There is a lot of contradictory information on the Internet related to the use of gravel, rocks, or pebbles below the soil when repotting a plant. Some people say it is beneficial, others say that is not. The main reason people add rocks below the soil is to promote better water drainage. But does it work?
Adding gravel under the soil is a bit of an urban myth. At least when it comes to potted plants. In nature, gravel can help with the soil’s drainage. They keep the soil loose enough for the water to slip between the crevices and drain down the soil. Good drainage keeps the plant’s roots in good shape and far from excess moisture, which can cause root rot. But does it work the same way inside a pot?
The problem with adding rocks below the soil in a pot is that the soil will eventually get into the crevices, get compacted, and turn into mortar. Eventually, cementing the rocks and not allowing the water to flow down.
If the pot does not have drainage holes of its own, the water that slips through the rocks or pebbles will flow down the pot with nowhere to go. This water will eventually rot and increase the moisture inside the pot. Not too good for the plant’s health.
The best use of rocks or pebbles is to place them on top of the drip tray and below the bottom of the pot. Then, when the pot drains out all the excess water, the water that falls into the drip tray will have no contact with the pot, the soil, or the plant's roots.