Herbicides play a vital role in weed management efforts in California.
For herbicides to work effectively, they need to be selected based on targeted weed control, spray equipment must be functioning properly, appropriate nozzles need to be used, and the application technique must be done in a manner that protects desired vegetation. When herbicides are not properly applied, weed control will be reduced and desirable plants can be injured.
Plants are particularly sensitive to postemergent herbicide injury from off-target movement (spray drift), so application technique is one area you don’t want to skimp on. Postemergent herbicides commonly associated with spray drift injury include the plant growth regulators (2,4-D, dicamba, etc.), amino acid inhibitors (glyphosate, imazamox, etc.), and cell membrane disruptors (paraquat, carfentrazone, etc.).
Since potential exposure of desired vegetation to herbicide spray drift is high, it’s important to keep sprays on-target, when treating weeds. The degree of herbiciderelated injury varies, depending on on herbicide type and dose, plant species and age, and other factors.
Symptoms from herbicide drift vary from mild yellowing of the foliage to plant death. Spray drift damage occurs mainly when spray droplets are carried from the application site downwind and deposited on sensitive plant surfaces, like buds, leaves, and fruit. Spray drift can also occur from vapor drift (evaporation of herbicide from a plant surface) and particle drift (spray carrier droplets evaporate, leaving concentrated herbicide droplets).
Spray drift is influenced mainly by droplet size. Spray droplets with a mean diameter of <200 microns are the most susceptible to drift. Since small droplets are lighter and remain airborne longer than larger ones, they travel greater distances from the treatment site. A high spray height or wide nozzle tip angle will also increase drift distance.
To maximize weed control and reduce the likelihood of spray drift injury of desired vegetation, consider the following application factors before spraying:
1. Label instructions – Read and follow all label directions carefully before spraying.
2. Environmental conditions – Spray when winds are <7 mph. Don’t spray on foggy days or when the air is still, because fine droplets can remain airborne a long time. Spray when the temperature is <80 F, because some products are volatile at higher temperatures. Consider spraying towards the end of day when it begins to cool. Use “amine” formulations of 2,4-D, sine “ester” formulations are more volatile under warm conditions.
3. Buffer zones – Leave an appropriate buffer zone to protect sensitive plants near the treated site and don’t spray if the wind is blowing towards a nearby sensitive crop, garden, waterway, or other sensitive area.
4. The sprayer – Use spray shields where possible, because they help reduce spray drift by 35 percent to 75 percent.
When using, low-volume (<5 gpa) sprayers, treat small weeds so the equipment can be lowered as much as possible. Likewise, boom or boom-less sprayers should also be set as low as possible to reduce drift. Practice good sprayer maintenance and calibration for efficient use of herbicides, time, and labor.
5. Application timing – Apply postemergent herbicides when weeds that are <4 inches so the boom can be lowered. Raising spray booms or angle spray nozzles upright to compensate for large weeds, risk of spray drift is increased.
6. Nozzle selection, spray pressure, and volume – Use spray nozzles with an orifice size of >02 for larger droplets less prone to drift. Use “Drift Guard”, “Air Induction”, “AirMix”, “TurboDrop”, or other nozzles designed to reduce drift. They work well with most pre- and systemic postemergent products. Keep the spray pressure as low as possible according to manufacturer recommendations. Don’t crank up the spray pressure for a larger volume of water (potentially more drift), rather select a larger nozzle size or a drift-preventing nozzle. “Extended Range (XR)” type nozzles produce larger droplets under a low (15 psi) operating pressure and fine-sized droplets at high spray pressures.
Refer to table 2 for various spray nozzle types and manufacturer’s recommended uses. Nozzle tip wear must be frequently checked to insure each nozzle is putting out the appropriate flow rate. Change nozzles when the flow rate differs by 10 percent from that of new nozzles. Tip wear depends primarily on the material used in the manufacturing of the spray tip: brass (wears quickly) > stainless steel > polyacetyl > hardened stainless steel > ceramic > carbide (little to no wear). Dry herbicides wear tips faster than liquid materials.
7. Boom and nozzle height – Operate nozzles at their lowest recommended height. For 80 degree tips, this is 18 inces, and for 110 degree tips, this is 12 inches. Orient nozzles forward to allow for further height reductions.
8. Travel speed – Use a travel speed of 3-5 mph, since speeds above 6 mph can create a wind-sheer effect on spray droplets, increasing drift.
9. Spray additives – Spray drift retardants can reduce drift up to 95 percent. Spray additives, like ammonium sulfate, can improve efficacy of postemergent materials, but may alter spray viscosity and flow rate. Check the flow rate and droplet pattern on water sensitive paper before adding herbicides to see if droplet size is adequate.