A multitude of species can be grown for decades in pots smaller than 1 litre. Exactly what species, though, will depend upon your climate. Tropical species can be grown indoors, but (generally speaking) only if you can provide artificial light approximating sunlight. In the line of tropicals, pemphis and small leafed figs (ficus) are two species that come to mind. Jade plants (crassula) do very well in low light indoors.

In the line of temperate plants, pinus thunbergii (Japanese black pine) is the classic bonsai tree, and can even be grown in the tropics. There are a multitude of small azaleas that are relatively carefree to grow in small pots.

I am a bonsai enthusiast, living in USDA plant zone 8. I have many varieties of maples (acer palmatum, acer ginnala, acer platanoides,and acer rubrum) that I have kept in pots smaller than 1 litre for 5 or so years now. I also have Douglas firs, lodgepole pines, pinus parviflora, pinus strobus, pinus mugo, spruce (Colorado blue, picea abies, and picea abies 'nidiformis'), cedars (Atlas and libani), elms (ulmus parviflora 'Suiju' and Zelkova serrata), cotoneasters, pyracantha, forsythia, varieties of prunus, malus and pyrus, stewartia, boxwoods, junipers, escallonia, ceanothus, and a few others that I'm not recalling right now. For bonsai we look at species with short internodes and small leaves or ones that will exhibit this behavior when confined to small pots.

But, there are a number of slow growing miniature cultivars that are sold in the U.S. for 'rock gardens'. Chamaecyparis obtusa (hinoki cypress) 'chirimen' is one that comes to mind that will do just fine with little attention for about 5 years in a 1 litre container. Cedrus libani 'Kenwith' and 'Green Prince' are a couple more of this type.

The issue really isn't what plants can be grown in pots smaller than one litre (just about any species), but the materials and techniques to do it successfully. Otherwise, just look for slow growing, miniature, 'rock garden' plants at your local garden center.

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.

• https://www.southernliving.com/home-garden/gardens/creative-container-gardens
• https://www.countryliving.com/gardening/garden-ideas/g4098/best-plants-for-container-gardening/
• https://www.familyhandyman.com/list/best-plants-for-container-gardening/
• https://www.gardengatemagazine.com/articles/containers/all/meet-8-of-the-best-container-plants/

A few specific plants that no one else has suggested yet:

• Starflower (Pentas lanceolata)
• Lantana
• Million bells (Calibrachoa sp.)
• Heliotrope
• Ornamental peppers (the flowers aren't showy, but then they have long-lasting clusters of multicolored hot peppers)
• Angelonia (Angelonia angustifolia)
• Fanflower (Scaevola aemula)
• Nemesia
• Pansies
• Snapdragon (if you let them go to seed they may grow back next year)