Poison ivy, that toxic weed found in all parts of Missouri, can be particularly vulnerable to herbicide control in autumn, a University of Missouri agronomist said.
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“The use of herbicides to control poison ivy may usually be best achieved in the late summer to early fall, before the foliage loses its green color,” said Fred Fishel, director of the MU Integrated Pest Management program. “It is at this time of year that many perennial plants such as poison ivy translocate sugars to their root systems for winter storage. When herbicides are applied at this time, they also are carried to the roots along with this sugar flow.”
Poison ivy and its rarer cousin, poison oak, thrive in fence rows, roadsides, railroads, waste ground, low and upland soils or wet woods, alluvial wet soils along streams, in valleys, rocky wooded slopes, ledges of bluffs, and thickets, Fishel said. “It’s little wonder why this weed is so well adapted to the Show Me State.”
The most recognizable feature of poison ivy is the leaves, which are divided into three leaflets. “There are two variations in Missouri,” Fishel said.
“The most common variation, which has been recorded in all counties, contains leaflets which are coarsely and noticeably toothed. The rarer type, which is equally toxic, has leaflets that are mostly entire, or without teeth around the leaflet margins.”
He said many people often mistake poison ivy for poison oak. “The distinguishing characteristic of poison oak is its middle or terminal leaflet, which has a rounded or blunt tip,” he said. “In Missouri, this species is known primarily to occur in the extreme southern portion of the state.”
Whether it’s poison oak or ivy, the noxious weed adversely affects about 85 percent of the population by producing urushiol, the oil that causes skin rash. This oil can retain its toxic properties several years after plant death, Fishel said.
Several herbicides are labeled for poison ivy control, he said, including products that contain 2,4-D, dicamba or mecoprop. Roundup, an herbicide popular with homeowners, also can be used. Applied as a broadcast spray, it takes four to five quarts per acre for effective control. With a hand-held sprayer, a 2-percent solution applied to the point of run-off will achieve control.
“To maintain control, repeat applications may be necessary,” Fishel said.
“If the brush has been mowed or tilled, do not treat until regrowth has occurred. After treatment, allow at least seven days before tillage, mowing or removal.”