Pretty pots can enhance the look of a plant in the indoor or outdoor environment. It is well understood that the black plastic or green containers plants are purchased in are utilitarian and not really attractive at all. The first decorative, rather functional, pots are basic terra cotta. The value to the plant placed in a clay pot is that the earthen material from which the pot is made breathes. This breathing allows for a somewhat limited exchange of oxygen and transpires excess water to the atmosphere around the plant. This benefits the grower and the plant since 80% of all plants die due to over watering.
Even with breathable clay that lightens when dry we still over water our herbaceous friends. Over watering leads to water draining out the bottom of the pot onto the surface the pot is sitting on. This could be a family heirloom table, an Antique Road Show first prize Louis The XIV original or even the hardwood floor in the living room. Water dripping or flowing onto such surfaces leaves water stains that often cannot be fixed easily. Water can ruin the value of such tables, desks and floors making them unworthy of appraisal on prime time PBS.
To combat water running onto the floor decorative clay was glazed then painted. The result was a planter even more suitable to the environment in which it was placed. The result for the plant in such containers, often with no drainage holes, was death from.....you guessed it, over watering. Except in this case, drowning.
Logic has told indoor gardeners to combat the lack of drainage holes in such pots place a layer of stone at the bottom of the planter. This is supposed to create a pace for the water to collect and save the the roots by leaving them out of harms way. This is one of the oldest Old Wives' Tales in my Directory of Old Wives' Garden Tales. If this OWT were an old coin or piece of paper currency it would be worthy of incredible value just due to the low serial number alone. Drainage stone in pot bottoms to alleviate water damage to roots is cataloged as OWT #00001.1 . No subsection letter or number as my other OWTs warrant.
Drainage stone placed at the bottom of plant containers DOES NOT STAVE OFF DROWNING. In fact it could make it more likely to happen. This fact was discovered and written on way back in the 1930's in college level soil science texts. Here is why drainage stones in plants, no matter how well intentioned, is not doing anything except hurting your back with the extra gravitational pull of heavy rocks.
Water moves down in soil by gravitational pull for the most part. It is more complicated by simplicity in the garden rules in my yard. So if water is pulled down through soil by gravity what then makes soil go up, both into plant systems and evaporation?
Capillary action. The same way that our circulatory system works. Except in the plant world no pump, aka heart, is forcing the issue. Water is very cohesive meaning it stays together and generally where it is put. That is why sucking through a straw idseasier with liquid then a slurry of solids like mud. While soil does not really have vessels to create a physical capillary network what is does have is air space between every particle of soil. Water flows through the air space not through the soil particle. So in essence there exists in soil a vast network of capillaries in the form of air.
The continuity of these air pockets determines how far and how fast water moves any direction up or down in the soil strata. As long as the air pockets are of a similar size moisture flowing through this virtual capillary network is even and predictable. But....break that network open by changing the size of air space abruptly and the flow of moisture is interrupted drastically. This radical change in air space, drainage stones under a layer of fine soil, breaks the cohesion factor of water almost stopping the push me pull you flow action. Basically the moisture becomes confused, scared and stops moving.
Studies have shown, with dramatic photographic evidence (NSFW for faint office environs) that the soil in contact with drainage stones and immediate above it is saturated, has low oxygen content and is often anaerobic in content causing roots to rot. Think of it this way. Spill some water on the floor. Attempt to soak it up with "The Quicker Picker Upper" except that the Quicker Picker Upper" is fused on the upper half with a towel of poorer quality, like larger weave brown industrial paper towels, and you will see the water will not continue its' upward capillary action
into the cheaper "Slower Picker Nothing Upper."
Solution to the no drainage but the pot is pretty anyway? Careful watering using a moisture probe all the way to the bottom of the pot to ensure no swamp is being created. In addition frequently lifting the plant out of that Ming Dynasty Vase to check the health of the roots. If the roots are whitish to whitish yellow and you don't drop the antique you are fine. If the roots are brown, smelly and putrid looking choose the Ming or the Marigold but you can't have both.