Aquatic Plants

Aquatic Plants
News - 6/17/2020
Aquatic Plants

Aquatic plants are an essential part of a balanced lake ecosystem. They provide oxygen and consume nutrients in the water column, and provide habitat and food to microorganisms, fish, and birds. In a natural healthy lake, native plants exist in balance with the organisms they support. In an urban lake with unnaturally high nutrient inputs, they can grow excessively and create issues ranging from nuisances and recreational impacts to degradation of overall ecosystem health. Unfortunately, the challenge is compounded on Lake Oswego by the presence of invasive as well as native aquatic plants.

The overall amount of plant growth in the lake has notably declined in the past several years. As recently as a decade ago, many areas of the lake routinely had plant stands “topped out,” i.e. grown up to the surface, by early-mid summer, something we rarely see anymore. A large part of the west end became so thick with plants that people couldn’t waterski in September, in 2012. Some of our methods have since changed significantly as have general lake conditions, and overall growth is well under control at the moment. Plant and algae dominance in lakes fluctuates naturally over time, so we continue to monitor the plant community. Should plants regain a hold over the next few years, we need to plan ahead to be able to effectively manage them, preemptively reduce the percentage of invasive plants, and encourage the growth of native plants where possible.

Currently, there are four dominant plants in the lake. Two are native: Native Elodea (Elodea canadensis), and Coontail (Ceratophyllum demersum). These are plants that we want to see growing in the lake, assuming they pose no negative impact to recreation. In fact, we are looking into methods for encouraging their growth, particularly the Native Elodea. Two are invasive: Brazilian Elodea (Egeria densa) and Curly Leaf Pondweed (Potomogeton crispus). Left un-managed, these plants can spread prolifically and become difficult, expensive, or impossible to acceptably control. The LOC’s Pesticide Discharge Management Plan (PDMP) defines action thresholds for invasive and native plants differently. For native plants, we allow them to grow so long as they aren’t densely established in the “prop zone,” approximately 4 feet down from the surface. Once chopped up by boats, they become a surface debris issue, which increases the load on our debris collection system. When we find invasive plants, we manage them with a goal of full suppression, regardless of their height or density.

Aquatic herbicide options are few, considering the limitations of a well-used recreational water body which also serves as an irrigation supply for hundreds of homes. Not to bore you with details, but we have two primary options: Endothall, which controls Coontail and Curly Leaf but has little effect on either native or invasive Elodea. We use it where we hope to preserve native Elodea, and it is convenient as it has no irrigation restriction. We also use Diquat, which is a highly effective product that works on all plants (and algae), particularly the invasive Brazilian Elodea. Unfortunately, Diquat and a few other specialty products we occasionally use for invasive plants have irrigation restriction of three or more days.

When we schedule an application that uses a product that carries an irrigation restriction, you will receive notification via the email you have on file at the LOC at least 72 hours beforehand. This is why it’s especially important to keep your contact information updated and email checked regularly. Following such an application, you should not use lake water to irrigate until the specified restriction is over. Inconveniently, as the summer weather warms and your landscapes may require more watering, aquatic plants are enjoying the warm water and sunshine as well and are growing faster than any other time of year. New small plant stands establish, the makeup of established stands changes, and to properly respond we sometimes need to use irrigation-restricted products to best manage the situation. We know it can be frustrating to get an email telling you to turn your sprinklers off in mid-summer, but hope you can understand the circumstances that make that decision necessary. If you’ve had problems keeping your landscape healthy during irrigation restrictions in the past, you can plan ahead for future treatments. Modifying your irrigation system to be able to temporarily run on municipal water is an option, but you can also fill containers such as buckets or barrels ahead of time, and hand-water important plants during the restriction.

A crucial part of managing the aquatic plants in Lake Oswego is preventing the introduction of new species. We are somewhat lucky to only have two invasive plants to manage, as there are countless other problematic species in water bodies all around us. If a trouble plant like Eurasian water milfoil, a common invader in the Columbia and Willamette River systems, became established, it could seriously impact the usability and health of the lake, and would require herbicide treatments with even more inconvenient water use restrictions than our current treatments carry. The LOC maintains fish screens that also keep plant fragments out of the intake on the Tualatin River, and strictly requires boat and trailer decontamination at the ramp (very important, and not just from a plant management standpoint). An extra note to aquarium and pond or water feature owners: the aquatic plant market is poorly regulated, and many plants you can buy locally and online are invasive and would be disastrous if they ended up in the lake. While escapement seems unlikely, its been documented that aquariums are responsible for many invasive plant infestations across the country. Please dispose of aquarium water down the drain or toilet, and keep pond water completely contained. There are great resources for selecting and sourcing native options and we encourage you to use them, but keep all varieties of introduced aquatic plants away from the lake.

If you see plants growing in the water, take a second and try to identify them. It might be that outside your boat slip is a nice patch of native Elodea or Coontail, contributing to the health of the lake and not bothering anyone. Or it could be a new infestation of invasive Brazilian Elodea, highly likely to resprout when fragmented, waiting to be distributed by a boat propeller to spread to an adjacent corner. Either way, it’s good to be connected with what’s going on in your backyard! Feel free to call or email with questions about our aquatic plant management policy, send pictures of plants for identification, or ask about tips for how you can best manage your waterfront.