Suppose someone keeps a plant that must be watered daily in order for it to survive. May that person water the plant on Shabbat? Note that this is definitely allowed and even required for pet animals who must be fed daily; however, plants seem to me as though they might be different in this regard.
The reason not to water an animal is that it was banned by rabbis because it takes too much time and effort (tircha) (e.g., Aruch Hashulchan 324:1). They built exceptions into the ban in cases of need (such as, usually, when the animal depends on you for food) (e.g., 324 passim). The reason not to water a plant is because God said you can't make a plant grow on Shabas (e.g., 336:2). He didn't build in any exception for your own plants.
That said, consult your rabbi if this is a practical question for you, rather than relying on what you read on this site.