Technical resource bulletin. TRB-015, Use of hexazinone to control turkey oak competition in a young longleaf plantation

North Carolina Forest Service TRB — 015 Technical Resource Bulletin TRB-015 July 2020 The Use of Hexazinone to Control Turkey Oak Competition in a Young Longleaf Plantation Bill Pickens - Conifer Silviculturist, NC Forest Service, Raleigh, NC Hans Rohr - Forest Supervisor - Bladen Lakes State Forest, NC Forest Service, Elizabethtown, NC Sarah Crate - Outreach Communications Coordinator, The Longleaf Alliance, Andalusia, AL Abstract: The use of hexazinone in a longleaf stand is an effective method to control turkey oaks that compete for resources and shade the forest floor. Studies show competition control increases tree growth and allows the understory plant community to expand and flourish. In our demonstration study the broadcast application of hexazinone increased total average tree height in the treated plots by 9. 1 feet representing a 24% gain over the untreated trees. Average di¬ ameter at breast height (DBH) was 2.4-inches greater for the treated plots. Hexazinone application caused positive changes to the understory plant community resulting in substantial increases in the number and abundance of grasses and forbs present. A one-time application of hexazinone is a good management option to maximize timber production volume, facilitate pinestraw raking or to jumpstart an understory restoration project. Introduction Many degraded longleaf sites in the sandhills regions of North Carolina are invaded by turkey oak and other scrub oaks following a timber harvest or planting failure. Fire or mechanical removal is often not effective to control these thick bark scrub oaks, as they sprout vigorously if top- killed by fire or if cut by a saw. Herbicide treatment is a proven method to reduce the number encroaching hard¬ woods and at the same time favor expansion of the under¬ story herbaceous layer. A one-time application of herbi¬ cide can be used to compliment or jumpstart restoration efforts that includes a prescribed burn program. Control of competing woody vegetation is also effective to increase timber growth and yield, Pinestraw producers want to eliminate hardwood brush and trees to produce debris free bales and improve raking efficiency. Hexazinone is a broad spectrum, water soluble herbicide whose mode of action is photosynthesis inhibition. Hexa¬ zinone controls some grasses, many broadleaf forbs and some woody species. Oaks, sweetgum and sumac are very susceptible to hexazinone. Longleaf pine is tolerant of hexazinone at prescribed rates. Blueberry, legumes, grass¬ es, and many other forbs are largely unaffected or will re¬ cover from application. Hexazinone is soil active and so is mostly absorbed by plant roots with minor foliar uptake. It works best when applied to moist soil. Hexazinone ’s per¬ sistence in forest soils is relatively brief with a typical half -life of 90 days. It is highly soluble in water and leaches readily. Hardwood trees are significant competitors for resources required to optimize growth in pine plantations. Numerous studies have documented growth and volume increases from early vegetative control treatments in pine planta¬ tion' 6. The growth benefit derived from vegetative control is long lasting with differences in height and diameter con¬ tinuing to diverge into mid-rotation4. The removal of hardwood trees and brush also increases the number of legumes, native grasses, other forbs in the understory8. Longleaf pine ecosystems require some type of woody competition control to restore or maintain a fa¬ vorable understory plant community4. Without control, especially frequent fire (l-3 years), the dense canopy of hardwood will shade out desirable grasses and forbs. Low rates of hexazinone is very effective at controlling oaks on the sandy sites while selectively sparing the wiregrass, bluestems, and other native grasses associated with long¬ leaf pine ecosystems3. The purpose of our project was to demonstrate the benefits realized from the use hexazinone to control turkey oak in a longleaf pine plantation. We hypothesized that hexazi¬ none would provide effective oak control and the young longleaf trees would respond with increased growth. We also assumed a decrease in shading would encourage colo¬ nization of understory plants and stimulate the expansion of those already present, especially wiregrass. Methods The study was initiated in 2006 at Bladen Lakes State For¬ est located in southeastern North Carolina. The predomi¬ nant soil is a Wakulla series - a very deep, somewhat ex¬ cessively drained, rapidly permeable sandy soil of the Coastal Plain that is low in organic matter, nutrients and water holding capacity. Precipitation is abundant in the region averaging 49 inches per year. The soil has a site index of 7050 for longleaf. Prior to a clearcut harvest in fall of 1 999 the site was occu¬ pied by slash pine stand. In January of 2002 container grown longleaf seedlings were hand planted (Ю x Ю spac¬ ing - 435 seedlings per acre). No site preparation treatment