This time of year the undergrowth of Connecticut woodlands begins to show a tinge of green. This color is certainly welcome relief after a long winter. Too bad so much of this color is due to invasive Japanese barberry (Berberis thunbergii de Candolle). This thorny shrub dominates unmanaged wooded areas. Deer don’t eat it and birds spread it by eating and dispersing the shrubs’ prolific fall berries. Japanese barberry quickly grows into large thickets that provide cover for mice and an ideal environment for immature blacklegged ticks - the very ticks that carry Lyme disease. In their early life, ticks are susceptible to desiccation – they need high-humidity at the ground level to thrive. Japanese barberry accommodate young ticks by leafing out earlier than most native shrubbery. The early leaves help maintain moisture levels at ground level by blocking drying sunshine.
This connection may not be a big deal if a tick simply lived its entire life in a barberry thicket. They don’t. After ticks feed on mice they seek out host number two – commonly white-tailed deer. Fortunately for the ticks, Japanese barberry grows to a height that allows them easy access to passing deer. Ticks climb barberry stems where they hitch a ride on passing deer. Deer stroll from woodlands to yards and gardens, bringing their tiny passengers along for the ride.
We know this, in part, because of research from the Connecticut Agricultural Experiment Station in New Haven. Scott C. Williams and Jeffrey S. Ward have been monitoring the number of mice, the number of ticks and Lyme-infected ticks, and ground-level humidity in three geographic areas of Connecticut. In each area they have test plots of uncontrolled, controlled, and no Japanese barberry. For control, they used one of three methods.
One form involved torching the base of each shrub until the main stems carbonized and glowed – in effect girdling main stems to stop nutrient transfer. The dead shrubbery was left standing. The other two forms of control involved mechanically cutting the shrubs – usually by brush hog – and mulching cut plant material in place. They controlled regrowth with herbicides or flame from a propane torch applied directly to new sprouts. Note: Torching is acceptable control method for organic land care. It involves heating new sprouts by sweeping the flame back and forth over leaves until their cells burst – torching does not involve turning a large patch of land into a flaming inferno.
With a total of three years of data now collected, Williams and Ward estimate plots without barberry have 30 Lyme-infected ticks per hectare (the equivalent of 2.471 acres). By contrast, plots of uncontrolled Japanese barberry contain 280 Lyme-infected ticks per hectare. They report decreased humidity and suspect this led to “a near 60% reduction in the number of B. burgdorferi-infected adult blacklegged ticks,” in controlled plots.
They speculate continued barberry control will result in continued decline in tick populations and, therefore, suggest all landowners, managers, stewards “immediately initiate a management plan to address this alien invader.”
As Scott Williams explained for my report on his and Ward’s previous Japanese barberry-tick study, mechanical control takes vigilance and follow-up. Effective eradication requires proper identification, mechanical removal of all above-ground portions in late-spring or early summer, allowing the shrubs’ roots to use starchy reserves to force out new growth, then killing new growth in later summer. (Again, propane torch flaming is an acceptable organic land care practice.)
Williams also advises against pulling a barberry shrub from the ground unless it is small and you can get all the roots. Any little rootlet may re-sprout. Again, re-checking for re-sprouting is vitally important.
I urge homeowners to take a few minutes to check their properties for Japanese barberry and, if found, to follow the control methods described above. This, and not planting any more Japanese barberry, may just cut your chances for Lyme disease.
Read more on Japanese barberry:
- Connecticut’s Nursery and Landscape Association called for a voluntary phase-out of 25 Japanese barberry cultivars.
- The Invasive Plant Management Guide from UConn offers more detailed Japanese barberry obliteration methods.
- Additional photos from Invasive Plants of the Eastern U.S.