Stephen Henning wrote:

If you are spraying a
field with established rhizomes, then you will need to keep at it since
the rhizomes can extend many feet under ground.

I should have said many feet laterally under ground. They never go very
deep. When I plant Christmas trees, I pull out long poison ivy
rhizomes. They are usually never more than 3 or 4 inches below the
surface. I am allergic to poison ivy so I wear gloves, long sleeves and
wrist guards. When I come in I put my clothes in the wash and
immediately take a good shower with a strong soap. I seldom get much of
any rash.
--
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18,000 gallon (17'x 47'x 2-4') lily pond garden in Zone 6
Cheers, Steve Henning in Reading, PA USA