Answer: I'm so glad to hear you enjoy the Burpee site! It sounds like you are a container gardener. Unfortunately, there is no rule of thumb for plant root area. An easy guide is to choose plants that are labeled as container or dwarf varieties. Seed packets and nursery labels usually recommended spacing, and you can often space them a little closer if there are plenty of resources (sun, nutrients, moisture) to support healthy growth of all the plants. The only way to know for sure is to learn more about the plants that interest you. Peruse books on container gardening at your library or bookstore for guidance on choosing plants. And, as always, feel free to experiment.
At any rate, don't use pots smaller than 1 gallon capacity - they can be long and shallow for fibrous rooted plants like alyssum and portulaca, but anything with a tap root will need a deep container. I hope this helps!
Q&A Library Searching Tips