Recently, I attended a symposium where current research findings on the link between Japanese barberry (Berberis thunbergii) and Lyme disease were presented. Over the past few years we’ve learned that this highly invasive non-native shrub is not only impacting our regional ecosystems, it’s also indirectly affecting our public health. While Japanese barberry is considered invasive in at least 20 states and the District of Columbia, it is also still available for sale in many nurseries across the US.
According to recent studies by scientists Jeffrey Ward and Scott Williams at the Connecticut Agriculture and Experiment Station (CAES) eliminating stands of Japanese barberry from forested areas can reduce the number of Lyme disease-infected ticks on the property by 80%.
The Problem With Japanese Barberry
Once established, Japanese barberry tolerates a wide array of site conditions, from full sun to shade and it’s not overly picky about soil conditions. It reproduces through seeds, rhizomes or layering and forms dense thickets that choke out native wildflowers and tree saplings.
Berberis thunbergii is a prolific seed producer and it’s seeds are estimated to have a 90% germination rate. Birds eat the seeds and carry them for a few hundred yards, depositing them in areas where the Japanese barberry can take hold and quickly choke out most other plants.
Breeding programs to develop ‘sterile’ Japanese barberry cultivars may be misguided since past history with other sterile cultivars of invasive plants has shown they sometimes cross-pollinate with the wild species.
The Link Between Japanese Barberry, Ticks and Lyme Disease
Ward and Williams’ research has shown that Japanese barberry infested forests have approximately 120 ticks infected with the Lyme disease bacteria per acre. Compare that to approximately 10 infected ticks per acre found in forested areas with native trees and shrubs and no Japanese barberry.
It seems that stands of Japanese barberry retain humidity. And ticks need an environment with about 80% humidity to actively feed, quest and reproduce. By measuring humidity levels above and at ground level under the barberry foliage, Ward and Williams found humidity levels under Japanese barberry dipped below 80% for only one hour per day. In open, sunny areas with no Japanese barberry, the exact opposite is true. The humidity levels rise to 80% for only one hour each day.
White-footed mice, a known apex host for Lyme disease also thrive under the canopy of Japanese barberry. The combination of mice and a tick population this is active almost all day long appears to be a potent mix that is leading to a public health epidemic in many states.
Why You Should Care
At this point, you may be thinking so what? I don’t have Japanese barberry in my garden and I don’t live near or spend time in a forested area with lots of Japanese barberry so how does this affect me.
In addition to creating the ideal environment for ticks, stands of Japanese barberry also tend to increase the levels of nitrogen in the soil. At the same time, the number of earthworms in the nearby soil almost doubles.
In our gardens, earthworms are welcome visitors. But in Japanese barberry infested forests, they are devouring the leaf litter which should act as a protective covering for the soil. Less leaf litter means loose soil, more erosion, less tree regeneration and less wildflowers.
Nitrification and low levels of leaf litter also add to storm water run off which affects the quality of water in our reservoirs. If you drink water, you should care about Japanese barberry infestations.
Controlling Japanese Barberry
The team at CAES worked on 150 acres of land divided into 28 different study areas. They evaluated 56 different treatment and timing combinations for controlling Japanese barberry and found that a fairly simple two-step process is most effective.
First, kill all of the plant that is above ground by cutting it down or applying heat with a propane torch. Before you run out and buy an industrial flame torch, please read these safety guidelines from CAES or check out this video on controlling barberry.
Next, once the roots begin to push out new growth, treat this growth with heat or a chemical herbicide. Here in Connecticut, starting this 2nd step in mid-October seems to be ideal. The plant is already stressed so the second attack will likely kill it. If the barberry you’re eliminating is mature, you may need to continue step two a few more times.
The size of the area you’re treating, the density of the Japanese barberry and your comfort level using propane and/or chemicals should dictate how you actually proceed. Ward and Williams also found using a chemical herbicide worked best for controlling plants growing in full sun.
Be Ready to Plant
The third step in controlling Japanese barberry is to be ready to re-plant the area with native plants. This is an important step for increasing biodiversity.
If you don’t re-plant quickly then it’s quite possible other invasives will move into the area and you will have traded one problem for another. If deer are a problem, plant only deer-resistant natives or fence off the area.
Some native shrubs that are good alternatives to Japanese barberry include highbush blueberry (Vaccinium corymbosum), New Jersey tea (Ceanothus americanus), Virginia sweetspire (Itea virginica), and dwarf witchalder (Fothergilla gardenii).
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