Yellowflag iris (Iris pseudacorus L.), native to Europe, is an emergent invasive of pond margins, ditches, and other wetland sites in much of the United States. It forms dense stands which displace native sedges and rushes, reducing waterfowl habitat and water flow. Yellowflag iris reproduces by seeds and through rhizome fragmentation. The rhizomes make it hard to remove mechanically. Accessing an infestation for making herbicide applications can be problematic. Because of its height and density, and because it grows in shallow water and mud, yellowflag iris is difficult to treat with hand-held booms or mounted equipment.

 

 

 

 

 

 

 

 

 

The drizzle application technique was developed at University of Hawaii for treating hard-to-reach invasive plants along forest trails. Low volumes (2 to 5 gpa) of concentrated herbicide solution are applied using a spray gun with a fine orifice disk. The spray gun puts out a thin stream of solution with a range of 20 ft. The operator waggles the gun to create a drizzle of spray droplets on the target. This technique allows directed treatment from a distance. The large droplets reduce drift, and the low volumes allow fewer tank refills, thus greater portability and savings in labor costs.

 

 

 

 

 

 

 

 

 

 

 

 

In 2008-2011 we did two trials at UC Davis comparing yellowflag iris control with shoreline drizzle applications vs. hand-held boom broadcast applications. Applications were made in September 2008 and September 2010, a time of year when iris is starting to senesce and send carbohydrates down to the roots.

The boom was a 5-ft boom with three 8004 nozzles, mounted on an extended 10-ft wand. Even with the long wand, we weren't able to reach the outer edge of the infestation without getting stuck in the mud.

 

 

 

Although we were concerned about getting adequate coverage with low-volume drizzle treatments, we were able to get nearly complete control of yellowflag iris. We used the aquatic-registered herbicides Rodeo (glyphosate) and Habitat (imazapyr). All treatments included 30% aquatic-labeled Competitor surfactant.

Chemical	Rate, % product	Volume (gpa)	Iris % cover (SD)
glyphosate	20	5	3.7 (5.5)
glyphosate	20	10	3.7 (2.3)
imazapyr	10	5	1.3 (0.6)
imazapyr	10	10	1.0 (1.0)
imazapyr	20	2.5	0.7 (0.6)
imazapyr	20	5	1.7 (2.9)
untreated	---	---	100

 

 

 

 

 

 

 

 

 

Drizzle application is a practical method for control of yellowflag iris. Imazapyr is most effective in this usage. These results suggest that drizzle applications are more adaptable than we had previously thought, and they may be an option in other large patch situations where broadcast treatments would normally be used.