Useful wild plants: Bracken Fern
As self-reliant people, we want to use species in a sustainable way by either gathering them or growing them. Making soap is an intensive activity that requires either growing a lot of oil bearing plants and harvesting them and pressing out their oil, or hunting a lot of animals for fat, or raising a lot of animals and harvesting it; since that takes so much effort, in the beginning we want ways to do that easier. Way easier, for in a collapse, those oils may be good for calories to consume for bare minimum survival.
The good news is that many plants contain sapponins, natural soap substances. This doesn't mean that you get a bar of strong soap from them, but it does mean that you can swish around their roots in water, and it will create mild soaps that you can use for your body and hair.
The three main ones that I know of are: yucca which grows in arid places, soapwort which can be found in gardens in America and grows wild in Europe, and bracken fern which grows almost everywhere east of the Mississippi an a bit over that border.
Bracket fern is the same fern from which we harvest fiddleheads. Here are some links to help you identify it:
[link to en.wikipedia.org
[link to www.pollenlibrary.com
[link to www.pfaf.org
Dig up and clean the roots, press the roots to get out the sapponins and mix that with water. It's very simple, and will do a reasonable job of cleaning without using lye from wood ashes. This is a far gentler means of cleaning.