Every year I receive several calls from people wanting to know whether or not they can plant a garden in an area that was field corn last year. My first question for them is related to the herbicide application. What herbicide was applied? Generally, this information is easy to obtain. However, the response I give may not be as easy to deal with because of herbicide rotation restrictions.
A herbicide's rotation restrictions may be found in the general use precaution section of the label or it may be a separate section in the label. These restrictions tell the applicator what crops can be planted the year of application in case of a crop failure as well as the crops that can be planted the following year because of the residual effect of the herbicide. In the past, corn ground was the main concern for vegetable growers because atrazine was used on most of the corn acres. The concern with products containing atrazine was because of the potential carryover with this chemical and the fact that it had the following rotation restriction. "Do not plant sugar beets, tobacco, vegetables (including dry beans), spring-seeded small grains, or small seeded legumes the year following application, or injury may occur." This meant vegetables could not be planted in ground treated with an atrazine product because the label is the law, and not following label directions is breaking the law.
Now days, in addition to atrazine treated corn, vegetable growers also need to be concerned with soybean ground because of recent introductions of herbicides such as chlorimuron or imazethapyr.
Products containing chlorimuron have a rotation restriction for most vegetables that consists of successfully growing to maturity a test strip of intended crop(s), before planting the vegetables you intend to harvest. This is called a field bioassay, which the label states should be initiated 9-18 months after the application.
Products containing imazethapyr have increased the vegetable rotation interval for applications made in 1993. Last year an 18 month rotation restriction was stated for vegetables other than edible beans and peas, lettuce, and sweet corn. This year that restriction is 26 months.
Therefore, if you intend to grow vegetables on ground that was corn or soybeans last year, find out the herbicide(s) applied and remember the label is the law.
This article originally appeared in the March 31, 1993 issue, p. 37.