the first few centimeters of the soil should be dry before watering, just poke your finger into the soil if dry then its time to water.

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centimeters? I am thinking inch at least even in small cup, make those babies reach for the bottom of the cup!

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Letting the soil dry up almost completely, then watering as needed makes the roots search for water, getting longer and stronger as they do. When you do water it is then taken up by the thirsty roots and you see quite noticeable growth spurts. I grow in soil and only water about every 7-10 days, as dictated by the dryness/wetness of the soil. Overwatering is a very common mistake that almost everyone makes. Let those babies dry out a bit, makes them stronger plants overall. When they're young you don't want to make them work too hard, keep the soil just barely moist, but not wet and you can see growth of up to two to three inches in a week's time.

dont get to scientific with them at the end of the day their just plants.
if your useing a 3 to 4" cup you dont want to disturb the roots so dont be poking about too deep.

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Logic will get you from A to B. Imagination will take you everywhere