Is Craig Harper’s FSI Cocktail Safe for Non-Target Trees?

January 9, 2023 By: NDA Staff

You can quickly restore sunlight to a closed-canopy forest to fuel deer forage production – a technique called “Forest Stand Improvement” (FSI). We recommend girdling and spraying select trees to kill them standing using an herbicide blend we named the “Craig Harper Cocktail” after its creator, Dr. Craig Harper of the University of Tennessee. One of the ingredients is imazapyr, which is soil active, and that often leads to questions about whether this blend kills nearby non-target trees.

The answer is – No – not in the small amounts applied to a girdle or stump. This technique is a surgical strike on individual trees using minuscule doses of chemical. In fact, Craig’s ongoing experiments are showing evidence the blend can kill one trunk of a multi-trunked tree without harming the others.

“I imagine there will be people who doubt that,” Craig told us, “so I thought I would send you pictures to confirm.”

Craig Harper treated one trunk of this split-trunk yellow poplar with his custom herbicide blend in fall 2021. In 2022, the treated trunk was dead but the untreated trunk was not harmed.

The point of Craig’s experiments with multi-trunked trees was to test the safety of his blend and the girdle-and-spray technique when it comes to killing select trees while leaving nearby, desirable non-target trees unharmed. Not only is it safe for nearby non-target trees, it appears to be safe for the untreated trunks of a multi-trunked tree!

The one documented exception is with tree species that clone themselves by sprouting new trunks from their roots, such as aspens and sweetgums. In a formal study of forest stand improvement, Mark Turner of the University of Tennessee applied FSI to four, 8-acre patches (32 acres total) using Craig’s herbicide blend. Gathering data afterward, he documented only one case of non-target mortality out of 440 trees sampled, and it was a non-treated sweetgum tree located next to a treated sweetgum.

Craig girdled and sprayed one trunk of this three-trunked willow oak in December 2020. Visiting the site again seven months later in July 2021, he found the treated trunk was dead but the other two trunks were alive and well, and showing no harm whatsoever.

Most likely, these were clones sharing one root system. Most tree species, including oaks, do not do this. Unless you somehow spill significant amounts of imazapyr on the ground in your work area, evidence suggests you will never see non-target mortality using stump-treatment or girdle-and-spray at these rates.

Below is the recipe for a Craig Harper Cocktail. Always read and follow the complete label instructions for safe and effective use of any herbicide, including precautions for your personal safety.

FSI is a powerful tool for rapidly increasing the amount of quality deer forage and cover where you hunt. We’ve got plenty of guidance to help you learn more:

Dr. Craig Harper was a guest on NDA’s Deer Season 365 podcast, episode 14, on January 5, 2022, where he covered using FSI to create great deer habitat from start to finish. Download and listen to this episode on our website or wherever you listen to podcasts.

We’ve written a two-part article covering the complete FSI technique. Start reading part one of “Deer Habitat in a Squirt Bottle” now.

We have several videos in our Deer Habitat playlist on NDA’s YouTube channel that cover how to choose the right herbicide blend for you, how to girdle trees, and more about FSI techniques. Watch one of them right here.