An extremely simple way of thinking about the green world is to divide the vascular plants into those that are woody and those that are not. The diversity of nonwoody plants, such as wildflowers and grasses, is staggering!
Dividing plants into nonwoody (herbaceous) and woody categories has little to do with their true relationships with each other. Many plant families include both nonwoody and woody species. For example, the rose family includes woody cherry, apple, hawthorn, and plum trees; woody rose and blackberry shrubs; but also herbaceous strawberries, avens, Indian physic, and cinquefoils.
Some nonwoody plants, as they mature, can develop stalks that are fairly woody, and many woody plants are nonwoody when young.
Ferns and fern allies are nonwoody, but unlike most familiar nonwoody plants, they are nonflowering; instead, they reproduce via spores. Meanwhile, grasses, sedges, and rushes are nonwoody plants that reproduce with flowers, although their flowers are not very showy.
To botanists, categories like "wildflower" and "weed" are artificial. What blossom is pretty enough to be called a wildflower? What species is undesirable enough to be called a weed? These categories have nothing to do with true plant relationships and are based entirely on human perception.
Many kinds of nonwoody plants are nonvascular: mosses, liverworts, and hornworts are small plants that lack veinlike vascular tubes. These plants, with lichens, are grouped in another section of the online field guide.
Grasses: big bluestem, fescue, corn, wheat, rice
Lilies: onion, dogtooth violet, hyacinth, daffodil, trillium
Asters: sunflowers, coneflowers, ragweeds, thistles, goldenrods, asters, dandelion, yarrow, crownbeards, thoroughworts, blazing stars, ironweed, marigolds, lettuce
Legumes: beans, clovers, peas, alfalfa, honey and black locust, redbud, vetches, tick trefoils, lespedezas, partridge pea
Mustards: cresses, rockets, alyssum, broccoli, cabbage, watercress, radish, horseradish
Dogbanes and milkweeds: Indian hemp, butterfly weed, common milkweed, sand vine
Mallows: hibiscus, cotton, hollyhock, okra, rose mallow
Nightshades: jimsonweed, horse nettle, potato, eggplant, tomato, garden peppers, tobacco, ground cherry
Mints: peppermint, henbit, basil, salvia, coleus, skullcap, germander, wild bergamot, horsemint, dittany
Carrots: celery, parsley, Queen Anne’s lace, parsnip, fennel, cilantro, golden Alexanders