Can I change the genetics of a group of plants naturally?
Of course, you can place a population of anything (incl. a population of plants) into a specific environment and you could expect this population to slowly adapt to it.

Most plants don't have a particularly fast generation time and therefore, the evolutionary experiment might take longer than your lifetime. The experiment would be much faster if you choose some unicellular algae such as Chlamydomonas reinhardtii for example.