They have no drainage.
Any plant should have access to proper drainage holes; otherwise, it will face the problems of overwatering mentioned above, caused by the accumulation of water. To avoid these issues, you have to make sure to provide a couple of holes in the bottom of the container before planting and use the proper potting soil for each and every species.
For instance, cacti and succulents need the best drainage and a light substrate (try this one) that doesn’t retain moisture because they don’t tolerate moisture-rich soil.
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