Herbicide application in drought conditions

[Christopher Sprague]

Six or seven years ago when we first developed contract language concerning herbicide application, we included the following clause which we borrowed from the PA DCNR:

"No herbicide application may take place when the Palmer Drought Index drops below negative two (-2). The Palmer Drought Index map is available on the internet at: www.cpc.ncep.noaa.gov/products. Scroll down to and select: Palmer Drought Severity Index Graphic (current).  The supervising forester may allow or prohibit spraying if local conditions differ from the Palmer Drought Severity Index."

-2 is the threshold from near normal conditions to moderate drought, which is why I believe that PA DCNR chose to use this number. Until this summer, we've never had to be concerned about crossing the -2 threshold, but if you look at the map now, most of the state (and country) is below -2.  Our region (eastern plateau) went below -2 just this past week, but data does not include Sunday's rain which was 0.25-0.5 inches depending on the location. 

We presently have multiple herbicide applications in progress and several recently completed. My concern is whether or not we need to shut down the spraying operations.  I'm not necessarily concerned about tightening our contract language because it gives us plenty of wiggle room, at our discretion.  I am more concerned with balancing the need to get the applications done in the short window of time that the contractors have to do the work, and the need to be sure that the treatments will be effective--without the need for follow-up treatments.  Ultimately the contractor is responsible for meeting our targets at their cost, so it is in everyone's interest not to spray if the treatment will not be effective. 

I've read a couple of different glyphosate labels and they indicate not to spray drought stressed plants, but nothing specific about the Palmer Drought Index (PDI).  Obviously things have been very dry, but I haven't noticed any visible symptoms of drought stress in beech or fern yet.

I presume the PDI is geared towards agricultural crops, exposed to full sun, so doesn't it stand to reason that it would take a higher PDI reading to impact understory vegetation in a forested environment, given the cooler temps and moister environment?  Secondly, if the application is cut stump or hack and squirt, as opposed to a foliar application, does the PDI reading have the same ramifications?  In other words, does the method of herbicide application on a drought stressed tree impact the uptake and effectiveness of the herbicide? 

Thanks for any input you can provide.

Chris