Greetings, Elaine, and thank you for your excellent question to AskExtension/Ask a Master Gardener. 

The short answer is: The mixture you propose will have some effect on the leaves of poison ivy, causing them to brown, curl, and possibly die, but it will not be likely to kill the plant.

The recipe you gave, and many similar to it, have been spreading like weeds (pun intended!) across the internet and are touted as “safe” or “natural” alternatives to chemical herbicides. As you probably already know, poison ivy is a resilient plant! When treated with this mixture, the roots of the plant will almost certainly survive and will send out new shoots. Additionally, the salt and vinegar might affect nearby plants and could make the soil inhospitable to other or new plants. The salt/vinegar/Dawn recipe, when made with “grocery store” vinegar rather than the stronger horticultural vinegar, has been shown to be effective on small annual weeds, like dandelions, but ineffective on perennial or biennial weeds, such as ivy. Its overall effect on the soil of lawns, landscapes and gardens is a topic of much discussion. You might be interested to read more research based information here, here, and here. (Complete URLs for all links are also at the end of this response.)

As to recommendations: If you seek a chemical recommendation, glyphosate, triclopyr and 2,4-D herbicides are effective. If you choose to use chemical control, I would recommend using a product labeled specifically for use on ivy. We are not allowed to recommend specific brands, but the helpful folks at your local garden center should be able to help you find a product that fits the bill. Please be sure to read and follow all label safety and application instructions.

If you don’t want to use chemicals, manual removal is an option. It can be difficult to remove poison ivy by hand, especially at this time of year, because of the plants' oils that cause dermatitis (sometimes very severely) for most of the US population. Additionally, if removing by hand, you need to be diligent about finding and removing any new shoots that emerge. It is not a one and done approach. There is a good section about manual control in this article and this article.

Whether or not to use chemical control in gardens is a personal choice; at Ask A Master Gardener we do not try to persuade or dissuade other gardeners in either direction, but I will share my personal point of view for your consideration. Like many gardeners, I don’t like using chemical herbicides in my garden. I prefer to weed by hand, and I use some corn-gluten based pre-emergents. Poison ivy is one of the few "weeds" for which I will use chemical control. Like the author in this article, I am constantly scouting for those tell-tale “leaves of three”, and I use a glyphosate spray labeled specifically for ivy control as soon as I spot them. I try to thoroughly drench the ivy without getting the product on nearby plants, as much as that is possible, covering nearby plants if needed. The herbicide is absorbed into the plant and travels to the root to kill the plant. So far this season, I haven’t found any poison ivy in my garden (knock wood!). I hope this point of view is helpful to you.

In preparing a response to your question I verified information with research-based sites to be sure that my answer was accurate and current. The links to the best of those articles are cited in the response and/or listed below.

If you need more information or have additional questions about poison ivy control, please feel free to write back. If you respond to this email your response will come directly to me. I would be happy to hear from you! And if you have a few minutes, we would be very grateful if you would complete the very short survey at the end of this response. I hope our service has been of help to you.

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Resources for you:

Ohio Perennial and Biennial Weed Guide - The Ohio State University Extension  https://weedguide.cfaes.osu.edu/singlerecord.asp?id=113

Poison Ivy Scouting – The Ohio State University Extension https://u.osu.edu/bhanr/2017/06/13/poison-ivy-scouting/

Poison Ivy – Purdue Extension https://www.extension.purdue.edu/extmedia/ho/ho-218-w.pdf

How to Control Poison Ivy – Iowa State Extension https://yardandgarden.extension.iastate.edu/how-to/how-control-poison-ivy

Home Garden Remedies: Helpful or Harmful? – Florida State Extension https://blogs.ifas.ufl.edu/highlandsco/2020/07/24/home-garden-remedies-helpful-or-harmful/

Vinegar: Is it a “Safer” Herbicide? The Ohio State University Extension https://fairfield.osu.edu/news/vinegar-it-%E2%80%9Csafer%E2%80%9D-herbicide

Garden Myths - Kansas State  https://www.shawnee.k-state.edu/lawn-garden/Gardening%20Myths.pdf

Thank you for replying, Elaine! It is so nice to hear that the response was helpful!

If you need any further help, please write back any time. I will be standing by.

Happy gardening and happy Independence Day!