| Habit | Same as growth habit. |
| Half- or sub-shrub | A plant with stems that are woody at the base, usually dying back to the woody stems or even back to the ground after severe winters. Suffrutescent. |
| Hardened | A plant condition created by various factors enabling it to withstand environmental stresses; contrast with succulent growth which is very vulnerable to environmental stress and damage. |
| Hardy | Used in the horticultural sense, enduring winter conditions. |
| Herb | A plant dying to the ground at the end of the season; one whose aerial stems are soft and succulent without appreciable parenchymatous xylem tissue, a plant not woody in texture. |
| Herbaceous | Not woody; having no persistent woody stems above ground. |
| Hesperidium | A fleshy berry-like fruit with a hard rind and definite longitudinal partitions. Ex. Orange |
| Hip | The fruit of a rose. |
| Hippocrepiform | Horseshoe-shaped. |
| Hirsute | Pubescent with coarse or stiff hairs. |
| Hoary | With a close white or whitish pubescence. |
| Husk | Outer covering of a fruit or seed. |
| Hybrid | A plant resulting from a cross between two or more other plants which are more or less alike. |
| Imbricate | Said of scales which overlap like shingles; the opposite of valvate in which the scales meet along a line without overlapping. |
| Imperfect flower | A flower that lacks either stamens (pollen producing structures) or pistils (seed producing structures). |
| Incised | Cut by sharp and irregular incisions more or less deeply, but intermediate between toothed and lobed. |
| Included | Not protruding as stamens not projecting beyond a corolla; as opposed to exserted. |
| Incomplete flower | One which lacks any one or more of these parts; calyx, corolla, stamens, and pistils. |
| Indehiscent | Not opening, as applied to fruits. |
| Indeterminate | Said of those kinds of inflorescence whose terminal flowers open last, hence the growth or elongation of the main axis is not arrested by the opening of the first flowers. |
| Indurate | Hardened. |
| Inequilateral | With unequal sides. |
| Inflorescence | A flower cluster. |
| Infra- | Below. |
| Internode | The part of a stem between two nodes. |
| Involucrate | With an involucre or cluster of bracts. |
| Involucre | A cluster of modified leaves about a flower cluster. |
| Jointed | Having nodes or points of real or apparent articulation. |
| Juvenile | An early phase of plant growth, usually characterized by non-flowering, vigorous increase in size, and often thorniness. |
| Keel | A ridge on the back of a leaf or bud scale. |
| Key | A small indehiscent fruit with a wing. |
| Knees | Pointed or dome-like outgrowths from cypress roots rising above the water. |
| Lactiferous | Milky. |
| Lanceolate | Shaped like a lance-head, as applied to leaves. |
| Lateral | Said of buds which appear along the sides of the twig; borne at or along the side. |
| Lateral bud | A bud borne in the axil of a previous season’s leaf. |
| Latex | Milky sap. |
| Leader | The primary or terminal shoot; the trunk of a tree. |
| Leaf | The foliage appendages of the stem, though not always serving as foliage; sometimes metamorphosed into a spine (barberry), or tendril (clematis), or reduced to a scale (juniper). Leaves originate at and mark the node or joints of the stem. Buds normally occur in the angles or axils above leaves and are correspondingly alternate, opposite or whorled on the stem. |
| Leaf scar | Scars from which leaves have fallen. They usually occur characteristically either singly (alternate) or paired (opposite) or in groups of more than 2 (whorled) at each node. Leaf scars differ greatly in size and shape, and offer some of the best winter characteristics. The points where woody strands of vascular tissue passed up into the leaf are usually evident, and are called bundle scars or bundle traces. Typical leaf scars are essentially at the level of the stem; but they are raised o a pronounced base or leaf cushion in some cases, or the buds are covered by an articular membrane in other. |
| Leaflet | One of the divisions of a compound leaf. |
| Legume | A pod; the characteristic fruit of the pea family. |
| Lenticels | Very small wart-like structures, breaking through the bark of most young twigs. Corky in texture and made of loosely packed cells, providing gaseous exchange between the inner tissues of the stem and the atmosphere. |
| Lignified | Woody, hardened. |
| Ligule | 1. A strap shaped organ. 2. A minute projection from the top of the leaf sheath in grasses. 3. The strap shaped corolla in the ray flower of Composites. |
| Linear | Long and narrow. |
| Lineate | Lined; bearing thin parallel lines. |
| Lined | Lightly ridged or ribbed. |
| Lobed | Divided rather deeply, as applied to leaves. |
| Lustrous | Having a slight metallic gloss, less reflective than glossy. |
