I agree. It suggest the roots of the plant more than the word "absorb" which could mean that the plants absorb moisture through their leaves, stalks and stems. If it's specifically roots, I would go for "soak up" or "draw up".
At the beginning of your sentence, it should either be "Plant roots..." or "Plants' roots..."
Salt grass was planted along the coastline, but since it has a long deep root, it soaks up all water and brings death to other plants.
When I have to paraphrase, I have to use other verbs. Obviously, you see the verb ''absorb'' more than ''soak up'' in any context, but as SD and emsr2d2 said, it would be acceptable for the test raters.