The classification of a small plant depends primarily on its internal structure and biological makeup, not just its height. A small plant is generally defined by its short stature and low-growing habits, which prevent it from developing into a large, tree-like form. Plant taxonomy primarily distinguishes species based on the presence or absence of persistent woody tissue above ground, leading to various scientific and common terms for these low-lying organisms.
Herbaceous Plants
The largest group of small plants are herbaceous species, defined by their soft, non-woody stems. These plants lack the rigid, lignified tissue found in trees and shrubs. Their above-ground growth often dies back completely at the end of a growing season, ensuring they remain perpetually small since they do not accumulate persistent, tall growth.
The life cycle of a herbaceous plant refines its classification into three groups. Annuals, such as basil, complete their entire life cycle—from seed germination to seed production—within a single year before dying. Biennials, including plants like foxglove, require two years to complete their cycle. They typically establish foliage in the first year and then flower, set seed, and die in the second.
Herbaceous perennials survive for more than two years. Their defining feature is that their stems die back to the ground each winter, while the root system survives to sprout anew in the spring. Plants like peonies maintain a small stature despite their long lifespan. The term “herb” itself, frequently used for culinary plants like dill, cilantro, and mint, is a common name for many of these non-woody species.
Small Woody Plants
Small plants that possess persistent, lignified tissue are generally classified as shrubs, marking a structural difference from herbaceous types. A shrub is a perennial woody plant distinguished from a tree by its size and growth habit, usually remaining under 10 meters in height. Unlike a tree, which develops a single main trunk, a shrub features multiple woody stems that arise from or near the base. These multiple-stemmed, bushy forms define the shrub. Very small woody plants, often under two meters tall, are sometimes called subshrubs. These include species like lavender, which maintain woody growth at the base but produce softer, partially herbaceous growth at the tips.
Dwarf Varieties
Horticulture utilizes the term dwarf varieties to describe plants, both woody and herbaceous, that have a significantly reduced size due to genetic mutation or cultivation. These dwarf forms, such as miniature conifers, are intentionally bred to maintain a compact stature for use in gardens and containers. This provides a way to have the characteristics of a larger species in a consistently small form.
Non-Vascular Organisms
The very smallest plant-like organisms are the non-vascular plants, scientifically known as Bryophytes, which include mosses, liverworts, and hornworts. These organisms lack the specialized internal transport systems, the xylem and phloem, that define all other true plants. This absence of a vascular system is the primary factor limiting their size, keeping them close to the ground. Bryophytes cannot efficiently transport water and nutrients against gravity over long distances. Instead, they rely on simple processes like diffusion and osmosis for material exchange, which are only effective over very short distances. Consequently, most mosses and liverworts remain only a few centimeters tall, often forming dense, low mats or cushions. Their reliance on direct absorption of moisture also dictates that they thrive almost exclusively in damp or moist environments.
Functional Groupings and Common Names
Beyond scientific classifications, many small plants are grouped according to their growth habit and function in a landscape. The term groundcover is a practical designation for any small plant species that spreads horizontally to form a dense layer over the soil. This helps prevent erosion and suppress weeds, defining groundcovers by their function rather than their botanical family.
Creepers
Plants that trail or spread along the ground are often called creepers, describing their low-lying, vine-like growth pattern. These include species like creeping thyme or low-growing sedums. Creepers are valued for their ability to fill in spaces or cascade over retaining walls.