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?
We are talking about gardening :) useful plants, such as root vegetables, leaf vegetables, fruits, and herbs, are grown for consumption
Do people have to go to school to work in this field?
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
, lawn grasses, shade trees, ornamental trees, shrubs, vines, herbaceous perennials and bedding plants are used. Gardens may cultivate the plants in a naturalistic state, or may sculpture their growth, as with topiary or espalier. Gardening is the most popular leisure activity in the U.S., and working with plants or horticulture therapy is beneficial for rehabilitating people with disabilities. Plants may also be grown or kept indoors as houseplants, or in specialized buildings such as greenhouses that are designed for the care and cultivation of living plants.