From IPM in the South by Rosemary Hallberg
By: Chris Marble, University of Florida
Proper use of herbicides is one of the cornerstones of a good weed control program – but bad things can happen when they are not used correctly. Compared to insecticides or fungicides, herbicides have a much greater potential to damage valuable ornamentals or turf, in some cases even when small mistakes are made. Luckily, if you follow a few key principles, damage can be avoided.
- Applying the wrong amount of herbicide is the number 1 reason for causing damage to turf or ornamentals and the number 1 cause of weed control failures. Herbicides are designed to work within a narrow range – apply too much and damage can occur. Apply too little and weeds will not be controlled. The best way to prevent causing damage (also called phytotoxicity) is to properly calibrate equipment and use herbicides only in sites where they are labeled.
- Identify the weed species. Properly identifying the pest you are trying to control is the first step in an Integrated Pest Management Program (IPM). Herbicide labels will contain tables showing the rates needed for a particular weed species or for a particular size of the weed. The lowest rate should be chosen for the weed species you are trying to control to improve the safety profile of that herbicide.
- Know how far tree (and shrub) roots can extend. Many herbicides that are commonly used in turf areas contain warnings such as “Do not apply in areas that contain roots of desirable trees or ornamentals.” How far do tree roots extend? On average they extend close to three times the spread of the drip line (tree canopy) – which in some cases means well into turf areas. In these situations, choose herbicides that are not as prone to leaching or being absorbed by tree roots. Many turf herbicides are also prone to causing damage to nearby ornamentals through drift or volatility (i.e. 2,4-D and others) and should only be used in recommended areas.
- Know how long it takes the herbicide to work. Damage can occur due to multiple applications being made in a short period of time. This is sometimes done because the herbicide seems to not be working, and the applicator thinks a repeated application is needed. For some herbicides, symptoms may not be noticeable for a week or longer depending upon environmental conditions. Know how long it will take the herbicide to work and then re-apply as needed. Note that some weed species will almost always required repeat applications for 100% control – each application should be made at the proper rate. Never apply higher than the labeled rate to try and achieve better or faster results. Following label recommended split applications (applying a lower rate at multiple timings) has also been shown to improve weed control and reduce phytotoxicity with many different herbicides.
- Use adjuvants and surfactants carefully. Adjuvants and surfactants are often needed and/or recommended for use with certain herbicides but they may also increase the risk of temporary phytotoxicity to the turf. Only use the specific types of adjuvants or surfactants recommended on the product label and do not use more than the intended amount.
- Use contact herbicides if spraying close to ornamentals. Glyphosate is a great product but if even a little gets on an ornamental it may never recover. Be careful using systemic herbicides around ornamentals (including thin barked trees and plants with green stems) or try to use contact action herbicides for small and annual weed species. Even if the ornamental is contacted by these herbicides, it will not move throughout the plant and is likely to recover.
- Use good judgment. It seems obvious but it still happens – avoid herbicide applications in strong winds and use lower pressure to avoid potential herbicide drift damage. Always clean and flush hoses and tanks, especially when applying fungicides, insecticides, and herbicides from the same equipment. Only a small, residual amount of a herbicide such as atrazine left in the bottom of a spray tank can kill many different types of ornamentals. Also note label recommendations pertaining to weather and other environmental conditions. There are many herbicides that can cause unwanted damage to turf at high temperature (above around 90°F) but are safe during cooler times of the year.
- Always read the label. We all know we are supposed to read and follow pesticide labels but the time invested reading herbicide labels is the most valuable time you can spend in developing and implementing your weed control program. Reading and following all parts of the label is the best way to ensure applicators are safe, the product performs as it was intended, and that we are protecting the environment