How to Plant & Grow Marsh Mermaid Weed (Proserpinaca palustris)

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Photo by: Rudi77 / CC BY-SA

Marsh mermaid weed belongs to the water milfoil family Haloragaceae. Not to be confused with Proserpinaca pectinata, known as comb-leaved mermaid weed, marsh mermaid weed (Prosperpinaca pectinata) has larger, serrated leaves as opposed to the smaller, comb-like leaves of pectinata. Their habitats quite often overlap, making it easy for one to mistake them until looking more closely at the leaves.

However, both can be grown in your pond. For the purposes of this article and to avoid undo confusion, we will focus on P. palustrus, but know that both P. pectinata and P. palustrus are suitable for fish ponds and much of the care and maintenance for these two plants is the same.

Facts, Benefits & Uses of Marsh Mermaid Weed

They are known for providing valuable habitat to fish and aquatic macroinvertebrates such as caddisfly and dragonfly larvae, as well as oxygenation and purifying water with their submersed leaves.

This plant can grow tall enough to exist above water, and this portion of P. palustrus will produce incredibly small, green-white flowers that develop directly on the stem, typically at the bases of leaves. Though typically green, P. palustrus’ leaves can become red-orange in the presence of iron. Having access to plenty of sunlight can have this effect on the leaves, as well.

Marsh mermaid weed is considered an at-risk species, meaning that it’s of conservation concern in North and South America and is closely monitored to avoid further population damage via human activities. Its preferred natural habitats include the clean, slow-moving waters of fens, lakes, ponds, the more sluggish areas of rivers and streams, riverbanks, and wetlands.

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Marsh Mermaid Weed Fact Sheet:
Herbaceous Aquatic Perennial
USDA 5 – 11
Full sun
White, green
July – September (Summer)
Height up to 30 cm (12 in)
1.2 cm (.5 in) or free floating
pH 6.5 – 7.5

Marsh Mermaid Weed Growth, Hardiness & Climate

Multiple flowering marsh mermaid weed underwater
Marsh mermaid weed has long, serrated leaves and produces small, bell-shaped green-white flowers.

If placed in full sunlight, marsh mermaid weed is a moderate to fast grower. Planted in partial shade, and it will be a slow to moderate grower. Your pond’s location, as well as whether or not you wish to trim P. palustrus back every few months, will determine lighting conditions and how quickly it grows.

Height can range anywhere from only a few inches to over a foot, again depending on light conditions. More sunlight will equate to greater growth. Sometimes, P. palustrus can grow a couple of feet in order to reach the surface of the water. Each leaf can be up to 4 centimeters long if submersed, or up to 8 centimeters long if above water. Leaves found below water will appear more fan or feather-like, while those above water will be ovate with serrations along the edges.

It’s a relatively hardy plant, able to grow just as well in Canada as it is in Cuba. However, it is not winter hardy (zones 5 through 11 are best) and will die back once frost hits. Fallen seeds can sprout the following spring. Other than that, however, with ample sunlight and moisture, marsh mermaid weed is considered a hardy plant, suitable even for novice ponders.

How to Plant Marsh Mermaid Weed In Ponds

Marsh mermaid weed growing in an emergent pond edge
Michael Ellis / CC BY

Marsh mermaid weed prefers slightly acidic soils, as they’re most often found in fens, bogs, and other moderately acidic wetland habitats. When planting, you can take plant trimmings and either place them in up to a half inch of soil, or simply anchor them to the bottom of the pond. Their roots will form quickly and take hold without any issue.

If planted marginally in shallow water or damp soil without standing water, plant them in closer to an inch of soil as they won’t have water about them to help hold them upright. You can place substrate or mulch around the roots to both hold them in place and help retain moisture. Aquatic planting baskets can also be used, either marginally or anchored to the bottom of the pond.

How to Care for Marsh Mermaid Weed

A shallow pond edge with proserpinaca palustris marsh mermaid weed
cassi saari / CC BY-SA

As state previously, marsh mermaid weed requires as much sunlight and moisture as possible. If not planted directly in a pond or lake, be sure to keep the soil consistently saturated. In some cases, you may need to supplement carbon dioxide and phosphate for this plant, as it uses it up quickly. However, it should be noted that this is usually only necessary if L. palustris is in an aquarium – ponds and lakes should have enough naturally occurring nutrients for marsh mermaid weed.

If grown in ample sunlight, marsh mermaid weed can grow quickly. You’ll want to prune the foliage as needed, though this shouldn’t really be necessary more often than once per year. As always, be sure to clean up any dropped or trimmed foliage to maintain healthy water quality. If not planted directly in water, place mulch around the base of the plant to help hold moisture in and encourage growth.

How to Winter Marsh Mermaid Weed

L. palustris is not tolerant of the cold, and will die down to its rhizomes in the winter. You can either allow this to happen, as new plants will emerge from rhizomes and seeds in the spring, or gently dig up marsh mermaid weed and transfer it indoors for the winter. You can also simply trim it and plant the trimmings indoors rather than digging up the entire plant.

Is Marsh Mermaid Weed Toxic, Poisonous or Invasive?

Though native to the Americas, marsh mermaid weed is not considered invasive outside of these areas. This is due to its non-aggressive growing nature, as it doesn’t spread very far vertically or horizontally. Its rhizomes are weak, not prone to choking out other plants or breaking through walls and lines like many other plants with rhizomes (such as cattails and phragmites). As a result, it’s considered non-native but not invasive outside of the Americas, meaning it’s alright to grow in the U.K. so long as you ensure it doesn’t spread to natural waterways.

Marsh mermaid weed is non-toxic, and in fact is listed as a “fair” food source for waterfowl, such as dabbling duck species like the unique Northern Shoveler, by the U.S. Fish and Wildlife Service.

Is Marsh Mermaid Weed Edible? Will Fish Eat it?

Marsh mermaid weed is edible, but most pond fish don’t seem to find it overly interesting or palatable. Some ponders report that their goldfish nibble on it, but not enough to cause much damage. If your fish do eat it, rest assured that they won’t be harmed by it. In the event that your fish really like this plant and are damaging the population, you can easily obtain more mermaid weed by simply clipping an inch or two off the top of the plant and placing it in the substrate.

Where to Purchase Marsh Mermaid Weed & Seeds? (UK & US)

Due to its threatened nature, it’s more likely that you’ll find marsh mermaid weed via online outlets rather than in a physical store such as a nursery or pond retailer. A quick online search yielded zero results from plant nurseries and home improvement stores near me, but you should check nonetheless as your area may be different.

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