Milfoil on Swains Lake.
Updated 06/20/2026
Protecting Swains Lake from variable milfoil and other aquatic invasive species is one of the core missions of the Swains Lake Association. Through a close partnership with the New Hampshire Department of Environmental Services (NHDES) and professional lake management specialists, we work year-round to keep our waters clean, navigable, and healthy.
📢 Latest Status: Summer 2026 Update
We are thrilled to share an incredibly positive report from our Spring 2026 lake survey!
On May 8, 2026, Georgia Bunnell (NHDES Exotic Species Program Coordinator) completed a thorough survey of Swains Lake and reported zero milfoil sightings.
Key Takeaways for this Season:
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🚫 No Spring Herbicide Treatment Needed: Because the lake is currently clear of target growth, Swains Lake will not require an herbicide treatment this spring.
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🎉 Two Years Clean: This marks our second consecutive season with a completely clean spring survey report!
🔬 Understanding the Science: Cautious Optimism
While two consecutive clean seasons is an amazing milestone, it is vital to understand that our job isn’t done. When asked if a two-year clean streak means we have permanently eradicated milfoil, NHDES provided some important scientific context:
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Post-Treatment Trends: It is highly common to see little to no weed regrowth for a year or two following a successful herbicide treatment.
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Dormant Seeds: Variable milfoil is resilient. Viable seeds can remain dormant in the bottom lake sediment for years, meaning plants can suddenly pop back up in future seasons.
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Transient Boaters: Swains Lake remains at constant risk of re-infestation from outside boats, trailers, and fishing gear that carry fragments from infested waterbodies.
Eradication requires years of zero growth, meaning continuous monitoring is our absolute best line of defense.
📅 What’s Next for the 2026 Season?
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Late Summer Re-Survey: NHDES biologists will be back out on Swains Lake later this summer to conduct a second comprehensive survey. This will catch any late-season growth that sprouted as water temperatures peaked.
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Diver Deployment (If Needed): If any small fragments or patches are found during the summer survey, the SLA will coordinate with weed-control divers to manually pull them before they can spread.
🤝 How You Can Help
Keeping Swains Lake milfoil-free is a community-wide effort. Here is how you can do your part:
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👀 Keep Your Eyes Open: If you notice any suspicious, feather-like green weeds while out swimming, boating, or paddling, do not attempt to pull them yourself (as fragments can drift and root elsewhere). Note the location, take a photo if possible, and report it immediately. 👉 CLICK HERE
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🧼 Clean, Drain, Dry: Always ensure your boats, trailers, kayaks, and fishing gear are entirely clean of mud, plants, and water before launching into Swains Lake and immediately after pulling out.
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⚓ Support the Lake Hosts: Our SLA Lake Hosts stand guard at the public boat launch all summer to inspect incoming vessels. This program is entirely funded by your $25 annual membership dues.
👉 Click Here to Renew or Join the SLA for 2026 and Support Our Lake Protection Efforts!
What is Milfoil?
Milfoil is a submerged aquatic plant with fine, densely packed, feather-like leaves whorled around a main stem. It can grow up to 15 feet and may exhibit a three- to six-inch green spike-like flower above the waterline in late June or in July. A cross-section of the stem will reveal “pie-shaped” air chambers. This exotic species of milfoil has been in the state since the late 1960s, and can currently be found in over 75 waterbodies in New Hampshire. There are six native milfoil species present in the state that do not cause problems, as they are low growing and do not form monocultures, unlike variable milfoil. Eurasian milfoil is another non-native milfoil found in New Hampshire, but it is less of a threat than variable milfoil due to our water chemistry (Eurasian milfoil tends to be found in waters with higher pH ranges than those found in New Hampshire).
How Does Milfoil Spread?
Boat propellers chop milfoil plants into small fragments. These fragments float on the surface and are at the mercy of the wind and lake currents. In a short time, roots form on these fragments. If washed ashore, these plants eventually take hold, creating a new colony of milfoil. The cycle goes on until every suitable area is filled in with these weeds. An alternative form of the plants develops during low water.
This vegetation type is more succulent than the submersed form and can persist for moderate periods of low water. NHDES has recently collaborated in a study to evaluate the viability of milfoil seeds and the research showed that milfoil seeds are very viable and have a high regeneration rate, though survival of the seedlings is thought to be relatively low. Regardless, seed production in a dense milfoil bed is high, and seeds are thought to be a probable source of new plants, even following extensive control measures (seeds are resistant to herbicides). Data suggest that long-term monitoring and appropriate follow-up activities are needed to truly reduce or potentially eradicate infestations. Regular surveys by NHDES biologists or volunteer Weed Watchers are needed to find new growth early, so that small-scale control measures can address the problem before it spreads.
It is unlikely that seeds are responsible for lake-to-lake spread; fragments are still the primary cause of that problem.
Information taken directly from NH DES Fact Sheet on Variable Milfoil: https://www.des.nh.gov/sites/g/files/ehbemt341/files/documents/2020-01/bb-23.pdf