Tuesday, December 5, 2023

Battling Buckthorn: Research Continues

Common buckthorn, Rhamnus cathartica, fills the understory of this hardwood forest. Buckthorn retains its green leaves longer than most other woody plants.

Anyone who’s tried to manage invasive common buckthorn (Rhamnus cathartica) knows it’s a battle. Pull it out and seedlings take its place. Cut it and it grows back, especially if the cut stump isn’t treated with herbicide. Buckthorn baggies or other covers for cut stumps are an option, but not a practical one for large, dense infestations. The search for insects that target buckthorn hasn’t succeeded yet, either.

Although these difficulties are frustrating, researchers at the University of Minnesota haven’t given up. Just the opposite: They are leading several investigations of potential solutions. Their studies of biodiversity, fungi, goats and other possible buckthorn remedies are shedding light on what works, what doesn’t and what to try next. Here are summaries of some of their efforts.

Using biodiversity to control buckthorn

The University’s “Cover It Up!” studies investigate the use of plants to suppress regrowth of common buckthorn after it is removed. Two phases are completed and a third is in progress, all asking questions about which plants are best at outcompeting buckthorn, how best to plant them, and how managing fire and deer browsing affects the success of using plants to control buckthorn.

The researchers were surprised by one of their findings: Most buckthorn seeds remain viable in the soil for only 1-2 years, not the commonly thought 6 years. That means seedlings will be abundant for the first year or two after buckthorn is removed but should taper after that, assuming new seeds aren’t introduced.

Phase 3 of the project is underway. When it’s done, researchers hope to offer buckthorn solutions that are both affordable and practical. For more information, see the Cover It Up! project page.

Can fungi help?

In some parts of Minnesota and Wisconsin, buckthorn is dying, possibly from canker rot, root rot and other fungal diseases. Researchers are trying to identify the fungi infecting the trees and how they may be used to manage buckthorn stands. Employing fungi would reduce the use of herbicides, an important benefit when working near water or other places where protection of other resources is paramount. The project began in January 2023 and will continue for another two years. See the project website for more information and updates.

Orange growths of crown rust on common buckthorn.
Another fungus being investigated for control of common buckthorn is Puccinia coronata, the rust fungus that causes crown rust of oats. The fungus uses buckthorn as an intermediate host and is visible during summer as orange, fuzzy-looking spots on leaves and stems. Some negative effects of the fungus have been found on buckthorn, so researchers have started identifying those strains and studying their effects on buckthorn growth and mortality. The study began in January 2023 and continues for two more years. More information is on the project website.

What about goats?

A study to evaluate the effectiveness of goat grazing on buckthorn ended in 2021. Researchers found that although goats can control buckthorn, the benefit is temporary. Buckthorn can rebound after grazing unless other control measures follow.

Grazing goats are also at risk of eating snails or slugs that are intermediate hosts for the brainworm Parastronguloides tenuis, which can cause fatal neurological disease. Co-grazing waterfowl with goats has been suggested to reduce that risk, so the study also examined the effect of waterfowl on the abundance of snails or slugs. In a 2022 paper published in EcoHealth, the researchers found that where goats grazed alone, the abundance of snails and slugs increased, but where ducks and geese were included with goats, the increase didn’t occur. They also found that waterfowl didn’t affect the overall diversity of the snails and slugs, which is important to protect populations of native gastropods.

The researchers point out that while waterfowl can lower the numbers of snails and slugs and therefore reduce the risk to goats of acquiring brainworms, more study is needed to learn if this also reduces the incidence of the disease.

More challenges of goat grazing are discussed in this paper and several others linked at the project website.

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