Scouring Rushes

2016 June 4

In addition to the Common Horsetails, we also have quite a number of their close relatives, the Scouring Rushes. There are two species of scouring rushes growing alongside of our road: one with stems about the diameter of a pencil and that get almost 3 feet tall:


and one with stems more the diameter of spaghetti, and that only get to be about a foot tall:


The big ones are most likely Equisetum hyemale, while the smaller ones are probably Equisetum scirpoides.

Aside from size, both of these plants are very similar. They have long, thin, unbranched stalks, with obvious joints between the stalks.


They pull apart readily at the joints.

Large scouring rush:

Dwarf scouring rush:

And both species have a spore-bearing head at the tip.



The stalks are hollow, with smaller channels running along the wall of the stalk, making them resemble corrigated cardboard. The stalks are surprisingly strong and stiff as a result. One point is that the larger scouring rushes are a bit deceptive, they don’t contain as much biomass as they look like because they have a very large hollow space inside:


Compare this with the dwarf scouring rush, which has a relatively miniscule hollow, and so the stalks have much greater solidity.


There is a bit of an advantage to the hollow stalks. Since these plants don’t have separate leaves, all of their photosynthetic area is on the stalk, and so the greater the area, the better they can photosynthesize. And the easiest way to increase the stalk area is to increase the diameter of a hollow stalk.

The stalks also feel extremely rough. This is because of silica (SiO2) crystals that the plant deposits in its tissues. They contain a lot of this, apparently as much as 20 to 30% of their dry mass consists of silica. This is extremely high compared to most other plants. Grasses, which are considered to be pretty high in silica as plants go, only contain 2 to 5% silica. In the case of the scouring rushes, the silica is likely to be both a stiffening agent, and a partial defense against being eaten. Anything with teeth that tried grazing on scouring rushes would wear out its teeth very quickly. And, unless their teeth could regrow, this would leave them starving due to being unable to chew their food. Of course, there are ways around this. Some animals can regrow their chewing surfaces, while others (like birds) can use gravel-filled gizzards to “chew” their food. But still, the silica certainly does make the stuff unpleasant to eat, and apparently makes it more difficult to extract what nutrients are there. I understand scouring rushes are a bit toxic as well, which probably accounts for me not yet seeing anything eating them.

Anyway, the roughness of the stalks is the source of the common name of scouring rushes. They are essentially naturally-growing equivalents of sandpaper, and have been used for centuries for scrubbing and polishing a variety of surfaces.

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