1. How Scouring Rush and Field Horsetail Look Different but Belong Together
Field horsetail grows along roadsides and field edges. Its green stems carry rings of thin side branches at each joint, so the plant looks feathery. A Japanese river office describes these green stems as about 30–40 cm (12–16 in) tall [4].
Scouring rush is different. Its stem is a plain, dark green tube that is usually unbranched and stands about 30–100 cm (1–3 ft) tall, with a small cone at the tip that makes the spores [3]. The two plants look nothing alike, yet both are in the same genus, Equisetum. Like all horsetails, they are ferns, so they spread by spores rather than flowers and seeds [6].
One more thing before we go on. In spring, field horsetail sends up a pale, spore-making shoot that is often called tsukushi in Japan. A popular idea says that it grows into the green plant, but the river office says that this is not so [4]. The two kinds of stems come up separately from the same underground stem. That is a different story from the one on this page, which is about the silica in the green stems.
2. Why a Soft-Looking Stem Feels Like Sandpaper
The "whetstone" name comes from a plain observation. The Japanese encyclopedia entry says that scouring rush was named because its stems, which contain silicates, were used to polish things instead of a whetstone [3]. People dried the stems and used them to polish wood and metal [3]. The thesis from the University of Potsdam says that horsetails were used to scour kitchen utensils and polish wood because of the silica built into the skin (the epidermis) [1].
That thesis also looked at where the silica is, using several measuring methods. It found silica laid down all over the outer skin, with the most on the small knobs that dot the surface. The abstract reports a silicon content of about 33% at the knob tips and about 17% on the skin beside them (it does not say whether this is by weight or by atoms), and almost none was found in the stem's inner parts [1]. Most of this silica is amorphous, meaning that its atoms are not arranged in a neat crystal; it is closer to glass than to quartz [1].
So the hardness is not spread evenly through the plant. It is concentrated in a thin, rough coat on the outside, the part that touches whatever you are scrubbing.
3. How Much Silica Scouring Rush Holds, and When It Builds Up
The same thesis abstract says scouring rush can hold silica up to 16% of its dry weight [1]. That would be 16 g in 100 g of dried plant. This is a ceiling, not an average: the plant does not always carry that much, and the abstract does not give typical values.
The stiffness is not there from the start. In 1973, a team of American plant scientists studied young stems of a North American form of scouring rush. They numbered the sections between joints (internodes), with higher numbers meaning more growth. Silica showed up first around the stomata, the tiny pores that let air in and out, starting at internode 3. It then appeared in small bumps on the skin (internode 8), and finally in the side walls of the long skin cells (internode 10) [2].
The last step happens after the stem has stopped stretching. The authors describe it as one of the final stages of building a mature stem, strengthening the tough, fibre-like (cellulose) frame of the cell walls [2]. A scouring-rush stem, then, is not born hard. It hardens as it matures.
One caution: this order was measured in a North American form, and I did not find a matching study of the Japanese plant, so I would not claim that every scouring rush follows exactly the same steps.
4. Does Field Horsetail Have Silica Too, and Which Is Harder?
The Potsdam abstract describes horsetails as a group that people used for scouring because of silica in the skin [1], so silica is not a feature of scouring rush alone. But I could not find any source that measured field horsetail and scouring rush side by side under the same conditions. That means I cannot say which one has more silica or which one is harder.
What the sources do show is a difference in habit. Scouring rush became known as a polishing tool, and its name says so. Field horsetail has been known mostly for its green branches and its spring spore shoots. Whether the cousins differ much in silica remains an open question for anyone willing to check the research papers.
5. How to Tell the Two Plants Apart Outdoors
Scouring rush is used as a garden plant, and in the wild it grows mostly in damp ground such as mountain wetlands and stream valleys [3]. Wherever you find either plant, compare these three things:
- Branches: field horsetail has rings of side branches. Scouring rush has a plain stem and rarely branches [3] [5].
- Height: a scouring-rush stem can reach about 1 m (3 ft), while the green field horsetail stems described above are about 30–40 cm (12–16 in).
- Feel: look at the plain, unbranched stem without touching. If you want to feel it, check with an adult first, and do not pull, snap or scrub the stem. Leave plants in other people's gardens and in parks alone, and look without picking.
If you want to go further, read the Potsdam abstract and the 1973 paper listed below, then look for a study that compares silica in field horsetail and scouring rush directly. If you find one, you will know something this page cannot tell you.