PART 2 By Joanne Burghardt, SLCA president May 2025
Control Measures
Source: CRVA, Invasive Species Centre)
1. Manual:
Does not require a permit if certain benchmarks are met, see details here: https://www.ontario.ca/page/remove-invasive-aquatic-plants`
a. Hand Removal:
i. Advantages: Simple, effective and selective with minimal impact on native aquatic plants. Can reduce biomass and percent cover. Potential for increased community awareness and involvement.
ii. Disadvantages: Not always effective as it is very difficult to remove all plant material; does not eradicate. Time consuming and labour-intensive, only practical for isolated patches.
b. Raking:
In certain situations (i.e. deeper water) raking may be combined with hand pulling to try and remove all floating plant fragments. In deeper water, guide the rake along the plant and spin the rake so the stems get wrapped around the rake before pulling it up from the water.
“Worldwide perhaps the most used form of aquatic plant management is hand pulling. The goal of hand removal is to remove the whole plant including the root; this can sometimes be difficult, though, as many aquatic plants are brittle. Operation costs for hand removal are minimal; that fact coupled with the highly selective nature of hand removal makes it popular.” – Lakes Environmental Association, Maine USA https://mainelakes.org/invasives/milfoil-control/physical-removal/
Once the plants have been removed, they are placed into a mesh dive bag and taken off site to be composted. The greatest challenge during hand removal is to remove as much root as possible and to keep the plant from breaking apart, or fragmenting.
– Lakes Environmental Association, Maine USA
https://mainelakes.org/invasives/milfoil-control/physical-removal/
2. Benthic Barriers:
Requires a permit, see details here: https://www.ontario.ca/page/remove-invasive-aquatic-plants
Benthic barriers, bottom screens, or benthic mats are covers laid on the bottom sediment of a water body to block sunlight, preventing plants from photosynthesizing and suppressing their growth. These barriers target the rhizome, the main method of spread, and can restrict growth in small, localized areas.
Difficult to install, require ongoing maintenance.
3. Mechanical:
Requires a permit, see details here: https://www.ontario.ca/page/remove-invasive-aquatic-plants
a. Mechanical Harvesting:
i. Advantages: Direct relief and immediate results. Effective when repeated, moderately inexpensive; removes plant biomass in the short term (weeks to months) and can reduce plant growth the following year.
ii. Disadvantages: Labour intensive. Non-selective control measure will indirectly harvest other plant, fish, and macroinvertebrate species. Harvesters can also fragment rhizome pieces, contributing to further spread. Any fragments left behind are at risk of re-rooting and growing. This problem can be mitigated by containing and removing fragments during mechanical harvesting. Milfoil can also return at a faster rate than untouched areas due to nutrient release during dredging and aeration of the bottom.
b. Dredging or Suction Harvesting:
i. Advantages: Longer-term solution.
ii. Disadvantages: Disruptive; Very limited areas, usually small areas within larger waterbodies. Slow, expensive. Increases water depth. Milfoil can return at a faster rate than untouched areas due to nutrient release during dredging and aeration of the bottom. Algal blooms can also result from nutrient release. Non-selective; water quality impacts.
4. Biological: ( Indications are that biological remedies such as milfoil weevils have largely been discontinued)
Requires a permit, see details here: https://www.ontario.ca/page/remove-invasive-aquatic-plants
Biological control is the use of an herbivore, predator, disease or other natural enemy to reduce established populations of invasive species. Most invasive species have no natural enemies in their new habitats. Biological control aims to re-establish an ecological balance between the invasive species and its natural enemies by selecting highly host-specific natural enemies from the country of origin and moving them to the country where the invasive species is a problem.
5. Chemical: Aquatic Herbicide:
Requires approval from the Ontario Ministry of the Environment, Conservation and Parks, see details here: https://www.ontario.ca/page/remove-invasive-aquatic-plants
Advantages: Currently two herbicides in use – Reward (Diquat) and the newer ProcellaCOR FX. Effective and fast acting, requires only a short exposure time. Robert McGowan, Aquatic Project Specialist with the Ontario Federation of Anglers and Hunters, says he has used all methods of removal, including herbicides.
ii. Reward: Reward works by killing off the parts of plants it touches. As a broad-spectrum herbicide, diquat can harm many non-target species. In a study that tested the sensitivity of native and non-native aquatic plants such as Eurasian water-milfoil to a range of diquat concentrations (4.7 – 1,153 ug/L), diquat was found to be highly lethal to all plant species. A low concentration of 74 ug/L (below the label recommendation) resulted in 100% mortality of all aquatic plant species. Indirect effects also include lethal effects on invertebrate species using aquatic plants for cover. In addition, plant dieback can lead to problems with dissolved oxygen and eutrophication. It is not effective in turbid waters. Areas that have been treated cannot be used for recreation or human consumption for at least five days, which could create a negative public perception. It may also be less effective on hybrid water-milfoil. “
https://www.greencastonline.com/products/reward-landscape-and-aquatic-herbicide/turf
iii. ProcellaCOR FX: A newer herbicide, is touted as a selective, systemic aquatic herbicide used for controlling invasive aquatic plants like Eurasian Water-Milfoil. It's approved by Health Canada and Canada.ca and is works by mimicking plant hormones, causing abnormal growth and ultimately killing the plant from the inside out. It’s also very expensive. https://sepro.com/aquatics/procellacor-fx
David Lean, a retired freshwater biologist, is not fan of using herbicides to kill EWM. Born in Apsley, Lean is a long-time cottager on Jack Lake.
In an interview with outdoorcanada.ca, he raised his objections to using the herbicide Reward (Diquat Dibromide) to get rid of weeds.
Among his concerns: it doesn’t kill the roots, the plants could come back; herbicides could get into drinking water, other native plants will be killed as well as organisms that fish eat.
In the outdoorcanada.ca article, Lean recommends using a rake to remove weeds, then letting nature take its course for the rest. It could take up to 10 years, he says, but nature will balance itself out. “Every invasive species has an advantage for a short time because their natural enemies aren’t present yet,” Lean says. “If you go out and spray, you affect this balance, and you’ll prolong the problem.” https://www.outdoorcanada.ca/whats-the-best-way-to-combat-eurasian-watermilfoil-opinions-are-divided/
Farlain Lake Community Association (FLCA) in Tiny Township was the first freshwater lake in Canada to use ProcellaCOR FX to treat Eurasian Water-Milfoil since it's registration by Health Canada in 2023. In 2018 they were awarded a $199,800 Grow grant from the provincial government’s Ontario Trillium Foundation (OTF) for the treatment of 17 sites. The lake was treated in September 2024. The treatment exceeded their expectations. Four weeks later EWM was dead and other native plants remained healthy and untouched.
https://www.midlandtoday.ca/local-news/aquatic-invasive-plant-appears-to-be-eradicated-in-farlain-lake-9556676
https://farlainlake.ca/our-lake/eurasian-watermilfoil
The Mazinaw Property Owners Association in Bon Echo also researched ProcellaCOR FX: estimated cost to treat their lake was $125k for the application and 3-year management of 23 known patches over 15 hectares of the 1630-hectare lake.
An excellent news article on the decision-making process Mazinaw used when deciding their plan of attack: “Mazinaw Lake Property Owners Plan To Use Chemical Treatment On Eurasian-Milfoil” Frontenac News, Sept. 4, 2024.
https://www.frontenacnews.ca/addington-news/item/17506-mazinaw-lake-property-owners-plan-to-use-chemical-treatment-on-eurasian-milfoil
For comparison, Fairlain Lake’s surface area is 109.6 hectares, Steenburg Lake 281 hectares, Mazinaw 1630 hectares.
IMAGE: Buck Bay in Mazinaw Lake, thick EWM masses
Disposal of EWM:
- Dispose on dry land, above the high-water mark, at least 30m from water to prevent material from re-entering the water.
- Small amounts can be put on land to dry and then be mulched, buried, composted or left to decompose.
- Gardens or farm fields are excellent disposal sites.
- Plant material can be sealed in a black plastic bag and left in direct sunlight for about one week then discarded in household garbage.
- Limerick Township does not allow large amounts of Milfoil to be disposed in the waste site. Small amounts mixed in with regular household garbage is permitted if the weeds have first been left to sit for 1-2 weeks in the sun in a construction grade garbage bag.
- Tudor & Cashel Township will accept milfoil at their waste stations if it is dried. They urge you to exercise caution to ensure it doesn’t accidentally get dispersed on the way to the dump.
Restoration:
In some invaded systems, planting a reclaimed area with native vegetation can help prevent the reestablishment of the invasive species. Once an invasive aquatic plant is removed, EWM can return or something else can become established. This can be curtailed by planting the treated area with native plants. In one study on Cedar Lake, transplanted species were chosen that would match present species in the lake and cover a large surface area.
More information:
Sources: Ontario Invasive Plant Council, Crowe Valley Conservation Authority, Limerick Township, Tudor & Cashel Township, Lake Associations
