Wax Myrtle Biology and Control in Pastures
Southern wax myrtle (Morella cerifera), more commonly known as wax myrtle or simply myrtle, is a commonly found evergreen shrub/small tree (Figure 1). It is a native plant often found in fencerows, pond and marsh edges, pine plantations, hammocks, rangeland and pastures. Although many see this plant as a viable landscape option as well as important for wildlife habitat, dense infestations of wax myrtle can completely eliminate bahiagrass growth and survival (Figure 2). Therefore, it is important to know how to identify southern wax myrtle and the options for control.
Biology. Woody shrub/tree, that is capable of reaching a height and spread of 35 feet, but 15 to 20 foot tall plants are most common. Leaves are simple, alternate, olive-green in color, and are dotted with small rusty-looking glands on both sides of the leaves. Overall, the leaves have a wavy appearance (Figure 3) and are aromatic when crushed.
Young stems are light green to gray and are hairy. The bark, even with age, is relatively thin and is grayish-white (Figure 4). Individual plants typically have multiple trunks and new stems appear frequently from the trunk base and roots.
Wax myrtle flowers in the spring. Flowers are green, very small and are arranged in cylindrical clusters around individual branches (Figure 5) and immature fruit are also green (Figure 6). Mature fruit are less than 0.5 inches in diameter, round, fleshy and blue in color at maturity. The fruit attracts birds, which is the main dispersal mechanism for wax myrtle.
Wax myrtle does not grow well in sites containing extremely dry or wet soils as well as sites that are extremely nutrient deficient. Many of our soils in Florida are sandy, acidic and infertile, requiring at least some level of fertilization for forage production. Considering that low-level fertilization enhances wax myrtle growth, wax myrtle tends to be more problematic in improved pastures than in rangeland.
Controlled Burning. If wax myrtle has become problematic in rangeland, it is likely that the level of controlled burns is not sufficient. Research conducted in the late 1970s showed that burning rangeland ever 2 to 3 years provided excellent suppression of wax myrtle growth. This interval of burning allowed for sufficient fuel to carry the fire up into the wax myrtle canopy. In pastures (especially bahiagrass), however, there is generally not enough fuel to carry the fire into the way myrtle canopy, especially in late winter.
Mechanical Control. Prior to the use of herbicides, roller-chopping or some other method was used for wax myrtle control. Mechanical treatment, as with other weeds, only provides temporary control. It has been documented that waxy myrtle plants grow at least 2 inches every 30 days; therefore chopping will result temporary control, but plants can be at least 12 inches tall 6 months after treatment.
Chemical Control. Control of wax myrtle with herbicides can be a daunting task, considering that there are typically many different sizes of wax myrtle within a given pasture or rangeland. Additionally, it has been determined that wax myrtle plants over 30 inches tall are more difficult to control compared with smaller plants. Therefore, if a pasture contains a number of plants over 30 inches tall, it is best if the wax myrtle clumps are chopped prior to treatment with herbicides. The fast growth rate of wax myrtle allows for spring chopping or mowing followed by herbicide treatment in the fall. Our research shows that fall applications (after spring chopping) of 1 quart/acre (using a water volume of 20 to 30 gallons of water per acre) of Remedy Ultra provides 80% control 12 months after treatment.
For very large plants where broadcast herbicide application is not feasible, it is advisable to use basal or cut-stump applications of Remedy Ultra. For basal bark applications to work successfully, the trunk size should be less than 6 inches in diameter. Mix 25% Remedy Ultra with 75% basal oil (1 qt Remedy Ultra + 3 qt basal oil) and apply the solution to the lower 12 to 18 inches of the trunk. Keep in mind that all trunks much be treated within a given clump to provide adequate control. For cut-stump applications, apply a 25% Remedy Ultra solution (in water) to a freshly cut surface. The herbicide solution must be applied to the cut stump within 15 minutes of cutting the wax myrtle to ensure penetration to the roots. For more information on techniques for woody plant control, see EDIS publication SS-AGR-260 (http://edis.ifas.ufl.edu/AG245).
Figure 1. Wax myrtle plants are often found in improved pastures, fencerows, pond and marsh edges, and native rangeland. Photograph by Brent Sellers.
Figure 2. Large, dense areas of wax myrtle can completely eliminate forage growth. Wax myrtle plants were removed approximately 7 months prior with a bobcat equipped with a chipper/shredder. Photograph by Brent Sellers.
Figure 3. Waxy myrtle leaves are typically wavy, 2 to 4 inches long with 2 to 3 blunt teeth near the tip of the leaf. Photograph by Brent Sellers.
Figure 4. Wax myrtle bark is typically grayish-white and is relatively thin, making basal bark applications feasible. Photograph by Brent Sellers.
Figure 5. Wax myrtle flowers are green and arranged in cylindrical clusters. Photograph by Chris Evans, River to River CWMA, Bugwood.org.
Figure 6. Immature wax myrtle fruits are green. Mature fruits are blue in color and are fleshy, attracting birds for dispersal. Photograph by Brent Sellers.