Horsetails Then and Now
Around 300 million years ago, Missouri was a warm, wet land covered by shallow seas and swamps. The marine animal fossils we find today in our limestone rock formations are testimony to that long ago environment. Amphibians and insects were abundant. Early reptiles also roamed the land, but the dinosaurs wouldn’t rule the earth for millions of years to come.
One of the dominant plants then was a primitive-looking tree that reached a height of 60 feet, with a trunk that was up to a foot in diameter. Future scientists studying the fossil record would give this tree the name of giant horsetail. They called it that because its fossils resembled a giant version of the horsetails we have today.
Although most of the giant horsetails would not survive the Carboniferous Period, these much smaller relatives did survive the intervening millennia and now are common plants in Missouri.
Horsetails were thought to belong to a group called “fern allies,” plants related to ferns. Recent studies, however, indicate that horsetails are actually primitive ferns.
Like other ferns, they reproduce by spores rather than seeds. Spores of horsetails are near-microscopic, single cells that are capable of reproducing plants the size of a pinhead, with just half a set of chromosomes. These tiny plants then produce sex cells that unite to form plants with full sets of chromosomes that we recognize as horsetails.
Horsetail spores are unusual in that each green spore contains four wing-like structures called elaters. When moist, the elaters coil around the main body of the spore. When dry, the elaters uncoil and help the spore to catch the wind for transport. This characteristic increases the chances that the spores come to rest in moist sites that are suitable for growth. The distinctive winged spores of horsetails are short-lived compared to spores of most ferns, but wind currents can transport them long distances.
The stems of horsetails are usually green and hollow and may or may not be branched, depending on the species. Photosynthesis takes place primarily in the stem, as the leaves are merely small, black or green teeth that occur in rings at each joint of the stem.
Three Missouri Horsetails
Missouri is home to three species of horsetail. The most familiar species, common scouring rush (Equisetum hyemale), grows in dense colonies along streams and rivers, at margins of ponds, in roadside ditches and on railroad rights-of-way. It is most prolific in sandy soils with plenty of available moisture, in full sun to moderate shade.
Its jointed, hollow stems are dark green and remain that color throughout the winter. The stems are rich in silica, which led to their use in cleaning pots and pans during colonial times. That accounts for the plant’s common name of “scouring rush.”
The rustling of the stems, when someone walks through a colony, produces a swishing sound similar to sheets of fine sandpaper being rubbed together.
When producing spores, the stems are capped by a yellowish, coneshaped “strobilus,” which comes from the Greek word for pine cone. The strobilus has an outer surface of densely arranged hexagonal plates that spread apart when mature, allowing the spores to waft away like fine dust. The finely ridged and jointed stems typically grow 3 to 5 feet tall.
Smooth scouring rush (Equisetum laevigatum) closely resembles common scouring rush, but its stems are smooth and they usually only persist for one growing season. There are other minor differences between the two species, and hybrids with intermediate characteristics do occur. Smooth scouring rush is less common than common scouring rush and usually grows in drier habitats, such as in the loess hill prairies of northwest Missouri.
Common horsetail or field horsetail (Equisetum arvense) has a strikingly different appearance from the two scouring rushes. It has two types of stems: a spore-producing one that is only briefly present in the spring and a sterile stem that is up and green throughout the growing season.
Its sterile stems, up to 2 feet tall, are shorter than the stems of scouring rushes. Their upper portion contains a series of slender, whorled branches resembling a miniature version of the giant horsetails of the past. The pale-colored, spore-producing stems are unbranched, have no chlorophyll and wither about the time that the green stems appear.
Common horsetail grows nearly throughout the state, usually along stream and river banks and other areas of moist or wet soil, but it is more common in northern Missouri.
Considering all the changes that the earth has undergone since their giant ancestors grew here, it’s a wonder that we have horsetails in Missouri today. It is somehow comforting to realize that this group of plants is still thriving despite millions of years of landscape upheavals and alterations.