Hi Penny. Well I try to consider 3 things. The height of the
plants, their
growing requirements, and color. For instance last year I put canna, marigolds (mixed colors) and nastrium all in the same pot.
I had to constantly prune the nastrium because it tried to crowd everything out. And then the canna started multipling and I was running out of room everywhere. lol. So this year I will make sure to use
plants that aren't quite as aggressive lol. Also I would stick to annuals since they bloom for longer periods of time and that is something I definitely want in a container
plant.
Also consider using some
plants that have attractive foliage to cover those periods when the other
plants haven't flowered yet.
I would probably be looking at snapdragons, petunias, and celosia. But that's just me. And I am still learning bunches.