Jason, thanks - that makes sense.
Geekgranny, be careful of the poison ivy oil - it's still active even after the plant itself is dead. In Virginia, we killed it and then bagged it for disposal - burning just makes a poisonous aerosol. We applied systemic killers (gets into the plant's system and kills the roots) on dry, calm days, and didn't water the poison off the leaves. We preferred to apply the poison sparingly to each individual plant rather than broadcast it; apparently, the stuff that holds the poison spray on the leaf without it running off, the surfactant, is more harmful to amphibians than the poison itself, not letting them form properly and otherwise clogging their gills. We had good luck with the general poison ivy killer at Home Depot or Lowes, spraying a couple of leaves on each plant each day over a few days. The leaves applied to started to shrivel, then the plant itself got unhealthy. As I said, many people make the mistake of "watering in" the poison after application - just the wrong thing to do. Good luck.