The two weeds that I have gotten the most questions on in the last two weeks are Canada thistle and Equisetum.

The best time to spray Canada thistle is in the next week or so, right before the plants start to show buds. Because of its extensive underground root system, it is one of the more difficult weeds to control. The aboveground portion of the plant is easy to kill with inexpensive products like 2,4-D, but a lot of re-growth is normally seen the year after application. In trials conducted on CRP ground in Keokuk County in 2004 and Johnson County in 2006, I saw the best long term control with products containing picloram (Tordon and Grazon P + D), clopyralid (Stinger and Curtail) and aminopyralid (Milestone and Forefront). Picloram and aminopyralid are not labeled in corn, but producers have had success with Stinger and Hornet (clopyralid + flumetsulam) in corn. Burndown applications of Roundup usually are made earlier than the optimum time to spray Canada thistles, but later applications in Roundup Ready crops can be successful.

Unfortunately Roundup is not an option for Equisetum control, since glyphosate has little effect on this weed. The weed has been around since the dinosaurs, and probably will still be around after humans are long gone. It occurs in a vegetative form, which looks like little Christmas trees, which some call scouring rush or field horsetail. Usually you can find the reproductive form nearby, which is a hollow jointed tube, which some call snake grass. These are the same species. It is often found in ditches and encroaches into corn and soybean fields from the edge of the field. The plants do not flower, but reproduce with spores and rhizomes. Most herbicides have little effect on the weed. The most effective herbicides are soil sterilants, which can only be used on non- crop ground. Last week I looked at an equisetum trial conducted in a road ditch by Ken Pecinovsky, farm superintendent at the NE Iowa research farm near Nashua. He had the best results with the soil sterilant Casoron (dichlobenil). Nothing is very effective in corn, but some have reported some suppression of the weed with Permit or Hornet. Tillage has not been very effective in controlling the weed and can actually help to spread it around. For more information and a picture of field horsetail, see the fact sheet by Bob Hartzler.

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