Successful Control of Lonicera maackii (Amur Honeysuckle) with Basal Bark Herbicide

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Lonicera maackii (family Caprifoliaceae), often referred to as Amur Honeysuckle, is a perennial shrub native to temperate Asia. In the Midwestern United States, L. maackii is an invasive shrub that was first made available as an ornamental plant and was later used for soil retention (Saxton 2012). The shrub now infests many savannas, woodlands, and grasslands in the region. Lonicera maackii has traits that make it a strong competitor. The shrub's leaves emerge very early in spring and stay green long into fall. Lonicera maackii can dominate the shrub canopy, leaving ground layer plants with much reduced sunlight. The shrub also produces allelopathic chemicals that inhibits other species (Hartman and McCarthy 2004). Its seeds are highly viable, germinate easily, and recruit readily, especially in areas with a lot of sunlight (Schulz and Wright 2015). The shrub rarely shows signs of disease.

Lonicera maackii is difficult to eradicate. Spraying the foliage causes herbicide to land on large areas of the ground layer, likely damaging desirable plants. Pulling the shrub from the ground when plants are small is effective, but even modestly-sized plants are a physical challenge to pull and leave the soil disturbed. Cutting the plant and applying herbicide to the cut stem produces high mortality; however, cutting can be laborious (Love and Anderson 2009). Mowing shrubs and prescribed fire can top-kill the plants, which will re-sprout.

In this study, we examined the efficacy of basal bark application in the month of May. We chose to study a basal bark herbicide treatment in May because in the few weeks after prescribed fire season (March–April), Nachusa Grasslands' herbaceous weed management season has not started, and it is obvious which shrubs have not been killed by prescribed fire or previously treated with herbicide. We conducted this 2014 work at The Nature Conservancy's Nachusa Grasslands preserve, Ogle County, Illinois (latitude 41°53'41.64" N, longitude 89°22'11.28" W, elevation 247 m). The site was in the Stone Barn Savanna unit with an over-story of Quercus alba (white oak), Quercus velutina (black oak), Quercus macrocarpa (bur oak), Carya ovata (shagbark hickory), and Carya cordiformis (bitternut hickory). The tract was purchased in 1999 and had a dominant understory cover of L. maackii shrubs. The study area was within a prescribed burn unit that has had almost annual fire since 1999; however, some of this area has had little direct fire due to the dense shading from the shrubs providing little fuel to carry fire. No fire occurred in the treatment plots from the herbicide application on May 22, 2014 to the data collection on September 13 and 14, 2014.

We randomly chose three treatment plots from 0.45 ha of a ridgeline running east-west. The plots were relatively flat with a slight southern aspect with a slope of 0–3%. The three plots were each approximately 30 m in the east-west direction and 15 m in the north-south direction. All three treatments plots were dense with L. maackii, with a ground layer of exposed soil and a few typical herbaceous plants such as Geranium maculatum (wild geranium), Ageratina altissima (white snakeroot), Alliaria petiolata (garlic mustard), Circaea lutetiana (enchanter's nightshade), and Pilea pumila (clearweed). One of the three treatment areas contained a retired, shallow, dolomite quarry.

The main treatment was basal bark herbicide applied to L. maackii. We also had a treatment of the mineral oil carrier without herbicide (to confirm the herbicide, not the carrier, was the lethal agent). We also had a control whereby no treatments were made to L. maackii. The shrubs varied in size with stem diameters ranging from 0.25–15 cm at ground level. Those that were multi-stemmed had...