Leaves of three, leave them be
August 27, 2004
Poison ivy grows in all of Missouri’s counties and it is well adapted to any soil condition. As a result, poison ivy can be found along streams, rock ledges, wooded slopes, fencerows, gardens, flowerbeds and wastelands.

“Unfortunately, it is a plant that adversely affects at least 85 percent of humans, many of whom don’t know how to identify poison ivy,” said John Hobbs, agriculture and rural development specialist, University of Missouri Extension.

The old saying “leaves of three, let them be, leaves of five, stay alive” is a good rule of thumb to follow according to Hobbs.

“Poison ivy is recognized by its leaves (each leaf has three leaflets). There are two species of poison ivy in Missouri. The most common kind has leaf margins that appear course toothed. The second species is without teeth around the leaf margin,” said Hobbs.

There is a cousin of poison ivy –named poison oak -- that is not as common. It also has three leaflets but it is found only in the extreme southern portion of the state.

A common vine found in the Ozarks – known as Virginia Creeper – is also sometimes confused with poison ivy. It has five leaflets but does not cause a rash.

Poison Ivy and Oak cause a rash on humans because of an oil called urushiol that the leaves release. How potent is the urushiol oil? The amount needed to cover the head of a pin could cause 500 people to itch. Worst of all, urushiol oil can stay active for one to five years on any surface including dead plants.

“The use of herbicides to control poison ivy is most successful in late summer to early fall. Be sure to treat before foliage loses its green color,” said Hobbs.

This is the time of the year perennial plants, including poison ivy, translocate sugars to their root systems for winter storage. When herbicides are applied during this time, they are also carried to the roots.

According to Hobbs, there are several herbicides used to control poison ivy. Roundup seems to be very popular with land and homeowners to control poison ivy. A two percent solution for hand spraying poison ivy will achieve control in about seven days.

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