The beautiful shades of green in PEI’s landscape are thanks to the 99.5% of our plants that photosynthesize – they make their own food from sunlight. Today’s plant is one of the very few that takes an entirely different path. This is Swamp Dodder (Cuscuta gronovii).
We’ve looked at Ghost Pipe (Monotropa uniflora) and Pinesap (Hipopitys monotropa), two unusual plants that sneak into the underground trading network between fungi and trees to steal some of the water, sugar, and nutrients for themselves (you can read more about them here: https://www.pei-untamed.com/post/pinesap). Dodder is much more overt, tapping directly into the above-ground stems of its host plants. If Ghost Pipe and Pinesap are thieves taking food from your kitchen while you aren’t looking, Dodder is the Vampire on your neck.
A surprising number of photosynthetic plants supplement their income by being parasitic on other species, but very few go all-in and eschew chlorophyll entirely. Swamp Dodder’s stems are orange thanks to the carotenoids that help protect its cells from sun damage. With no need for large leaves to catch sunlight, Dodder looks like orange spaghetti tossed on top of other plants (Photo 1).
Being first to the party can be an advantage, but not if you’re a parasite: there’s no point showing up before your host does. Swamp Dodder is an annual that germinates in late spring, giving nearby plants a head start. Although Dodder seeds start out rooted in soil, the new plants quickly find a host and begin wrapping around it (Photo 2). The parasite grows specialized roots called haustoria that penetrate its host, stealing water and nutrients from it. Within a few days of germination, Dodder is not rooted in the soil at all.
Dodder’s host selection and invasion are more nuanced than you might expect. Using light reflected from and chemicals emitted by potential hosts, Dodder avoids plants that are less fit – too small, diseased, or injured. The host also fights back with plant equivalent of scar tissue along with chemicals to try and repel the intruder. It’s been shown that during the infection phases parasite and host trade cellular messages; this “conversation” may encourage the host to lower its defences.
In late summer, Swamp Dodder produces lovely, five-petalled white flowers (Photo 3) that are pollinated by a variety of flies and wasps. These mature into seeds that drop to the ground to overwinter and continue the cycle or get eaten by birds and mammals to be spread to new locations.
Dodder doesn’t usually kill its host, but can keep faster growing, aggressive plants in check. In this way, it plays an important role in maintaining biodiversity within the coastal meadows where it’s typically found. Swamp Dodder is native but not common on the Island and is a fascinating part of PEI untamed!
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