Potted plants’ water needs vary with the weather and the seasons — plants need less water in cool weather, more in warm weather, and so on. Thus, even an automated system requires adjustment so that it waters less in spring and more in summer. Practice your powers of observation and make watering adjustments accordingly.
You can also take advantage of these other ways to tell when your plants need water or when containers are getting dry:
Pay attention to what your plants tell you: That’s right, plants can communicate with you. When plants start to dry out, the leaves droop and wilt. The plant may also lose its bright, glossy, green color and start to look a little drab. Make it your goal to water before a plant reaches that point (consider it a cry for help).
Dig in the ground: Stick your finger an inch or two into the soil in the top of a pot. If the soil feels dry, it may be time to water. For small containers, you can carefully slip the root ball out of the pot to see whether it’s moist.
Lift the pot: As soil dries out, it gets lighter. Compare how heavy a pot is right after watering thoroughly with how it feels a few days later. By simply tilting a pot on its edge and judging its weight, you eventually figure out how to tell when it’s dry or getting close to it.
Use a moisture sensor: Nurseries sell various devices for reading soil moisture. Most have a long, needlelike rod connected to a meter. You push the rod into the soil, and the meter tells you how wet the soil is. These sensors can be pretty handy, but don’t trust them too much right off the bat. Some can be thrown off by salts in the soil. To start, see how their readings compare to what you discover by feeling the soil and lifting the pot. Then make any necessary adjustments.
By taking these various factors into consideration, you’ll eventually get a sense of how often each container needs to be watered.