While doing some research on various skills used by our ancestors, I came across two plants that were used as soap in the early America. Since hygiene can be a major problem during a disaster, knowing which plants you can substitute for soap can be of great benefit both from a morale and health standpoint.
The first one Soapwort or Bouncing bet is an herbaceous perennial, 1-3 feet tall. The stems are erect and jointed with leaves which are 3-4 inches long and 1/2 to 1-1/2 inches wide at the middle. The plant has pretty white or pink flowers with five petals. Soapwort was brought over by the colonists to be used as a soap substitute.
Soapwort contains large amounts of saponins, which froth when stirred with water. Any part of the plant can be chopped up and used for soap. When bruised or boiled in water, the leaves produce lather with detergent properties that even removes grease. The plants are best collected in late summer to fall and dried for future use.
Soapwort can be toxic to humans and animals if taken internally. It can irritate the eyes if you get it in them.
The second is Yucca, also known as agave, Spanish bayonet, and Joshua tree. The plants contain saponins, with the strongest concentrations in the roots. If you stir some of the clean roots around in water, the water will get quite frothy. Use the soapy water as a shampoo or soap. To avoid killing the plant by using the roots you can get sufficient suds by pounding on the leaves and stirring them around in water.
Yucca can be found in California in the west, the southwestern United States, and into the drier central states as far north as Alberta in Canada. Yucca is also native to the lowlands and dry beach scrub of the Gulf and South Atlantic states from coastal Texas to easternmost Virginia.