Other Classification Systems (Operational)

A number of operational classification systems are employed simultaneously in the field of horticulture. The following are some of the major systems.

Seasonal Growth Cycle

Plants may be classified into three general groups based on growth cycle. Growth cycle refers to the period from first establishment (e.g., by seed) to when the plant dies. The three categories are as follows:

1. Annuals: An annual plant lives through only one growing season, completing its life cycle (seed, seedling, flowering, fruiting, and death) in that period. This group includes many weeds, garden flowers, vegetables, and wild flowers. The duration of a life cycle is variable and may be a few weeks to several months, depending on the species. Annuals are the basis of a major horticultural production group called annual bedding plants. These plants are produced largely for use in the landscape and also the vegetable garden. Popular annual flowers are geranium (Geranium spp.), zommoa (Zinnia elegans), and marigold (Tagetes spp.),and pansy (Viola tricolor). In cultivation, certain vegetables such as tomato (Lycopersicon . esculentum) are produced an annual cycle.
2. Biennials: A biennial is a plant that completes its life cycle in two growing seasons. In the first season, it produces only basal leaves; it grows a stem, produces flowers and fruits, and dies in the second season. The plant usually requires some special environmental condition or treatment such as exposure to a cold temperature (vernalization to be induced to enter the reproductive phase. Examples of biennials are sugar beet (Beta vulgaris) and onion (Allium cepa). Even though annuals and biennials rarely become woody in temperature regions, these plants may sometimes produce secondary growth in their stems and roots.
3. Perennials: Perennials may be herbaceous of woody. They persist yea"r-round through the adverse weather of the non-growing seasons (winter or drought) and then flower and fruit after a variable number of years of vegetative growth beyond the second year. Herbaceous perennials survive the unfavorable season as dormant underground structures (e.g., roots, rhizomes, bulbs, and tubers that are modified primary vegetative parts of the part of the plant. Examples of herbaceous perennials are turf grasses such as bermudagrass (Cynodon dactylon)and flowers such as daylilies (Lilium spp.) and irises (Iris spp.)
4. Woody perennials may be vines, shrubs, or trees. These plants do not die back in adverse seasons but usually suspend active growth. Although some perennials may flower in the first year of planting, woody perennials flower only when they become adult plants. This stage may be attained within a few years or even after 100 years. Woody perennials may be categorized into two types:
5. Evergreen: Evergreen perennials maintain green leaves year-round. Some leaves may be lost, but not all at one time. Example of evergreen perennials are citrus (Citrus spp.) and pine(Pinus spp.).
6. Deciduous: Deciduous plants shed their leaves at the same time during one of the seasons of the year (dry cold). New leaves are developed from dormant buds upon the return of favorable growing conditions. Examples of deciduous perennials are oak (Quercus spp.) and elm (Ulmus spp.). It should be mentioned that intermediate conditions occur in which some plants do not lose all of their leaves (semideciduous)

Kinds of Stems

There are three general classes of horticultural plants based on stem type. However, intermediates do occur between these classes.

1. Herbs: Herbs are plants with soft, non-woody stems. They have primary vegetative parts. Example include corn (Zea mays), many potted plants, many annual bedding plants, and many vegetables. In another usage, the term herbs is associated with spices (plants that are aromatic or fragrant and used to flavor foods or beverages).
2. Shrubs: A shrub has no main trunk. Branches arise from the ground level on a shrub. It is woody and has secondary tissue. Shrubs are perennials and usually smaller than trees. Examples   of  shrubs   are   dogwood   (Cornus  spp.),   kalmia   (Kalmia   spp.)   and   azalea (Rhododnedron spp.).
3. Trees: Trees are large plants characterized by one main trunk. They branch on the upper pert of the plant, are woody, and have secondary tissue. Examples include pine (Pinus spp.), oak (Quercus spp.), cedar (Cedrus spp.), and orange (Citrus sinesis).

Common Stem Growth Forms

The criterion for classification is how the stem stands in relation to the ground. There are several types of stem growth forms, the most common ones including the following.

1. Erect: A stem is erect if, without artificial support, it stands upright (stands at a 90-degree angle to the ground level). Because of the effect of strong winds and other environmental factors, an erect plant may incline slightly. Trees have erect stems. To adapt crop plants to mechanized harvesting, plant breeders have developed what are called "bush" cultivars. These plants have strong stems and stiff branches.
2. Decumbent: The stems of decumbent plants are extremely inclined, with the tips raised. An example is peanut (Arachis hypogaea).
3. Creeping (or repent): A plant is described as creeping when it crawls on the ground, producing adventitious roots at specific points on the stem. Stems that grow horizontally in this fashion are called stolons. The strawberry plant (Fragaria spp.) has creeping stems.
4. Climbing: Climbers are vines that, without additional support, will creep on the ground. There are three general modes of the climbing. Twiners are climbing plants that simply wrap their stringy stems around their support, as occurs in sweet potato (Ipomea batatas). Another group of climbers develop cylindrical structures called tendrils that are used to coil around the support on physical contact. An example of a plant that climbs by this method is the garden pea (Pisum sativum). The third mode of climbing is by adventitious roots formed on aerial parts of the plants, as found in the English ivy (Hedera helix) and Philodendron.