Q: What is the scoop on using coffee filters to cover the drain holes in pots rather than small rocks or broken pieces of pots?
A: The coffee filter idea seems reasonable to me. If you don’t cover the hole, potting soil will slowly dribble out. The one thing NOT to add to a pot is a layer of rocks at the bottom – it only lessens the amount of soil available for root growth.
I’ve mentioned before that folks who put rocks in their pots have rocks in their head. The reason may not be obvious but it is based on soil physics. Imagine the manner in which water moves in a pot. When you water a potted plant, the moisture initially soaks lower and lower through the soil. In a contest between gravity and the absorption capacity of the soil, some water drains out of the bottom. Most, however, stays in the soil at the bottom of the pot. The soil there becomes saturated with water.
As you know, roots suffer when they are surrounded by water-filled soil. Root rot and plant death follow when roots can’t breathe their gasses into, and out of, the surrounding soil. When rocks are placed in the bottom of a pot and then covered with soil, there is less soil in which the roots of your plant can grow. Water still moves in the manner described above but it now has less distance in which to drain away from the plant roots. The bottom line is that when you add rocks to the bottom of a pot the result is a higher proportion of harmful, water-logged soil.