Habit Terms, adapted from Woody Plants of the Blue Ridge by Ron Lance. Used by permission.

Tree. Typically has a single stem (trunk) at least 3 inches in diameter at breast height (4 1/2 feet above the ground), a height of 12 feet or more, and a single, rather well-defined crown.

Large tree. Mature size of over 60' in height; often over 18” diameter.
Medium-sized tree. Mature height of 25 to 501; diameter of 12 to 18”.
Small tree. Mature height mostly 12 to 20'; diameter rarely over 10”.

Shrub. Typically has smaller stature than a tree; usually with multiple stems and a less-defined crown.

Vine. Lacks a supportive trunk and climbs, sprawls, or creeps over other plants or on the ground.

Groundcover. May be a shrub or vine; prostrate on the ground or weakly ascending above the ground; often obscuring the ground surface from view.

Colonial (colony-forming) plants use underground stems called rhizomes or elongated, running roots to send up many shoots. Such rhizomatous or suckering plants may ultimately form colonies or beds, all from the same original plant. Stands of one species may be formed by rhizomes or root suckers, or be due to heavy seedling germination in specific areas.

Juvenile wood is found in all parts of young plants and sprouts, and lower limbs of older plants. It has not yet reached the capacity to produce flowers, and frequently exhibits foliage or twig characteristics which may differ from that of normal older wood. Mature wood includes branches and twigs which have reached reproductive age. The drawings in the book Woody Plants of the Blue Ridge, unless otherwise specified, show features found on mature wood.

Form:

Twining. Pertaining to vines; stems twist around objects for support.
Reclining or straggling, Leaning or lying over other plants or other objects; weakly self-supporting.
Trailing. Elongating stems which lie on the ground surface.
Fastigiate. Having a narrow crown of slender, up-turned branches.

Woody vs. Non-Woody:

Woody plants are generally distinguished from non-woody or herbaceous plants by the presence of woody stems — those remaining alive above the surface of the ground throughout the year. Recognition of a woody aspect is straightforward in the case of trees and large shrubs, but not all plants fit so conveniently in the woody or non-woody categories. Seemingly woody plants may die to the ground from radical climatic fluctuations during Winter in one area or particular year, or the same species may have woody stems in the southern parts of its range and be herbaceous in the north. Deciding whether a rhizomatous stem lying under leaf litter or a short, succulent stem qualifies as a woody plant stem also serves to illustrate the rather arbitrary nature of the woody plant category. Low-growing species like Chimaphila, Epigaea, Gaultheria, Hudsonia, MitcheIIa, Pachysandra, and Sibbaldiopsis are recognized as woody species by most people familiar with them, but these and other herb-sized sub-shrubs and groundcovers may not be so readily recognized by the beginner or student as belonging to the woody plant category.

Tree vs. Shrub:

The line of separation between trees and shrubs is even less obvious than the categories of woody vs. non-woody. While no problem may arise in identifying most of our canopy species as trees and very low thicket-forming species as shrubs, there is considerable middle ground. Many of our understory species vary from "shrubby" to "treelike" growth habits, depending on locale and other factors.... The obvious shortcomings of the tree vs. shrub concept will be seen with prolonged observation of many of our understory species, and the recognition of objective opinion (and less concern with the importance of the separation) of the tree vs. shrub category will inevitably be perceived.