How do I seal clay pots?

It takes a long time for water to erode the clay of a pot; the white stuff you see is mineral buildup. (Now, being left out through freezing-thawing cycles where they can get wet, that will get you a big pile of cracked pots in the spring, ask me how I know :/ ) Some plants love the airflow and drainage afforded by unglazed clay --- anything from a Mediterranean climate, and especially succulents (for beginners), I strongly suggest unglazed clay.

But my African violets, cyclamen, mandevilla, most of my tropicals, really --- anything that wants to stay "moist but not wet" --- they get plastic or glazed clay/ceramic, because they *want* the water retention. (It is perhaps even more important to make sure that these plants get well-draining soil with lots of perlite or grit, as even the most water-loving plants still need oxygen to their roots.)

Choosing the right pot and soil for the particular plant can be the most important factor in that plant's success this season. If the plant's needs are "fast-draining," if they tell you to let it dry out between waterings, these are indications that unglazed clay will do beautifully. If it has that magic "consistently moist" requirement, though, you want something non-porous.

Regardless of which type, it's important when watering containers to thoroughly drench, until water runs out the bottom. Otherwise, minerals from your water can build up in the soil and block the roots from getting what they need. This also means that you need to fertilize more frequently, as the nutrients get washed out more quickly.

"Tropical" = non-porous. "Mediterranean" = porous. I know a lot of cacti enthusiasts tear their hair out over the admonition to put succulents in unglazed clay, but for beginners, I think it's perfect. You can keep Mediterranean-climate plants in plastic or other non-porous pots, but it's super important to have a really, really well-draining soil and to let their soil dry almost completely before watering. They'll survive a wilt or two much more easily than being soggy.