• Family: Hydrocharitaceae
• Life Form: perennial, submerged, and rooted vascular plant.
• Tier: 4 

History & Introduction 

There are two kinds of Hydrilla verticillata found throughout the US: a dioecious strain (male and female flowers are found on separate individuals) and a monoecious strain (male and female flowers are found on the same individual). The dioecious strain of Hydrilla was first introduced to the US in the early 1950s near Tampa, Fl when an aquarium plant dealer dumped 6 bundles of the plant into a canal near his business. The monoecious strain was introduced a few decades later in Potomac Basin. Since then, Hydrilla has rapidly spread through much of the United States where it has done great environmental and economic damage. 

 Identification & Biology

 can grow in a wide variety of habitats, from waters as shallow as 0.5m to as deep as 10m if water clarity is good, with the ability to grow up to an inch per day. It can tolerate low light and moderate salinity, and is most often found in ponds and streams. It has been found growing well in oligotrophic and even eutrophic waters, and can even tolerate high levels of raw sewage. While it prefers organic substrates, it has been found growing well on sandy and rocky substrates. 

It forms dense mats along the water’s surface, with 20% of the plant being concentrated in the top 10cm of the mat. Plants can reproduce sexually as well as asexually, with a single plant fragment containing a single node being capable of producing a new plant. It may be seen as a submerged rooted plant, but can also exist as floating fragments. 

Leaves: bright green, pointed, about 5/8 inches long with serrated margins. Grows in whorls of 3 – 10 along the stem, most often 5 whorls.

Flower: thin stalks from the stem end in a single, small, floating white flower at the water’s surface as females can only be pollinated through the air. 

Male flowers lack a hypanthium and float up to the surface of the water as an air-filled bud. Eventually the anthers dehise and release pollen within a 10cm radius around the flower. If male Hydrilla flowers are present, you may notice the water surface becoming greenish-white from the pollen of the male flowers. 

Roots: ≤1.5” long tuber (turion), dull white to yellow in color. Tubers grow 2 to 12 inches below the surface of the sediment at the ends of underground stems. These tubers form at the end of the growing season and act as food storage reserves so that Hydrilla can overwinter. Tubers Can stay viable for many years.

This plant is often confused with several plants including the native Elodea canadensis (waterweed) and the nonnative Brazilian elodea (Egeria densa)

Damage

Environmental: Mats of Hydrilla provide poor habitat for native aquatic wildlife, and shade out native aquatic plants, ultimately reducing biodiversity and ecosystem resilience. Water slows down where large infestations of Hydrilla are present, allowing sediments to accumulate, decreasing turbidity and increasing habitat for mosquito breeding. 

In 2021, a neurotoxin was discovered on this plant and has been linked to a neurological disease and death in bald eagles. The disease, called vacuolar myelinopathy (VM), has been known since the 1990s, as eagles had been found losing control of their bodies and developing holes in their brains. In 2005 Susan B. Wilde, a professor at the Warnell School of Forestry and Natural Resources at the University of Georgia, identified a previously unknown cyanobacterium, known as Aetokthonos hydrillicola on the leaves of Hydrilla, and was only found in locations where birds were affected by VM. They found that this cyanobacteria produces the neurotoxin, aetokthonotoxin, which was ultimately linked to the cause of VM. The neurotoxin is consumed by primary consumers, such as small invertebrates and herbiverous birds such as coots, and is bioaccumalued up the food chain, having the most devastating impact on these large predatory birds.

Economic: Thick mats of Hydrilla have been known to cause immense damage to underwater irrigation and hydroelectric structures. Plant material builds up and prevents water flow, which not only reduces efficacy of these operations, but also damages the equipment. At one such hydroelectric power plant in Moultrie, South Carolina, Hydrilla clogs caused operations to cease for 7 weeks while machines were repaired and cleaned, costing $2,650,000 of expenses attributed to repairs, dredging, and fish loss, along with an estimated $2,000,000 loss in power generation for the plant. Hydrilla invasions can also impact aquatic recreation, as the plant matter can get caught in boat propellers, make paddle sports a tiring task, and swimming a generally unpleasant experience. In Florida alone, control of Hydrilla between 2008-2015 cost the state $125 million. It can cost as much as $1000/acre to treat. 

Due to the lack of habitat for commercially important fish species where Hydrilla is present, the size and abundance of these fish can be severely impacted as well. 

Management & Control

The best way to manage Hydrilla is to prevent its spread in the first place. You can do so by following the principles of Clean, Drain & Dry:

CLEAN: Remove visible aquatic plants, animals, and mud from all equipment before leaving water access

DRAIN: Drain watercraft bilge and other water containing devices before leaving water access.

DRY: Let boats and gear dry in the sun for at least five days OR wipe with a towel before reuse.

For established populations, mechanical harvesting and herbicide spraying are only moderately effective in controlling Hydrilla. Manual removal is extremely costly, and tiny fragments that are not removed can easily produce new populations of Hydrilla. Chemical control is also costly, and permits can be difficult to acquire. Where permissible, herbicides may only be used on small enclosed bodies of water, but can still have off-target impacts on native species.