My son studies Engineering and I just have a NUS card and we both signed up no problem, so not just for those studying plants x
The membership application form makes it clear that you qualify if you meet any of the three criteria.
Pasted off link "RHS student membership is open to students living in the UK who are: Horticultural students and apprentices, full or part time Students in full time education (16–19 years) Students in full time education (19+)" From the way it is laid out, I think if you fulfill any of those criteria you are eligible. So full time students would qualify too
Also available to: Students in full time education (16–19 years) Students in full time education (19+)
If this is open to only horticultural students then the title is misleading.