When creating plants, we must know their climate, which is covered in Creating Places, (The Art of World Building, #2). But unless ours is an ice world, for example, it can be assumed that all climates exist somewhere, which means that we don’t need to know which continents or regions our plant is found in just yet. It can be invented for a climate, and when we decide where on our world those climates exist, we’ll know if it could be found there. We may want to name it after a place, or vice versa, but that’s easy enough to accomplish later.
Classification
There are broad categories that plants fall into, but we’re most likely interested in only a few.
The seedless plants include algae, liverworts, mosses, and ferns, with only mosses being something we’re likely to use in our stories partly because no one thinks algae or ferns are interesting, and no one knows what a liverwort is (and it’s not interesting or useful when you do). If we have sea dwelling species, algae can be more useful if there’s a dangerous or useful kind that can develop.
The usefulness of moss is debatable, but it can be needed by wizards or have properties to make it deadly or otherwise cover a landscape with a color different than the green we expect on Earth. Mosses grow in damp areas and need plentiful water to reproduce. They can grow on rocks, trees, or discarded items. A special kind of moss, called sphagnum, can form floating islands found in bogs, where trees and other plants are growing in the shifting mat of clumped-together moss.
Among the plants with seeds are cycads, conifers, and flowering plants. An internet search on cycad will reveal plants that look like a palm tree, or an evergreen fern with very large leaves atop a branchless tree trunk (sometimes quite tall) and with cones in the middle of these leaves at the top. They grow slowly and live up to a thousand years, so they could be admired by a long living, humanoid species. They are in tropical and subtropical climates. These large cones can be imagined to contain useful material in them and to have predators who desire them.
A conifer also has cones but prefers colder climates and often forms enormous forests. Conifers include pines, cedars, Douglas-firs, junipers, redwoods, spruces, and more. Most are trees but some are shrubs. Their conical shape helps them shed snow and their wood is soft.
Then there are the flowering plants that dominate temperate climates; unless you live somewhere always cold or hot, this is what you see when you look out the window, and as such, these are the most common plants you’ll be inventing. These include not only flowers, shrubs, and vines but trees like the oak, maple, elm, aspen, and birch.
Regarding trees, the deciduous variety lose their leaves in autumn while the evergreens lose them continuously all year in such a way as to appear, by contrast, that they never lose their leaves, hence the name.
What does this all mean to a world builder? Not much other than having a better understanding of what we probably want to invent: mosses, conifers, and flowering plants, with algae and cycads bringing up the rear.
