Field Horsetail ID and Management in Field Corn

Chris Boerboom, Extension Weed Scientist

Field horsetail (Equisetum arvense) is a unique, native perennial weed. The plant is unique because it grows brown fertile shoots early in the spring that produce spores, not seeds (photo 1). As the fertile shoots begin to withering, sterile shoots with the familiar whorled branches emerge and grow (photo 2). These shoots are the plants often identified as “miniature pine trees” and grow from 6 to 24 inches tall (photo 3). While these shoots are seen above ground, a deep underground system of rhizomes and tubers allow horsetail to spread and persist from year to year. The rhizome growth is greatest in the summer and the tubers develop in the late summer and fall. It is unlikely that the spores result in the generation of many new plants or contribute to the spread of horsetail. Most spread is from creeping rhizomes or when rhizomes are moved with tillage equipment.

Photo 1. Fertile shoots of field horsetail.

Photo 2. Sterile shoots of field horsetail emerging.

Photo 3. Branched shoots of field horsetail.

Field horsetail and scouring rush (Equisetum hyemale) are often confused. These weeds are cousins and are similar, but scouring rush grows much taller, has thicker stems, and does not have fine branches (photo 4). The shoots of scouring rush are like “stove pipes” and can be pulled apart at the joints. Another difference with scouring rush is that it does not have the separate growth stages of fertile and sterile shoots of field horsetail; the green scouring rush shoots can be tipped with a cone of spores. 

Photo 4. Green shoots of scouring rush growing in a mixture with the brown, fertile shoots of field horsetail (photo credit J. Polenske).

Several important aspects are known about field horsetail’s biology. Typically, it is found more frequently in wet or more acidic soils. It is more productive in full sunlight and its growth is significantly suppressed by the shade of taller plants. In general, higher nitrogen fertility favors other crops to help shade out horsetail. Based on these characteristics, a competitive field crop should help to suppress field horsetail. This is likely why it is more common to find the highest densities of horsetail in disturbed areas along the margins of fields where it receives more sunlight. Wisconsin growers are also seeing field horsetail increase in no-till systems because tillage is not damaging its network of rhizomes and tubers. Furthermore, burndown herbicides typically applied in no-till corn such as glyphosate and 2,4-D are not effective in controlling horsetail. 

Our current control recommendations are based on research from colleagues in Ontario who recommend using flumetsulam (active ingredient in Python and one of the ingredients in Hornet) in burndown applications or a postemergence tank mix of ALS plus growth regulator herbicides (e.g. comparable to Steadfast plus Status) in corn. I conducted a preliminary experiment with Python as a burndown treatment in Wisconsin and control was adequate, but some managers have not been satisfied with the performance when they tried this program.  

With the support of the Wisconsin Corn Promotion Board, we are testing several burndown and postemergence herbicide treatments for their control of field horsetail this year. To date, one of our biggest surprises is the slow emergence and growth of field horsetail with the cool spring. We even struggled to find a one known patch of field horsetail because it was so slow in emerging.  Although the field horsetail is not as tall as we hoped and has not yet branched, we are applying the burndown treatments now because the fields need to be planted with corn. This will likely limit the amount of herbicide uptake by the shoots, but we hope some treatments are still effective. We will provide more information on the results of these trials at our meetings this fall.