Waterthyme is an invasive rooted aquatic plant in the Hydrocharitaceae family and is native to Asia, Africa and Australia. It is the only species in the genus but many consider it to have several variations. It was introduced through the aquarium trade and was subsequently dumped into the waterways. It can now be found from Connecticut south to Florida, into the central USA, west to Texas and up the Pacific coast. It is found throughout NC in various freshwater systems. It is considered one of the worse invasive plants in this country and the world and is on the USDA Noxious Weed List . It is listed as invasive by the NC Invasive Plant Council and many other states. The stems are slender and branched, up to 25’ long. The leaves are in bottle-brush whorls of 3-8 with serrated margins, reddish midribs and undersides with small sharp teeth. Tiny white flowers have 6 petals and grow on long stalks. There are both monoecious (both male and female flowers on the same plant) and dioecious (male and female flowers on different plants) forms. The plants are spread both by seeds and vegetatively. They overwinter in colder climates by their tubers. In addition to forming large monoculture mats that crowd out native plants and alter the ecosystem, the plant harbors bacteria that cause a brain-wasting disease in waterfowl. It is very versatile and can grow in almost any freshwater system including springs, lakes, marshes, ditches, rivers and tidal zones in water a few inches and up to 20 feet deep. It can grow sun to shade Insects, Diseases, and Other Plant Problems: No significant problems. Invasive in many states including NC. See native alternatives to the left.
Curated pairwise relationships between this plant and others in the library — who feeds
whom, who competes, who confuses the pests. Green reads as a good pairing, red as one to avoid.
No curated interactions recorded for this species yet.
Also noted by NC State
Related species the NC State Plant Toolbox flags for this plant — look-alikes to tell
apart, and plants that play a similar role in a planting. Ones we carry in the library are linked.
Dappled Sunlight (Shade through upper canopy all day) | Deep shade (Less than 2 hours to no direct sunlight) | Full sun (6 or more hours of direct sunlight a day) | Partial Shade (Direct sunlight only part of the day, 2-6 hours)
Soil drainage
Frequent Standing Water
NC region
Coastal; Mountains; Piedmont
Growth rate
Rapid
Maintenance
High
Form & Appearance
Habit / form
Spreading
Leaf color
Green
Leaf type
Simple
Leaf arrangement
Whorled
Leaf shape
Lanceolate
Leaf margin
Serrate
Leaf size
< 1 inch long, < 1 inch wide
Hairs present
No
Leaf detail
Small, pointed leaves are arranged in whorls of 4 to 8. Margins are serrated and may have one or more sharp teeth under the midrib. About 1/2 inch long.
Flower form
Solitary
Flower size
< 1 inch
Flower detail
Small white flowers with 6 petals appear late summer to fall.