For further sources, R Yosef Tzvi Rimon in his (highly recommended) reference book on shmita adresses this question indirectly and writes (pp. 34-35)
Since the Sages permitted labors that are forbidden by rabbinic decree
in cases of financial loss, it is permitted to water one's garden when
necessary (but less frequently than usual) [...] Generally speaking, a
private garden can be maintained even without performing any other
labor, and therefore they are forbidden.
He holds that the prohibition of planting applies even to plants that do not bear fruit (p. 42 and 89) - see this other question for the background
He later writes (p. 44)
One may perform labors that are generally forbidden by rabbinic
decrees only when failure to do so will cause the plants to die