Water Arum

Have you ever run into an acquaintance you haven’t seen in a long, long time? The person looks unfamiliar at first, then something dawns, and finally you remember who it is! That was my experience in finding today’s plant – Water Arum (Calla palustris) – for the first time in about 25 years.

Although I don’t see it often, Water Arum is common on PEI in slow-moving freshwater habitats such as the edges of ponds, streams, beaver meadows, and bogs. In late winter and early spring, its rhizomes can be seen on the surface of water-logged soils, putting down new roots at each node to help it spread. Distinctive heart-shaped leaves emerge in May, looking like across between Arrowhead and Violet (left photo).

The most striking parts of this plant are the flowers and fruit. Water Arum is a member of the Arum Family (Araceae) and so is related to Jack-in-the-pulpit that I’ve written about before (https://www.pei-untamed.com/post/jack-in-the-pulpit).

Like that plant, Water Arum’s two-parted flower is unusual: it’s comprised of a leaf-like spathe designed to direct pollinators to the finger-like spadix (top right photo).

The spadix has many individual flowers that develop into berries once pollinated. Those berries start out green (as shown here) but become bright red as they mature, standing out in contrast to the green vegetation around them. Squish the unripe fruit and you’ll find bright red seeds surrounded by gel (bottom right photo). Once the fruit matures, those seeds become brown. The gel plays two roles: it expands, breaking the fruit to release the seeds, and serves to help seeds hitchhike to new areas via waterbirds and other wildlife.

All parts of Water Arum contain calcium oxalate in needle-like crystals (called raphides) bundled together in specialized cells (idioblasts). When chewed, the idioblasts release their raphides, causing a painful burning sensation in the mouth and throat of whomever was unfortunate enough to take a bite. The raphides also pass along chemicals from the plant that stimulate an inflammatory reaction, further contributing to Water Arum’s anti-grazing defence system.

Because of this, Water Arum is considered toxic, although it does have a history of edible and medicinal uses. The effects of the calcium oxalate can be neutralized by proper processing of the plant which usually includes a combination of drying, grinding, and cooking. Starchy Water Arum roots have been used as an emergency food during times of famine, but I can think of a long list of plants I’d turn to first!

As a final note, the Calla Lily houseplant is related to Water Arum and has the same calcium oxalate raphides; if you have one, do be sure household dogs, cats, or humans don’t eat it. Our perennial, native Water Arum is a strange-looking but interesting part of PEI Untamed!