Water mimosa, thought to have originated in South America but now found in rainforests around the world, is a perennial aquatic legume with many different species. Belonging to Fabaceae, the legume family, it should be no surprise that this plant is exceptional at fixing nitrogen like many other members of its family. Also known as sensitive neptunia after the Roman god of freshwater, Neptune, water mimosa grows to about six inches tall with a stem and leaf spread of up to five feet.
Any stems that grow in water form spongy, hairy, soft structures called aerenchyma that serve to give the plant buoyancy and aid in efficient gas exchange. With leaves that very closely resemble that of Mimosa pudica, or sensitive plant, and ball-shaped yellow flowers that remind one of a rounder version of dandelion flowers, water mimosa is easy to recognize and identify. As a defense mechanism, water mimosa leaves will close up when touched, much like its semi-terrestrial counterpart, the sensitive plant.
Depending on location, various species of fish, turtles, geese, and ducks may feed on water mimosa. There aren’t any insects that are known to feed or rely on this plant, but bees and other pollinators may stop by the flowers from time to time. Some types of bacteria have formed a symbiotic relationship with water mimosa’s aerenchyma and rhizomes, which are what make nitrogen fixation possible. The nitrogen is used both by water mimosa and can also be utilized by nearby plants. It roots from its nodes and has creeping stems, making this plant quite easy to grow – and a potential nuisance as it spreads into thick mats if not kept in check.
Water Mimosa Growth, Hardiness & Climate
Water mimosa grows eagerly, its shoots growing as much as 7 centimeters per day if conditions are favourable and there is enough room. It does best in regions with air temperatures of 77 to 95 degrees Fahrenheit (25-35° C), but can tolerate temperatures as low as 50° F (10° C). Water temperatures between 70 and 85° F (21-29° C) are ideal, but a few degrees higher or lower can be tolerated for short periods of time.
As a tropical plant, water mimosa will only survive in zones 9 and above with ample sunlight. They can tolerate some shade, but majority or full shade will kill the plant. Flowers begin to bloom in early summer and will continue until frost occurs. In its native tropical regions, water mimosa is able to flower year-round.
How to Plant Water Mimosa:
Quite simple to propagate, you can plant water mimosa roots along the damp margins of your pond where it will then spread out to slightly deeper water. Another approach is to take a clipping from a live plant and simply allow it to float freely, and after a time it will put out nodes and, possibly, roots if it finds a shallow edge that it likes. They can also be placed in pots in a few inches of water.
In the wild, water mimosa prefers slow moving or still water, such as that of ponds or wetlands, ranging from 6 to around 30 inches in depth (though the plant itself will typically only extend up to 6 inches deep).
How to Care for Water Mimosa:
Since water mimosa spreads swiftly, it will need to be trimmed periodically – the exact amount depends on your personal pond aesthetic and function goals. If not trimmed at least twice during the growing season, neptunia will form dense floating mats that crowd out other plants, reduce sunlight and oxygen in your pond, promote mosquito breeding, and reduced fish activity. Make sure to properly dispose of any clippings either in the garbage or, preferably, via composting. Leaving the clippings will result in re-propagation in your pond, or worse, the plant finding its way into a natural waterway where it’s likely to be invasive (more on that below).
How to Winter Water Mimosa
Water mimosa cannot survive temperatures that stay below 50° F for longer than a few days. If you live in a region that experiences frost or snow, you’ll need to bring your neptunia indoors for the winter. It can be easily transplanted into a tank, either free-floating or in an anchored pot in a shallow area. Alternatively, water mimosa isn’t overly expensive and can simply be purchased again the following Spring.
Is Water Mimosa Toxic, Poisonous or Invasive?
Water mimosa is not known to be toxic to wildlife, and in fact the peoples of Asia have widely cultivated the plant for vegetable dishes for centuries. This plant is, however, quite invasive outside of its native ranges of the tropical portions of Asia, Africa, and South America. As such, it is imperative that you make absolutely certain that it’s legal to have this plant where you live, and if it is legal you should take precautions to ensure that the plant does not get into nature waterbodies.
In the United States, neptunia is, as of right now, considered a “noxious weed” and not yet invasive, as it isn’t widespread and has managed to naturalize in some ecosystems (you should still take precautions and make sure that you consult with your local aquatics or fisheries office!). In Australia, however, particularly Queensland, this plant is incredibly invasive and also illegal to have. Its escape into the wild has led to blocked waterways, loss of native aquatic plant and fish biodiversity, and stagnation of water due to depletion of dissolved oxygen.
Is Water Mimosa Edible? Will Fish Eat it?
Your fish may nibble on water mimosa, though it doesn’t seem to be highly desirable to most common pond fish species. If they do happen to eat it, no worries – neptunia is edible and won’t harm them. The fruits, or legumes, stems, leaves, and roots are often eaten raw, stir fried, or roasted into various dishes. The roots are also used medicinally in Malaysia and Thailand for things such as headaches, earaches, and skin wounds.
Where to Buy Water Mimosa?
Easily obtained, water mimosa can be purchased in most aquarium/pond stores, aquatic plant nurseries, as well as online.
