Herbicide Carryover Could Impact Where You Site Your High Tunnel

While this is a good practice it can present some unique challenges if the high tunnel is going to be erected in a field that recently grew an agronomic crop like corn.

Herbicides are used routinely on most farms and farmers carefully plan their agronomic crop rotations around the stated restrictions or guidelines that appear on the pesticide label. When a high tunnel is to be erected in a cornfield the farmer or grower may not be cognizant of the rotational restrictions that are to be followed when a high-value horticultural crop is to be grown.

On a recent visit to an area vegetable grower, we looked over a cornfield that had just recently been cut for silage. The soils were deep, the land was relatively level, and the field was located near his water source so irrigation could be easily deployed. When I asked the grower about the herbicide history for the site, he indicated that he thought that the herbicide atrazine may have been used in the field, but that he would have to check with another family member. The next day the grower shared with me that atrazine, metolachlor, and rimsulfuron had been applied to the cornfield.

A review of the herbicide labels revealed that the rimsulfuron product required an 18-month restriction on planting vegetable crops. A review of the atrazine plus metolachlor label revealed that a full two years needed to elapse before the soils in that field could legally be planted with vegetable crops like tomatoes.

High tunnel tomato crop. Photo: Tom Ford, Penn State

Pesticide labels are the law and in this case, the label is trying to prevent growers from making a serious mistake that could result in crop failure due to herbicide carryover. Atrazine is a very persistent photosynthetic inhibitor that can remain in the soil for a relatively long time. Atrazine's persistence in the soil is dependent upon rainfall, soil pH, and even microbial activity. Under certain environmental conditions, atrazine can experience enhanced degradation in fields when it has been used continuously, but even if the chemistry does degrade more quickly, you are still bound by the restrictions on the pesticide label.

Growers often ask Extension Educators if they can test the soils where herbicides have been applied to "clear" the field for planting high-value horticultural crops. There are private labs that can test for herbicide residues, but variability in field soil conditions and plow depth can impact the accuracy of these results.

One grower that I worked with years ago was intent on planting an area that had been planted in corn the year before. He contracted with a private lab to test for residues and thought that he was safe to plant his strawberries when the returned test results appeared favorable. After planting the strawberries, some of the plants became chlorotic in appearance and did not establish very well due to atrazine carryover in a few sections of the field.

Another grower wishing to defy the label conducted a bioassay in which he took soil from the field at varying depths and filled several flowerpots and planted snapbeans in each pot. This grower also filled two flowerpots with soil from an alfalfa field that had not been treated with an herbicide in the past four years. He planted snapbeans in these two control pots as well. The pots were watered, and the beans sprouted and emerged in each pot which gave the grower some hope. As the first true leaves emerged from each plant the grower started noticing a pronounced yellowing and then browning on the bean plants planted in the soils where herbicide carryover was expected. The snapbeans in the control pots grew vigorously and even began to flower, while the other snapbean plants were essentially stunted, chlorotic, and in some cases even dead.

This simple bioassay cost this grower very little to administer but provided him with the answer he needed. As a result, the grower decided not to plant this specific field with vegetable crops or cut flowers and instead opted for another year of corn. The grower elected to use glyphosate-tolerant corn the next year which allowed him to plant vegetable crops the following year with no fear of herbicide carryover.

If you are a beginning vegetable or cut flower grower, or a seasoned professional, carefully consider the field history before developing a cropping plan. Herbicide carryover is a real threat so review each herbicide label carefully so you don't lose your next crop to herbicide injury.

High tunnel cut flower production. Photo: Tom Ford, Penn State