One of the best survival greens is wild dock. Wild dock species are native all around the world. The leaves of most dock plants are long and narrow, and the plants are most easily identified by their distinctive seed stalks. The seed stalks start out green but quickly turn to maroon and then a rusty brown.
Dock leaves are a wild green that can be used in any recipe where you'd traditionally use spinach or collards. As one would expect the youngest leaves are the least bitter. You can boil the leaves in a couple of changes of water to help with the bitterness if you must use the older leaves.
Another lesser known use is eating the seeds. Collect a bunch of the plants seeds after they have dried on the plant, then use a rubbing motion in your hands to rub off a good bit of the husk. Separate by winnowing then grind the seeds. For best taste let them soak in water for a few hours to remove the bitterness then dry the seeds and grinding the dock seeds into flour. The flour is best if mixed with other flour. It is a great way to extend any other flour you might have available.