I have a balcony with a number of pots all of which are < 30cm in diameter. I struggle to work out what I should be putting in them. Most plants seem too big to go in these containers and are grown for gardens with a lot of space.
There are bedding plants, but these seem to spread out a lot. They also tend to be quite low. Is there a category of plants that are small but still have flowers are grow up? So I can search for the right things on garden centre websites.
The search terms "flowering outdoor container plants" will get you to plants that do well in containers or pots, and don't need a lot of room to spread out. You can swap out "potted" for "container" and "blooming" for "flowering." Other helpful terms to include can be "patio" and "windowbox." In some regions the term "pot plant" will also get you plants that grow well in pots (in other regions "pot plant" means marijuana). Also look for "dwarf" or "compact" varieties of bedding plants. These varieties are often developed specifically for growing in containers.
There are plenty of guides to "container gardening," that will include tips as well as lists of plants that work well. Here are a few that I found, but you may want to find a guide specific to your region.
A few specific plants that no one else has suggested yet:
You can grow almost anything in containers; herbs, shrubs, or even trees (bonsai). But also vegetables or flowers. Try to look for plants which are sold in small containers. Examples for flowers are Marigolds (Tagetes), but also pot roses. These are just two examples of many possibilities. Have fun gardening!
Some bedding plants grow up as well as bushing out a bit - Pelargoniums (commonly known as geraniums), Fuchsia, Salvia splendens, Lobelia cardinalis, Petunia (these come in trailing or upright, as do Fuchsia), Impatiens New Guinea hybrids but you might consider just buying larger pots. Under 30cm means you are quite restricted to fairly small plants with probably only one plant per pot. You might get away with Lavandula stoechas in a pot that actually is 30cm, not smaller though.
