Keeping an Eye on Grass Herbicide PHI (Pre-Harvest Interval) – Vegetable Crops Hotline

Keeping an Eye on Grass Herbicide PHI (Pre-Harvest Interval)

Grass-selective herbicides are valuable to weed management in broadleaf vegetable crops.

As harvest season draws closer, it is important to note that not all grass herbicides have the same pre-harvest interval (PHI) requirements.

The two most commonly used grass-selective herbicides are clethodim and sethoxydim. While most PHI’s are similar between the two (Table 1), there are some big differences.

For example, cantaloupe and cucumber require 14 days between the last application of clethodim and harvest. The interval for sethoxydim is only 3 days. The opposite is true for okra.

PHI should not be the only factor in choosing which grass-selective herbicide you use, but it needs to be part of the consideration. If you’ve already applied a herbicide and realize that harvest is sooner than the PHI requirement, you should delay harvest until the PHI is reached. This is easier for some crops than others. For example, a couple extra days may be a minor inconvenience for crops like carrot and potato. However, for summer squash the crop may need to be harvested and destroyed to avoid excessively large fruit and a reduction in new fruit production. Harvesting prior to the labeled PHI could result in excessive herbicide residue.

Please consult your product label before making any application. To find the PHI for crops not listed in Table 1, consult the label or the Midwest Vegetable Production Guide (Midwest Vegetable Production Guide (


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