Subject: Managing Invasive Plants Near the Blackwater River

Dear Kathy,

Thank you for reaching out to the UNH Extension Infoline regarding your invasive plants.

As a follow-up to our correspondence on March 19, 2026, I am providing guidance on managing Vinca minor (Periwinkle) and Lily of the Valley. While neither is currently on the NH Department of Agriculture’s prohibited list, both are considered "watch list" species due to their invasive-like behavior.

Eradicating an acre of these plants near a sensitive waterway like the Blackwater River requires a strategic, multi-year approach to protect the ecosystem.

Recommended Eradication Strategy

Because these species form dense mats with extensive rhizomes, a one-time treatment is rarely successful. The goal is to exhaust the plants' energy reserves over 1–3 years.

• Mechanical Preparation: Mow or weed-whack the area to the ground in early spring. This removes the waxy leaf cuticle and forces the plants to use energy for regrowth, making new shoots more susceptible to treatment.
• Targeted Herbicide (Glyphosate): Glyphosate is a systemic herbicide that travels to the roots.

Timing for Lily of the Valley: Apply in early spring during flowering while leaves are tender.

Timing for Vinca: Treat between July and October. Since it is evergreen, you can also treat it on warm winter days (above 50°F) when native plants are dormant to avoid collateral damage.

• River & Wildlife Safety: To protect the Blackwater River, use only aquatic-safe formulas. Standard herbicides often contain surfactants toxic to fish. Ensure any product used is specifically labeled for aquatic use near riverbanks.

Finding a Certified Professional

Given the scale (one acre) and the proximity to the river, we recommend hiring a NH-licensed professional to ensure compliance with environmental regulations.

• NH Department of Agriculture: Use the Division of Pesticide Control to find licensed commercial applicators.
• UNH Cooperative Extension: We can provide further safety resources or help you find qualified professionals in your county.

Native Replacements

Once cleared, immediately planting native alternatives prevents the invasives from returning. For shady, sloped areas near a river, consider:

• Christmas Fern (Polystichum acrostichoides) for erosion control.
• Wild Ginger (Asarum canadense) or Virginia Waterleaf as ground covers.
• Native Pachysandra (Pachysandra procumbens) for deep shade.

Helpful Resources

The following links offer useful information about invasive plants .

https://extension.unh.edu/resource/spotlight-nh-invasive-plants

https://extension.unh.edu/sites/default/files/migrated_unmanaged_files/Resource000988_Rep1132.pdf

https://extension.unh.edu/sites/default/files/migrated_unmanaged_files/resource001358_rep1815.pdf

Using EDDMaps to map invasive plant

• https://extension.unh.edu/resource/using-eddmaps-map-invasive-plants

Please let us know if you have any further questions!

Best regards,

Haim
Master Gardener Volunteer
UNH Extension Infoline Team
extension.unh.edu/yard-garden