This is an open thought question for future xenobotanists. I read that NASA is hiring a new round of astronauts for their Mars project, and it made me think about botany that is not on Earth (xenobotany)
Xenobotany only applies if there is any botanical knowledge to be investigated and/or discovered. I don't think this is really applicable for the Moon or Mars, we would have known if there were anything as big as plants there.
However, if we are talking about a planetary-body where there were plant life it's a different story. I think the first round of projects for a xenobotanist in these cases would be to document as much as possible: photographs, tissue-samples, multi-spectral readings, nutrient exchanges etc. It will also be really important to include advanced security measures to avoid cross-contamination between our worlds. After all of these measurements and samples have been analyzed, more in-depth projects can be conceived, observations into: evolutionary relationship, biodiversity, plant abundances, plant-plant interactions, density dependence etc.
I read about that and also some thoughts about terraformation (try to transform mars to an habitable planet with earth parameters), if they intend to build bases there it would be easier to analyse mars soil and compare it with eart's one in order to detect the differences and know which elements we have to add to make it usable for agriculture under greenhouses. Then it may be possible to send some plants from earth and do trials on mars's ground. Its a very interesting perspective. However i don't think it may be possible at least not with the technology we have on our hands.