A slight droop is the sign that the plant needs to be watered. Alternately, even if they haven't drooped yet, if the leaves soften the next step is drooping, so you might as well water if you feel the leaves have softened. Those happen to look happy, like they are not in immediate need of water at all.Â
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You can avoid letting the plants get soft or droop by using the weight method. First, when you water, water thoroughly letting the water flow through the drain holes. THis way you will know the soil is thoroughly moist. When you're done and the water has finished coming out the holes, just lift the pot slightly off the ground and notice how heavy it is. When the leaves soften or droop, lift it again, and this time notice how much lighter the pot is from when you first watered it, then go ahead and water it again. It is this weight difference that you are looking for - when it is almost as light as it is when the leaves soften or droop, go ahead and water. In the future, you won't need to let the leaves soften or droop, just judge the weight as close. You may, however, want to go through a couple cycles of letting the leaves soften or droop to become more familiar with the weight difference.