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The P&PDL Picture of the Week
for 17 November 2003



The Ancient Horsetail (Equisetum spp.)

Glenn Nice, Botany and Plant Pathology, Purdue University

I often get phone calls and questions about this unique group of plants and as some of you in Indiana may already know, if I present information about plant identification, this plant will always be included. Horsetail is one of my favorite plants, not because of the fact that it is often presented in first year botany classes, but because it is a living fossil. This group of plants is what is left of plants that were as thick as a forest and had relatives as big as trees, that flourished during the Devonian period approximately 350 million years ago.

Commonly known as horsetail, you may also have heard them called “scouring rush”, “mares tail” (a common name used for another weed in Indiana), “horse pipes”, and “snake grass”. Horsetail is a plant that often gets overlooked, until it is on the margins of a pond, in a ditch, or encroaching on a producer’s field. But, if you look for it, you will find it is quite common.

Easy to distinguish from other plants, horsetail consists of hollow stems. The stems are jointed and can easily be pulled apart into sections. Its stems also have siliceous ridges making it rough to the touch. I have used these stems to clean fry pans on occasion while camping, hence the name “scouring rush”. The stem is the primary photosynthetic organ. Most of the horsetail we see in Indiana consists of branchless stems. There are examples of branched horsetail and branching can occur when the top is removed. One thing you will notice about horsetail is that it does not appear to have leaves. Leaves are present but they are reduced to being small and scale like.

Sometimes, just to be cantankerous, I will ask students what the flower looks like. In most cases I get the correct answer, but once or twice someone has tried to tell me that horsetail flowers are yellow or purple. In reality this group of plants does not have flowers. Horsetails reproduce by spores and have two separate living structures. The structure that we see is the structure that produces spores in a sporangia on reproductive stems. The structure we don’t see is called a gametophyte and it is a very small multi-cellular structure that is part of horsetail’s sexual life cycle. One often finds horsetail in ditches, and along ponds due to this sexual part of its life cycle. The gametophyte requires a wet environment for fertilization and survival. However, vegetative reproduction will allow horsetail to populate dryer environments in Indiana. Tillage can sometimes spread horsetail rhizomes into dryer parts of a field.

Being perennials, horsetail has a deep rooted system with rhizomes and tubers that can produce many terrestrial stems, giving it a colonial appearance. When you see horsetail, it is usually several stems, reaching to the sky looking like a branchless forest about as high as your waist.

Horsetail’s impact as a weed might be considered marginal. Most crops with proper growing conditions and proper drainage can compete with this weed. However, aesthetics sometimes becomes a concern. Also I have heard it said that the silica in the stems can dull combine blades; however, I have not seen any data that would support this. Horsetail can cause “equisetosis” in horses, sheep, and sometimes in cattle, when it is fed upon for long periods of time. Symptoms of poisoning include breathing and heart problems, fever, digestive, problems, convulsions, and death (Indiana Plants Poisonous to Livestock and Pets, http://www.vet.purdue.edu/depts/addl/toxic/cover1.htm ).

If horsetail needs to be controlled, it is no easy task. If possible it is best to prevent horsetail. This includes avoiding tillage in areas where horsetail occurs. If tillage occurs be sure to clean the equipment to assure that rhizomes don’t get transported to new areas in the soil. Also improve drainage in poorly drained areas near ditches, bodies of water, and low spots. Mechanical control is difficult. In a study conducted in Quebec Canada, Equisetum arvense was removed by hoeing 16 times and did not have any impact on regrowth (Cloutier and Watson 1985).

In the case of chemical control, there are not many options available. The lack of efficacy of many herbicides, specifically contact herbicides, is partially due to the fact that horsetail is a perennial with a deep root system rhizomes, and tubers. It is also due to the fact that many herbicides just don’t enter the plant at high enough levels to get the job done. The siliceous structure of its stem may inhibit the movement of herbicides into the plant. Furthermore, by not having leaves the small surface area of horsetail would decrease the amount of herbicide that might enter the plant. Adding to the limited success of herbicides, keep in mind that this plant is often on the waters edge or close to it, reducing the number of herbicides that can be legally used. In such cases, only herbicides labeled for application on or near water can be used.

Glyphosate products are often used to suppress horsetail. However, regrowth should be expected. Control may require several applications over several years. In many cases glyphosate control of horsetail is inconsistent. Casoron (dichlobenil) has also been reported to have activity on horsetail. MCPA has been used in small grains to suppress horsetail.

The term “weed” to some, means “beautiful” to others. It has been said that “beauty is in the eyes of the beholder”. Horsetail is not always considered a weed. It is mentioned in folklore as a medicinal herb. There are historical reports that tender young shoots were being boiled and eaten by the ancient Romans; however I would not put too much weight on that, for ancient Romans used lead to line their aqueducts and pipes and we know now what harm that could cause. Horsetail is also theorized to play an important roll in the ecology of a watershed. Its value may be misinterpreted without proper research and understanding. In the study of an Alaskan shrub wetland, March reported in 2000 that horseweed did have an important role in removing and cycling certain nutrients.

So if you should see this plant, give it a little respect and wonder about its roll in the scheme of things. Don’t hold your breath waiting for it to flower though, you will be waiting a long time.

Don’t just take my word for it, below are some other sites that talk about horsetail
1) http://ipcm.wisc.edu/uw_weeds/extension/articles/conhorsetail.htm
2) http://plants.usda.gov/
3) http://www.sidwell.edu/us/science/vlb5/Labs/Classification_Lab/Eukarya/Plantae/Sphenophyta/
4) http://aquaplant.tamu.edu/Emergent%20Plants/Horsetail/horsetail_control.htm
5) http://www.ridgetownc.on.ca/weeds/Probweed/Probweed_Frame.htm

References:
Cloutier, D. and A.K. Watson. 1985. Growth and regeneration of field horsetail (Equisetum arvense). Weed Science 33:358-365

Marsh, A.S., J.A. Arnone, III, and B.T. Bormann. 2000. The role of Equisetum in nutrient cycling in an Alaskan shrub wetland. Journal of Ecology. 88:999-1011

Click on the small image to view a larger image.

Drawings of horsetail fertile and sterile stems, strobilus, and spores.

Picture source: The world’s worst weeds 1991. LeRoy G. Holm, et al.

Horsetail growing in a ditch.

Horsetail close up of the
segmented stem and the small,
grey, scale-like leaves.

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Last updated: 14 November 2003/amd
The Plant & Pest Diagnostic Laboratory at Purdue University