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So I've been slowing pushing back on some poison ivy on my property using Round Up Poison Ivy Plus Tough Brush Killer. I normally try to go in two times a year, once in the spring and once in the summer and spray whatever I see growing. All the big growth seems dead now and I'm basically working on small shoots that I miss each time. This year I've been a bit behind the times and just am getting around to it. Will it be effective to spray poison ivy in the fall? Or should I just wait until next spring. The leaves on these plants are starting to orange. (I'm in New Hampshire and its October if that matters) This is what the website says but I'm not sure if they are still growing around now:

WHEN TO APPLY Apply when weeds are actively growing. For best results, apply during warm, sunny weather (above 60�). Spray when air is calm to prevent product from drifting onto desirable plants. Rain or watering within 30 minutes of application will NOT wash away effectiveness.

Also as a side question in a few years I plan on cutting down the other brush in the area and converting it to grass. How long do the vines from poison ivy need to dead before I don't need to worry about getting the rash from them?

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David, the best way I've found to kill well established areas of very tough weeds (black berry, chinese knotweed...etc.) was to smother them. For my blackberries, I had the tree company working for the electric company pruning trees messing up the electrical lines dump their chips on my acre of blackberry. Piles 4-6' tall. All winter these piles produced so much heat decomposing that there was fog constantly. By spring, these piles had decomposed to 4-6" depth and not a single black berry came back. I've used the brush glyphosate stuff on other projects and it sort of worked, looked awful for quite awhile and still stuff came back. Year after year.

On the Chinese Knotweed I used clear plastic and pinned it down well. Started mid summer left it alone until mid summer of the next year and it never came back. Chinese Knotweed is incredibly tough. Roundup did not work at all. Horse Tail or Equisetum took 2 years doing the same. Some of that did start coming back.

We quit trying to use any chemicals as they just did not work as well as physically starving the plant to death...or frying it under clear plastic. Glyphosate works best on VIGOROUSLY growing plants. Later in the year the plant is just skating by waiting to shed leaves and go to bed having plenty of food stored in the roots for winter. Just constantly mowing, hacking, weedwacking the green vegetative growth back works just as well. This takes twice weekly wacking for the whole season.

  • this question is specifically about poison ivy. Does your answer apply? – kevinsky Oct 14 '15 at 9:56
  • Yes...and no irritant will be left. – stormy Oct 15 '15 at 20:04
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As @kevinsky said, follow the directions.

As far as how long the poison ivy will remain an irritant, this excerpt from Wikipedia about it should give you an idea:

Urushiol oil can remain active for several years, so handling dead leaves or vines can cause a reaction. In addition, oil transferred from the plant to other objects (such as pet fur) can cause the rash if it comes into contact with the skin. Clothing, tools, and other objects that have been exposed to the oil should be washed to prevent further transmission.

Urushiol is the "stuff" which will cause the irritation. The leaves are bad enough, but the root and vines are 100x more toxic. Urushiol is water soluble. If it gets on the ground, it will soon be gone so no worries there. It's the vines/leaves/roots themselves which will remain the irritant. Once gone it should pose no more of a threat to you. That doesn't mean it won't come back. I don't know exactly how it propagates, but it does. I've had recurrence of it in my yard several times in the past 12 years after we built on an infested lot.

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Well I think the instructions answer your question:

  • the plant is not actively growing
  • you would be fortunate to get temperatures above 60 deg F at this time of year

You should spray in spring when the active agent will be more effective.

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