negative impacts due to weather variables.

There is the potential for government cost-share funding to assist with pesticide applications in forestry settings. Both the Natural Resources Conservation Service (under the USDA) and the South Carolina Forestry Commission have programs that may assist in reducing the financial burden associated with pine plantation management. Seek assistance from your local, state, or federal office to determine eligibility and program guidelines.

Invasive Plant Species Identification

Accurate identification of plant species for control assists in the selection of the appropriate herbicide. Smartphone applications are available to help land managers correctly identify plant species within reforestation sites. Some smartphone apps for plant identification include PlantNet, iNaturalist, and PlantSnap. A species composition list on-site enables verification (according to the product label) of the active ingredient(s) that are capable of adequate target plant(s) control. If multiple plant species (woody trees and shrubs and herbaceous plants) are present, control may require applications using a tank mixture of two or more herbicides chemistries. If the label does not list a particular plant, a different herbicide may be required in the solution.

Herbicide Compatibility in Tank Mixtures

The individual herbicide label presents information on compatible herbicides that may be used in combination to form a mixture. Mixing non-compatible herbicides may render the solution ineffective or reduce herbicide efficacy. Perform a “jar test” before combining herbicides not listed on the label as compatible. The jar test simply uses a clear glass container to mix different herbicides and enables visual observation of how well the chemicals “mix.” If the physical properties (“looks”) of the herbicides change once mixed, they are most likely not compatible. Physical changes or visual cues of incompatibility may include the formation of gels, foams, lumps, precipitates, sludges, or separation of liquids. Should such physical changes appear or form in the mixture, the herbicides are likely incompatible; dispose of the mixture according to label directions. The herbicide label details applicator instruction procedures. Wear all required personal protective equipment (PPE) listed on the label for the type of application made. Consider and adhere to these guidelines to avoid possible detrimental health and environmental effects resulting from improper application.

Tank Mixtures and Timing of Applications

Figure 2. Herbaceous release (30 days post treatment) over planted pine to control bracken fern. Image credit: Sean Bowers. Clemson University.

Results from multiple research studies evaluating the effectiveness of varied herbicides in mixtures and timing of herbicide applications guide recommendations made to forest applicators. Treatments can be applied to cover the entire area as a broadcast spray, or a portion of the site can receive banded spray applications in linear rows directly above and around the planted seedling rows. Banded applications are less costly because less herbicide is applied per unit area. Banded sprays can be equally effective as broadcast applications.⁸ A tank mix of either hexazinone (Velpar^® L at 32 ounces per acre) or imazapyr (Arsenal AC^® at 4 ounces per acre) with sulfometuron-methyl (Oust XP^® at 2 ounces per acre) herbicides is considered as the “industry standards” for herbaceous loblolly pine seedling release^{9, 10}. Various other combinations and rates can be applied, depending on the particular situation. Competing vegetation consisting of herbaceous forbs and wood plants can be controlled by herbicides found in table 1 (table 1 is available as a separate document for download). When grass species are the primary competitor for the crop trees, grass selective herbicides found in table 2 would be appropriate. The timing of application also can significantly impact invasive species control, as an early spring release using a mixture of sulfometuron-methyl and hexazinone improves longleaf pine first-year survival, volume production,² maximized seedling height growth,^{11, 12} and encourages height growth out of the grass stage.^{5, 6, 9} Early growing season release using imazapyr with sulfometuron-methyl can also maximize seedling performance for loblolly pine only. This mixed formulation will adequately control herbaceous competition such as bracken fern as depicted in figure 2.

Table 2. Grass selective herbicides for use in pine seedling release treatments in field conversion applications.

Applications conducted prior to bud break reduce seedling injury. Applications performed after bud break may lead to stunted or reduced growth during that particular growing season. A formulation of hexazinone pre-mixed with sulfometuron-methyl carrying the trade name Oustar^®, produced by Bayer^TM, also provides excellent results when applied as a herbaceous release treatment. Results from an Oustar^® applications are comparable to the “industry standards” for seedling growth parameters and also for maintaining higher percentages of bare ground out to 120 days after treatment.¹⁰ Oustar^® produced superior results to other herbicides for herbaceous release of slash pine.³ Metsulfuron-methyl can be added in tank mixes at a rate of ½ to 1 ounce per acre, if releasing loblolly or slash pine. In slash pine stands, metsulfuron- methyl should not be tank mixed with other soil active herbicides. Combined chemicals may still separate if not agitated for a lengthy duration. Periodic or continuous mixture agitation helps to re-suspend any sulfometuron-methyl particles that could settle to the bottom of the tank from a period of inactivity. Non-ionic surfactant (NIS) or methylated seed oil (MSO) should be added to the mix solution to improve herbicide performance. Either adjuvant should be incorporated at 0.25-0.5% of the overall mixture. Use NIS when applications are conducted during cooler or wetter periods. MSO addition is advised during periods of drier weather.

In some instances, Oustar^® may not be available for the applicator to utilize for herbaceous weed control treatments. The pesticide applicator may opt to manually mix a formulation of hexazinone with sulfometuron-methyl that is comparable to Oustar^®. Table 3 illustrates appropriate mix rates of three herbicide products containing hexazinone with sulfometuron to serve as a procedural guide.

Table 3. Comparative mix rates of individual hexazinone products plus sulfometuron-methyl products to simulate rates of pre-mixed Oustar® (contains both hexazinone and sulfometuron-methyl).

¹ fl. designates fluid ounce measures; wt. are dry weight measures.

² Maximum use rate of Oustar® is 24 oz. per acre. Use minimum application rates on coarse (sandy) soils or soils with low organic matter and maximum rates on finer soils such as clay loams or higher organic matter content.

In some post-planting forest settings, hardwood tree and shrub vegetation may become established in subsequent years after the initial growing season. Chemical woody release can be applied within the second growing season until crown closure to reduce plant competition and enhance growth of favored pine crop trees. Previous research yielded success using hexazinone^{13, 14} and imazapyr ^{13, 15, 16} to accomplish this objective. Table 4 presents potential herbicides and application guidelines for woody release of pine species (table 4 is available as a separate document for download)

Herbicide selection is determined largely by the vegetative community present on the site. Each herbicide has its own limitations and strengths as to the ability to control or suppress certain plant species. Table 5 lists some of the plants that are controlled and tolerant to various herbicides (table 5 is available as a separate document for download). However, this is not an exhaustive list, and the reader is advised to refer to the product label after plant identification and evaluation in the field.

The land manager has many options regarding vegetation management in areas recently planted into pine. Accurate assessment of the plant community and knowledge of pine species susceptibility is of great importance to properly determine the needed product. Application timing and methodology for a particular herbicide will also make the difference in a successful treatment that achieves favorable competition control or potential death of the planted pines due to applicator error. This publication is a tool to assist in the decision-making process involving both herbaceous and woody release around pre-commercial pine stands. The applicator should still read the product label to ensure legal compliance after acquiring the herbicide(s) in order to avoid a costly mistake.

References Cited

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