Drizzle Application Technique for Invasive Plant Management
BY CELESTINE DUNCAN
The drizzle method of herbicide application was developed at the University of Hawaii for directed treatment of hard-to-reach invasive plants along forest trails. This method uses higher herbicide concentration than conventional methods, but is applied at a lower total volume per acre. Field studies at the University of California at Davis found drizzle application provides cost-effective control of emerged aquatic and woody plants in natural areas.
Equipment and Application
An orifice disk is used in place of an atomizing nozzle on the spray gun for drizzle application (Figure 1). A fine mesh strainer (100 to 200 mesh) keeps the orifice clear. This combination provides a slender jet-stream which breaks up into large droplets that ‘drizzle’ onto target plants (Figures 2 and 3). The spray stream is aimed directly at a target plant or, for larger plants, waved over the canopy so that droplets fall on the leaves and stems (Figure 4).
The backpack sprayer must maintain about 30 pounds per square inch (psi) pressure either by a hand pump or compressed gas (e.g. carbon dioxide). The spray gun with orifice disk is attached with a quick-connect fitting on the end of the spray hose. The gun used in drizzle application can also be operated from a truck- or ATV-mounted sprayer.
The drizzle method is best suited to liquid herbicide formulations that translocate in a plant such as triclopyr (i.e. Garlon® 4 Ultra or Vastlan™ specialty herbicides) alone or in combination with aminopyralid (Milestone® specialty herbicide). Imazapyr (Arsenal and others) or glyphosate (Accord® XRT II herbicide and others) can be used with the drizzle method, but are non-selective herbicides that can damage desirable grasses (Kyser pers. comm.).
Drizzle application is useful for managing invasive plants in terrestrial and riparian sites that are difficult to access. Woody plants and thorny brush species (e.g. gorse, Scotch broom, blackberry, etc.) and emerged aquatic plants (e.g. purple loosestrife and yellow flag iris) are best suited for this application method.
Advantages of the drizzle method include:
- Reduced time to treat an infestation and refill sprayer.
- The drizzle nozzle will reach a target plant 15 to 20 feet away, compared to 2 to 3 feet with a flat fan nozzle.
- Minimal drift because of large droplet size.
- Increased capability of making discreet target applications.
- Reduced cost compared to conventional backpack sprayer application or manual removal because application time is less.
- Increased versatility: The wand of the drizzle unit can be waved for broadcast application, aimed for a very precise spot application or, with an oil-based surfactant, used for basal bark treatments.
- Increased safety: Heavy loads are a risk factor in injuries. Risk is compounded by fatigue caused by carrying multiple heavy loads, especially in rough or steep terrain. Low-volume methods reduce the weight and/or number of loads that applicators must carry.
Disadvantages of drizzle method include:
- Higher concentration of herbicide active ingredient in tank mixes could increase exposure to an applicator in case of equipment malfunction or spill.
- Not suited to all invasive plants such as large infestations of spotted knapweed.
- Not all herbicides are suitable for drizzle application. Dry formulations that are difficult to keep in solution may plug the orifice.
- Over-application of herbicide from improper treatment technique may occur due to higher concentration of spray solution.
IMPORTANT!! This is NOT a spray-to-wet application method, but a low-volume application that results in scattered droplets of herbicide solution on a target plant (Figure 5). Calibration is important to deliver the correct volume and herbicide rate to control a plant ensuring accurate, cost-effective control. A constant pressure of 30 psi should be maintained during calibration and during the actual application.
To calibrate your sprayer, reference the following general instructions, or for detailed information on equipment calibration for drizzle method go to https://www.ctahr.hawaii.edu/oc/freepubs/pdf/WC-8.pdf.
Calibration instructions for a total application volume of 5 gallons per acre (gpa).
- Mark off a 1000 square foot (sq ft) area (e.g. 10 X 100 ft or 20 X 50 ft etc.)
- Spray the entire area uniformly in 1 minute at a sprayer pressure of 30 psi, and nozzle/gun configured as shown in Figure 1.
- Follow the tank mix guide for a 20% solution (Table 1) and application volume of 5 gallons of total solution per acre.
Field studies conducted at the University of California, Davis on management of yellow flag iris (Iris pseudacorus), Scotch broom (Cytisus scoparius), and tree tobacco (Nicotiana glauca) found drizzle application can reduce the amount of total herbicide solution applied and similar control of the target plant when compared to higher volume broadcast foliar applications (Table 2).
* Drizzle and basal bark applications contained an oil-based adjuvant (e.g. Competitor). Motooka and others (1999) reported that the use of high rates of oil-based surfactant resulted in improved absorption, an important consideration in low-volume treatments.
The drizzle technique is a practical application method that is adaptable to both aquatic and terrestrial ecosystems, including relatively large infestations. Drizzle application was the most cost-effective control method, primarily because of the significant reduction in labor costs (DiTomaso and others 2016).
DiTomaso JM, Kyser G. 2016. Shoreline Drizzle Applications for Control of Incipient Patches of Yellowflag Iris (Iris pseudacorus). Invasive Plant Sci and Manag 9(3):205-213. Online http://www.bioone.org/doi/abs/10.1614/IPSM-D-16-00023.1.
Kyser G. 2017. Personal communication. University of California, Davis.
Motooka P, Powley J, DuPonte M, Ching L, Nagai G, Kawakami G. 1999. Drizzle herbicide application for weed management in forests. Pages 136–139 in Proceedings, Western Society of Weed Science. Colorado Springs, CO. Online http://www.cabi.org/isc/abstract/19992302113.
Motooka P, Ching L, Nagai G. 2002. Herbicidal weed control methods for pastures and natural areas in Hawaii. Depart. Of Natural Res. and Environ. Management and Human Nutrition, Food and Animal Sciences, Hawaii Department of Agriculture Publ. WC-8. 36 p. Online https://www.ctahr.hawaii.edu/oc/freepubs/pdf/WC-8.pdf.
Oneto SR, Kyser GB, DiTomaso JM. 2010. Efficacy of mechanical and herbicide control methods for Scotch broom (Cytisus scoparius) and cost analysis of chemical control options. Invasive Plant Sci Manag 3:421–428. Online http://www.bioone.org/doi/abs/10.1614/IPSM-D-09-00030.1.
Oneto SR, DiTomaso JM, Kyser GB, Garcia, S and Hale S. Control of tree tobacco (Nicotiana glauca). Online http://www.cal-ipc.org/symposia/archive/pdf/17788.pdf.
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State restrictions on the sale and use of Accord XRT II and Garlon 4 Ultra apply. Consult the label before purchase or use for full details. Vastlan is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state.
Milestone is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details. State restrictions on the sale and use of Milestone apply. Consult the label before purchase or use for full details. Always read and follow label directions.
Active ingredients for herbicide products mentioned in this article. Product (active ingredient): Milestone (aminopyralid); Vastlan and Garlon 4 Ultra (triclopyr); Accord XRT II (glyphosate); Arsenal (imazapyr).