Updated: Dec 20, 2020
I don't take credit for this concept; I learned it in college and also from a greenhouse that I worked at. Now I've been using it for years and it has never let me down. If you follow this simple trick, you will be able to stop over & under watering and level up your growing game!
When deciding when to water, go by the weight of the plant. Don't be lazy, just pick it up and see how heavy it is. Remember that each plant will have a different weight when it is dry depending on the weight of the pot and density of the soil. The key to this method is that every time you check your plants you pick them up to see how heavy they are, and over time you learn what they feel like when they are dry and what they feel like when they are wet. As you learn you can become more and more accurate with when you plant likes to be watered. Watch the plant's reaction to your methods and adjust as necessary.
The reason this works is because it tells you what is going on in the middle of the root ball where moisture is held the longest. Due to gravity, the top of the soil dries out first and should never be used as an indicator for how dry the plant is. It may seem dry on the top, but underneath still may be very wet. You may think you are over-watering when in fact you are under-watering!
Sometimes plants can have similar reactions to being over- and under-watered. An under-watered plant can become droopy and sad looking, and so can an over-watered plant! A lot of times an over-watered plant can also get burned tips due to a lack of oxygen in the soil. Another variable to throw into the situation is that some plants like their roots to stay more wet, and some like their roots to dry out significantly. You can usually do some research on your plant to learn what it likes best, either on the internet or with your own experimentation! Mistakes will be made and you have to be okay with that.
One of the most common watering mistakes is to rely on the amount of time since your last watering. Plants don't always uptake water at the same rate, in fact it is always changing as they grow and progress in their lives. Bigger plants drink more water because they have a lot of surface area on their leaves, and the more surface area, the faster they drink water. When the humidity is low they drink more water, and when the humidity is high they drink less. Over time you will begin to learn when your plant needs water and when it doesn't, so you can stop over or under-watering.
Check out my recent podcast "Advice from an Over-Watered Plant" for a little insight from the plant's perspective.