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Native Seeds in Restoration: Garlic Mustard Treatment Part 2 – Seed Mix & Site Prep

Updated: Mar 11

In fall of 2022 RNPP measured out a couple of vegetable-garden sized squares of bare earth along the Rogue River and initiated the trial of a specially blended riparian seed mix.

Our goal? To learn how to best apply native seeds to revegetate sites that have been smothered by invasive plants and treated with herbicide for over a decade.

Project background and site prep were covered in the first blog post in this series, but to recap: At these two garlic mustard “demonstration sites”, we put down a native seed mix that contains species that do well in shady riparian areas. After the seeds have germinated, we will monitor the seeded areas for species diversity and compare them to “control” area where no native seeds will be added. We predict that the results will show a better coverage of native plants on seeded areas.

At both the John Day Rd. site and the ODFW site, blackberry and garlic mustard have long suppressed natural regeneration of native plants. Garlic mustard itself is allelopathic, preventing native seeds from germinating. Finally, some of the treatment methods used involved broad-spectrum herbicides sprayed across all vegetation at the site, meaning only overstory trees survived.

In this blog post, we will discuss the seeding mix and methods used at the demonstration sites, as well as a new seeding technique we’re excited about involving divots!

Seeding Strategy Overview

The scope of this project is not to revegetate all of the 100+ acres that have been affected by garlic mustard. Instead, the goal is to trial a specialized native seed mix and seeding strategy that can achieve some of the land management goals at the site (stabilize soil, prevent new invasives from moving in, provide habitat), and can be applied to larger areas in the future.

The “demonstration sites” where this seed mix is being trialed measure 10×20 meters (roughly 32×64 feet). These plots are divided into two “treatment” areas (seeded) and two “control” areas (unseeded).

Species Selection

To choose which species to include in this seed mix and begin restoring native plant diversity to the treatment areas, we first had to consider seed availability. We knew we wanted to “go heavy” with a large number of seeds per square foot, to give the plants a better chance at competing with invasives. We also anticipate some herbicide drift occurring through continued spot-spraying for garlic mustard and other weeds, resulting in some unintended die-off of young native plants.

Through the Rogue Native Plant Partnership, we were able to secure seeds for 19 different species in large enough quantity to be sown over an area totaling 1/10 of an acre. Most of the seeds used were collected in the wild around the Rogue basin, and some came from locally farmed seed lots. Availability also determined the ratio of forb, grass and woody species seed; while we have access to plenty of milkweed seed, woody plant seeds are not so abundant.

We also spoke with the herbicide applicators who have worked at these sites for years. Their observations were valuable for learning what species have come back naturally in some of the areas sprayed for garlic mustard. These included some of the grasses used in the mix.

Combining this intel with general knowledge of the species that compose riparian understory communities in the region, we came up with the following species list:

Some of these species were also chosen for their ability to grow quickly and compete with invasive plants, for instance, lupine, snowberry and tarweed.

Seed Mix

To create the seed mix shown in the graphic above, we used the seed mix calculator linked to in this blog post. That post has a detailed discussion of how to determine your seeding rate, taking you from a species list in-hand to a calculated seed mix with a balanced proportion of each species – we highly recommend it!

For this demonstration seeding we determined that a fairly high seeding rate, 100 seeds/square foot (or 72 lbs/acre) would be most effective.

Prepping & Seeding

As described in the first blog post in this series, the areas to be seeded had been treated for weeds before we arrived. That’s ideal for restoration planting – because native plants do not compete well with invasives, pre-treatment is needed.

Photo on left of demonstration site after garlic mustard treatment. Photo on right after raking and lightly marking out rows to place divots.

To improve soil contact with the seeds, we raked the entire demonstration site (plot). Then we checked our measuring tapes to mark out the “control” (no seeds) and “treatment” (to be seeded) sub-plots. These sub-plots alternate because environmental variations like topography and shading from overstory trees on one end of the plot or the other could bias our results, and we wanted to reduce that bias. The alternating sub-plot layout is shown in Figure 1.

Having checked our layout and measurements, we proceeded to the treatment sub-plots to make our divots and sow the seed.

Divots? What are divots?!

Divots are small impressions in the earth that mimic the footprints of a large animal like an elk (which roam the riverbanks around our demonstration sites). The technique was developed for seed-based restoration projects in the harsh arid environments of the US southwest (Bainbridge, 2007). The theory is that by placing seed in the divots, the seed is less likely to be blown away by wind or washed away by rains, while the divots may help protect the seed from drying wind and sun. The technique has been used on other projects here in southwest Oregon with successful results.

Vesper Meadow site that was treated with a tarp to smother weeds, then replanted with pollinator seed mix in divots.

For these demonstration sites, we used a hoe to make small impressions in the soil (no more than ½ inch deep), spacing them about a foot apart. A total of 50 divots were created in each 5×10 meter subplot, and a handful (about 1/10th of an ounce) of seed was added to each divot.

Larger image of multiple divots in the soil, arranged in rows. Closeup image of the quantity of seed mix placed in each divot.

Finally, we raked each seeded sub-plot lightly again to provide some soil coverage to the seeds while maintaining the divot structure.

Now, We Wait

Habitat restoration with native seeds takes time; while it may look like nothing is happening on the surface, all winter long, seeds are responding to a thousand subtle environmental messages that allow them to germinate at just the right time. If conditions are right in spring of 2023, those seeds will send small leaves toward the sun. If conditions don’t align for growth, these seeds will bide their time until a future spring.

The benefits for those hoping to restore landscapes with native seed is that dollars spent on seed, when it is planted at the right time and with good attention to site preparation, is rarely wasted. Even if only half of the species placed in these demonstration sites as seed develop into mature plants, the result will be higher species diversity than if we had relied only on planting starts or plugs available from native plant nurseries. And, at $14/oz* of seed versus $2-$20 per individual plant plug, our restoration dollars are stretched further.

Stay tuned for the next blog post in which we’ll reveal our monitoring results and lessons learned from this project.

*2022 price for seed mix; individual species/lot prices vary widely

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