There is water on Mars, possibly more than we realized, and in a liquid state.
But our plants would have a really hard time there for a few reasons:
- High UV radiation, which kills cells.
- Very salty soil. Salt is not good for plants.
- Lower light. The amount of sunlight that reaches mars is quite a bit less than what earth receives. Throw in dust storms that last months and plants would in trouble.
- Very cold. Average temperature on Mars is -67 F.
- Thin atmosphere. While Mars atmosphere is mostly carbon dioxide, which would be ok for plants, it is very very thin. The atmospheric pressure on the Martian surface averages 600 pascals (6.0 mbar), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (1,013 mbar). The atmospheric pressure at the top of Everest is about a third of sea level pressure (337 mbar).
- Bad soil. What we call dirt is mostly alive, with a large mix of decomposed biological material, bacteria, etc. Mars soil is dead, and as such would have no nutrients or any of the other things that plants would need to grow there.
If you wanted to try this, you'd probably have to start with some kind of modified lichen, engineered to handle sub zero temperatures, low moisture, high salt content, low pressure, and that would be able to eat rocks.