A weedy problem: Commonly used herbicides can damage live oaks | Sally Scalera

If you have oak trees in your yard and a lawn service that applies broadleaf herbicides to your lawn, pay attention to the weather this spring. Oak trees can be harmed by some broadleaf herbicides during a hot, dry spring. The problem is usually described as sections of the tree, either large or small, turning brown, with the leaves staying on the limbs and not shedding. This same problem has occurred three times, years apart, over the last decade or more.

When this first happened, it was on the grounds of the Eastern Florida State College (when it was still Brevard Community College) campus in Cocoa. The large oak trees in the front of the main administration building had large canopy areas that had turned brown. In addition, Ligustrum trees were also turning brown and dying quickly. When it was all said and done, only two large oak trees remained in front of the administration building, and some of the Ligustrum trees were removed.

The second time, years later, it was at a home in Cocoa Beach. The homeowner was distraught because one of her two live oak trees in the front yard suddenly turned totally brown while the larger one, just to the south, was still healthy and green. For a mature oak tree to die this quickly, it would not be caused by an insect or a disease.  Because her lawn was so nice and weed-free, I asked her if her yard was treated for weeds. She said that it was, and I asked her if she knew what product was being used. After getting the slips left after each visit, I scanned the paperwork and found one of the products I was looking for: MSM or metsulfuron-methyl.

Metsulfuron-methyl, also known as MSM, is a systemic herbicide used to control broadleaf weeds and certain grass weeds (including Bahiagrass) in established bermudagrass, St. Augustine grass, and other warm-season turfgrasses.  It is also an effective control against some of the most problematic turfgrass weeds, such as dollar weed (Hydrocotyle umbellata), Florida betony (Stachys floridana), and field garlic (Allium vineale). Since trees and shrubs are broad-leaf plants, they could also be affected by metsulfuron-methyl. I have heard this product was originally created to kill oak trees in coniferous forests.

Trees, palms, and shrubs can be affected because their root systems grow underneath the lawn, where they can absorb the systemic herbicide, translocating it throughout the plant, resulting in the death of sections or the entire plant because it accumulates in meristematic tissues. Typically, stem die-back, brown “fried” or “scorched” foliage, and patches of necrosis (dead tissues) in the phloem (the plant’s vascular tissue) are observed. These symptoms are typically noticed two to four weeks following applications made during hot and dry weather (although not exclusively).

Suspected metsulfuron-methyl injury has been observed the most in oak species such as live oaks (Quercus virginiana), Nuttall oaks (Q. texana), Shumard oak (Q. shumardii), willow oak (Q. phellos), and Ligustrum (Ligustrum spp.) and several types of palms. Damage from soil-applied metsulfuron-methyl has also been documented experimentally on Foxtail palm (Wodyetia bifurcata) (Brochat & Busey, 2010).

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Two years ago, I received a call where I suspected MSM could be the cause and asked the homeowner to find out what products had been sprayed on the lawn. MSM can go by numerous brand names now because when the patent expired, it became the active ingredient in many broadleaf herbicides for turfgrass use. When she got back to me with the product name, I did an internet search and discovered that MSM is a Sulfonylurea herbicide, so my next internet search was on the Sulfonylurea herbicides trade names. As I scanned the brand names such as Alligare, Escalade, Clipper, Poast, and Celsius, I saw the product used in her yard, Dismiss Turf Herbicide. Dismiss Turf Herbicide has the active ingredient Sulfentrazone listed and says on the label that it provides “faster visible sedge control.”  Sadly, during hot, dry springs, oaks should be added to that label.

Metsulfuron-methyl was introduced by DuPont and was available for use in certain crops in the 1980s and ’90s. It was later labeled for use on turf.  It was protected by a patent and was expensive, so it was typically only applied to well-maintained commercial properties or golf courses. Once the patent expired, the price dropped, and product applications became more common throughout the year. Now, metsulfuron-methyl products are marketed and sold by numerous manufacturers, so more pest control companies and homeowners are using the product more often.

Oak species are very susceptible to injury when the product is applied improperly. Unpublished research by Drs. Ed Gilman and Jason Smith at the University of Florida has shown significant injury when herbicides containing metsulfuron-methyl are applied at the root zone of live oaks in accordance with labeled rates. In this study, metsulfuron-methyl was detected in the leaf tissues of affected trees. Injury appears as scattered stem dieback that turns brown, appearing “fried” or “scorched,” with patches of necrotic tissues in the phloem. Often, the damage appears suddenly, and the division between the green and brown portions within the crown is very distinct.

Palms that are affected may not show any symptoms for 6 months or more following metsulfuron-methyl application. Research has shown that metsulfuron-methyl causes leaflet puckering and necrosis of the central part of the rachis (stem) (Broschat & Busey, 2010). The tips of the frond will drop down and may become necrotic. Metsulfuron-methyl, like most systemic herbicides, will only affect one or two newly emerging palm fronds.

If you have a company providing pest control, which includes the use of a broadleaf herbicide, give them a copy of our bulletin, Effects of Metsulfuron-Methyl-Containing Herbicides Potentially Damaging Ornamentals when Applied to Turfgrass. It can be found at this link: https://edis.ifas.ufl.edu/pdf/FR/FR40000.pdf.  Also, keep an eye on the weather because if we experience hot and dry weather, that is when the damage can occur.

Sally Scalera is an urban horticulture agent and master gardener coordinator for the University of Florida’s Institute of Food and Agriculture Sciences. Email her at sasc@ufl.edu.

This article originally appeared on Florida Today: Yes, the weeds die — but herbicides can harm live oaks | Sally Scalera