This is the basic information you got in yesterday’s National Park Mystery Plant quiz:
It’s an alien from way down south. Intentionally brought into this country by people who needed to provide oxygen in confined spaces, it’s been loose in America for over a century. You can now find it in scattered locations from border to border and coast to coast.
Everywhere you find this plant growing where it wasn’t invited, you’ll find people who despise its nasty habits. Perhaps its worst habit is producing dense stands or mats that crowd out native vegetation.
To know this plant well is to appreciate its oddities. For example, it doesn’t need soil to grow, and even though it is a seedless flowering species, all of the problem plants are males.
Identifying this plant can be tricky. People often mistake it for a plant whose common name rhymes with the most common ice cream flavor.
The places where this plant causes the most trouble are places where drawdowns are most likely to play an important role in its control or eradication.
From this you should have been able to deduce that the mystery plant is the submerged aquatic perennial Brazilian egeria (Egeria densa Planchon). Formerly identified scientifically as Elodea densa (Planchon) Caspary, this member of the Hydrocharitaceae family is usually branded anacharis when sold for aquarium use. It has many colloquial names, including Brazilian waterweed, Brazilian elodea, giant elodea, leafy elodea, South American waterweed, dense waterweed, and large-flowered waterweed.
Having “weed” as part of this plant’s descriptor is certainly understandable. Although prized as an aquarium plant that looks pretty while oxygenating the water and supplying nutrients, Brazilian egeria (i-JEER-ee-a) inspires nearly universal hatred in people who’ve had to deal with this highly invasive plant’s impacts on freshwater impoundments, canals, ditches, slow-moving streams, pools, and sloughs.
Brazilian egeria is a tall-growing (up to 12 feet) plant native to Brazil, Paraguay, and coastal areas of Uruguay and Argentina. It was brought to America as an aquarium plant in the late 1800s, and it’s been loose in this country since at least 1893. It can now be found, usually rooted at depths of less than 15-20 feet, in bodies of fresh water throughout the United States except in colder areas of the northern, Midwestern, and north-central states. (It can survive under ice for only brief periods, and prolonged near-freezing temperatures will kill it.) Acadia National Park, Colonial National Historical Park, and Everglades National Park are a few of the NPS units that have documented Brazilian egeria within their borders.
This plant looks a lot like another noxious aquatic import, the hydrilla (rhymes with vanilla). However, the midrib on the underside of the Brazilian egeria leaf is smooth, whereas the midrib on the underside of the hydrilla leaf has small teeth.
Once it is well established, Brazilian egeria tends to grow into densely tangled subsurface mats or stands that can interfere with swimming, boating, water skiing, fishing, and other recreational pursuits. Hydropower facilities may suffer from loss of storage capacity as well as clogged or damaged grids, pumps, and intakes. In dense enough concentrations, this weed can even interfere with commercial navigation in canals and shallow channels. Ecological damage results when native aquatic plants are crowded out, the movement of anadromous fish is impeded, water clarity and quality are reduced, and the aquatic ecosystem’s natural flows of water, sediment, and energy are disrupted. Aerobic decomposition following diebacks or herbicide use can excessively reduce dissolved oxygen levels. This is not to mention the aesthetic insults of dense mats in shallow water in the vicinity of residences, vacation homes, and resorts. Some individual impoundments have tallied millions of dollars in damages and control costs.
Since Brazilian egeria can grow while floating in water, and its stem fragments can root at nodes, it is readily dispersed. If plants are chopped up or torn by boat propellers, the stem fragments can drift away to colonize new locations. When mechanical equipment is used to mow and remove plants from heavily infested areas, unharvested clumps can sink to the bottom and take root or drift to colonize new areas.
Once Brazilian egeria is established in a water body, controlling or eradicating it can be easy or maddeningly difficult, depending on the degree of infestation, the size of the area needing treatment, and other physical and cultural factors. If all you need is some local control, as in the vicinity of swimming areas or docks, covering the mats or sediment with an opaque fabric will block sunlight and kill the plants. If more extensive treatment is needed, the main tools available are pesticides (such as Floridone or diquat), mechanical removal (harvesting), biological controls (such as grass carp or a fungal disease), and in some case, lowered water levels (drawdowns). All of these measures have significant drawbacks, and none is simple, easy, or cheap.
Using drawdowns is an attractive, if limited option. While the managers of some reservoirs or lake systems are at liberty to lower water levels in order to control aquatic plants, others may be restricted to drawdowns done primarily for maintenance, repair, or safety reasons. Drawdowns for Brazilian egeria control tend to be most effective when they are done repeatedly and when sediments are either dried for prolonged periods or frozen to depths of at least 8 to 12 inches.
Postscript: The Brazilian egeria is a flowering species, with male and female flowers produced on separate plants. So far, all of the Brazilina egeria plants found in U.S water bodies have been male clones that reproduce vegetatively.