Question

The gardener waters the plant change to past tense

Answer 1

The gardener watered the plant

Answer 2

Changing to past tense requires that all phrases in the entire sentence be changed to past activities .Your sentence is in present continuous tense .

Your answer is THE GARDENER  WATERED THE PLANTS .This one means that the work is going to be done in the past .It means that in the current situation the plant are already watered by the Object which is the gardener .