I have found a wonderful way to take a stem of Milkweed and keep it fresh while the caterpillars are eating it. Take the green foam that you keep cut flowers fresh in and poke hole in it. Place in a plastic storage container and punch hole in the top of the lid. Submerge the foam block in water in the container. With lid on, put plant cutting through hole and into the foam beneath. It will stay moist and live but not droop or die. Will last till all eaten by the cats. I keep that container inside something else so they cannot crawl away. It has been a lifesaver for me.