Plants that are just green usually are called evergreen or foliage plants. Evergreens are trees and shrubs that lose their leaves or needles very gradually, replacing them as they lose them, so it appears as if they never lose their foliage at all. Foliage plants are grown for their leaves instead of their flowers. While both groups include some non-green flowering plants or foliage, most of the plants in these groups are just green.
Most evergreen trees are green in color, save evergreen fruit and berry trees. Some have needles, such as pine trees, fir trees, cedar trees and cypress trees. Some needle, or coniferous, trees may turn brown in the fall or when in drought conditions, but they will quickly "green up" again in the spring. Palm trees are just green in color as well.
Many shrubs have berries or flowers, which means they are not solely green in color. There are a few that have such insignificant flowers or that they hardly show, or flowers that are green in color. The boxwood is a shrub commonly used as a hedge. It does feature tiny flowers for a brief period of time, but they are greenish-yellow in color and they do not stand out against the green leaves. The Mediterranean spurge features long, graceful green needles and tiny, insignificant green flowers. Creeping juniper is an evergreen shrub that does not flower and is a popular groundcover.
Foliage perennials are often used to set off the bright colors of nearby flowers or to soothe the eyes. After all, green is the most easily seen color and is a calming color. There are many perennials that are grown just for their foliage and are green year-round, including ornamental grasses such as lemongrass, which has a pleasing, lemony fragrance, and palm sedge.
Many succulents are also just green, including medicinal aloe (which has green flowers) and the dramatic agave, which features a tall stalk of green flowers which can have a slightly yellow tint.
Ferns are a hugely popular foliage perennial. Ferns feature green, graceful fronds and come in a wide variety of sizes and patterns, including the delicate lace fern and the two-toned (both green tones) Japanese tassel fern. Many herbs, such as garlic chives, also just green.