The best way imho, to know when soil grown potted plants need more watering is by checking how heavy the pots feel. This method is very simple to learn once you get the feel for how light a ready for watering pot of soil feels.
Get an equal size pot and fill it with your soil. The medium should be about as moist as a new bag of potting soil. Use this planter as a learning tool to get a rough idea of how much the pots should weigh before watering again. The pots with your plants will feel only just slightly heavier when the soil is ready for more water. Pick up one of your planted pots, if its noticably heavy, do not water it until it feels "light". Next time you water a planter, pick it up and feel how much heavier it feels compared to the sample pot of soil.
It only takes a few times picking up the pots until this skill is like a "second nature" to you. You wont even need the sample pot after you get accustomed to the lifting method.
Sure your plants are always putting on more weight as they grow larger, but once you're proficient at lifting the pots, you'll also know how to compensate for the weight of the plants with ease.
I dont know how to better explain this method of knowing when to water, but believe me, anyone who learns this method, will always know when its time to water.
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