How to Water a Plant. When the soil is dry, water the plant thoroughly. In fact, water the plant until the water comes out of the bottom of the planter. This will guarantee that the bottom roots in the planter have gotten water as well.However, don’t let the pot sit in the water on the saucer.here is no easy answer to “how much water should I give my plant.” This amount could depend on the type of plant you have, where it is located, how old the plant is and the type and size of the pot.