One of the most challenging aspects of controlling invasive plants is that even when treatments are successful, a healthy, diverse community of native plants does not usually spring back to replace invasives. Most invasives require many years of targeted treatments to eliminate them from a site, and garlic mustard is a particularly challenging one. Meanwhile, native plant communities can all but disappear from the site over years of being sprayed with herbicides and suppressed by the invasive plant itself.
Garlic mustard (Alliaria petiolata) invades riparian woodlands in the western US, and was first introduced by settlers who enjoyed its leaves for food (another common name is “poor man’s mustard”). Garlic mustard has been present in the Rogue Basin for at least a decade, and treatment efforts have focused on using herbicides to limit its spread. However, no focused research has been done to understand the best way to restore native habitat after treatment.
In summer of 2022 Rogue Native Plant Partnership launched an exciting new collaboration with the Jackson/Josephine County Cooperative Weed Management Area (CWMA) and Oregon Department of Agriculture (ODA) to work on this problem. We’ll be creating two demonstration sites to attempt to re-establish a native plant community after years of herbicide treatment, using a seed mix developed for Rogue River riparian habitats. In a series of three blog posts we’ll describe our methods, and the results. But first, a little background about garlic mustard!
Garlic Mustard: More than just bad breath
Though it is widespread in the northern Willamette Valley, garlic mustard has a limited presence (net treatment area less than 100 acres) in the Rogue Basin. When allowed to overtake forest understories, garlic mustard greatly reduces the availability of native plants that are vital to wildlife, including deer, elk and pollinators (ODA, 2015). Its large, broad leaves effectively shade out all competing vegetation. To add to this, the plant also emits a compound that prevents the seeds of native plants from germinating (a strategy known as allelopathy) (Prati, 2004).
A “Clean Slate”
Garlic mustard was first discovered along the Rogue in 2010, and the plant has been the target of treatment efforts since then. Two treated sites are now the focus of the RNPP re-seeding project. They are less than three miles apart on opposite banks of the Rogue River, located between the cities of Gold Hill and Central Point, Oregon. One is a privately owned ranchette along John Day Road and the other is on Oregon Department of Fish & Wildlife lands. For consistency, we will refer to these sites as “John Day Rd. site” and “ODFW site.”
At the ODFW site, garlic mustard was first reported around 2012 and herbicide treatment began soon after. Herbicides used were broad-spectrum and sprayed widely, resulting in the total suppression of all new growth of plants including trees and shrubs. In 2017, the strategy evolved to spot treatments with herbicides specific to forbs, so that grasses could regrow at the site. The grasses did rebound but were non-native. Some small areas were seeded with native grasses in an attempt to add species diversity.
At the John Day Rd. site, the treatment history is much shorter. After the landowner removed blackberry near the river a few years back, garlic mustard “exploded” at the site. Herbicide treatment began in 2022, and as is the current practice at the ODFW site, only forb-specific herbicides are used in a spot-treatment method.
Despite the differences in treatment history, both sites looked pretty similar vegetation-wise when the RNPP contingent arrived in the fall of 2022 to plan our reseeding strategy. Under a shady canopy of mature bigleaf maple, Oregon ash and cottonwood was a nearly “clean slate” of rich, brown streambank soil, hardly a green leaf in site. However, we know that seeds of both garlic mustard and native plants are lying dormant in the soil. Our hope is that by adding native plant seeds in a targeted way, and continuing to spot-spray emerging garlic mustard, we can give native plants the advantage.
Why use seed for habitat restoration and not established plants? Reseeding is used as a restoration strategy because it is less expensive than planting containerized plants, requires little or no irrigation, takes less labor and is more realistic on large scales. Although it can take longer for plants starting from seed to reach maturity and “hold space” against invasive weeds, seeding mimics the natural regeneration process (existing native plants dropping seeds on the ground) and allows for more species diversity than restoration methods that rely on nursery stock.
In the next blog post, we’ll reveal which native plants were included in the seed mix for the project, and how they were chosen. We’ll also get into the nitty gritty on designing the layout, prepping the site, and making divots!
Meanwhile we’d like to thank Rogue Basin Partnership for being the umbrella organization for the CWMA and so much more, including connecting RNPP with this opportunity to get native seeds on the landscape. In addition, Chris Pirosko of Silver Fox Pond & Lake Management and Kyle Strauss of Strauss Ecological Services provided highly useful intel on garlic mustard and are out there doing the hard work of invasive plant control all year ‘round.
Prati D, Bossdorf O. Allelopathic inhibition of germination by Alliaria petiolata (Brassicaceae). Am J Bot. 2004 Feb;91(2):285-8. doi: 10.3732/ajb.91.2.285. PMID: 21653384.
Oregon Department of Agriculture, Noxious Weed Control Program. Garlic Mustard Profile. Accessed February 2023. https://www.oregon.gov/oda/shared/Documents/Publications/Weeds/garlicmustardProfile.pdf