This is the cultivation of plants as part of horticulture
It is a more specialized version of agriculture, right?
Yes It is ornamental plants are often grown for their flowers, foliage, or overall appearance; 
What kinds of plants are we talking about?
Gardening
Gardening is the practice of growing and cultivating plants as part of horticulture. In gardens, ornamental plants are often grown for their flowers, foliage, or overall appearance; useful plants, such as root vegetables, leaf vegetables, fruits, and herbs, are grown for consumption, for use as dyes, or for medicinal or cosmetic use. Gardening is considered by many people to be a relaxing activity.
Gardening ranges in scale from fruit orchards, to long boulevard plantings with one or more different types of shrubs
Ornamental plant
Ornamental plants are plants that are grown for decorative purposes in gardens and landscape design projects, as houseplants, cut flowers and specimen display. The cultivation of ornamental plants is called floriculture, which forms a major branch of horticulture.
Garden plants.
Commonly, ornamental [garden] plants are grown for the display of aesthetic features including: flowers, leaves, scent, overall foliage texture, fruit, stem and bark, and aesthetic form. In some cases, unusual features may be considered to be of