| imbibition | The process of water absorption, causing swelling. |
| immobilization | The process by which soil microorganisms use available nitrogen as they break down materials with a high C:N ratio, thus reducing the amount of nitrogen available to plants. |
| immune | A plant that does not become diseased by a specific pathogen. See resistant, tolerant. |
| incompatible | Kinds or varieties of a species that do not successfully cross-pollinate or intergraft. |
| incomplete flower | A flower lacking one or more of the normal flower parts. |
| incomplete metamorphosis | See simple metamorphosis. |
| incubation | A period of development during which a pathogen changes to a form that can penetrate or infect a new host plant. |
| indeterminate | A plant growth habit in which stems keep growing in length indefinitely. For example, indeterminate tomatoes are tall, late-fruiting, and require staking for improved yield. See determinate. |
| infection | The condition reached when a pathogen has invaded plant tissue and established a parasitic relationship between itself and its host. |
| infiltration | The movement of water into soil. |
| inflorescence | A shoot bearing clusters of flowers. |
| inflorescence collective | A group of individual flowers. The grouping can take many forms, such as a spike (flowers closely packed along a vertical stem, e.g. snapdragons), an umbel or corymb (flowers forming a flattened dome, e.g. yarrow), a panicle (a complex hierarchical arrangement of flowers, e.g. hydrangeas), or a capitulum (tightly packed disc flowers, e.g. the center of a daisy). |
| inoculation | The introduction of a pathogen to a host plant’s tissues. |
| inoculum | The parts of a pathogen that infect plants. |
| insectary plant | A plant that attracts beneficial insects. |
| insecticidal soap | A specially formulated soap that is only minimally damaging to plants, but kills insects. These usually work by causing an insect’s outer shell to crack, resulting in drying out of its internal organs. |
| insecticide | Any material that kills insects. Includes numerous botanical products, both organic and synthetic. |
| insectivorous plant | A plant that captures and digests insects as a source of nutrition. |
| instar | The stage of an insect’s life between molts. |
| integrated pest management | A method of managing pests that combines cultural, biological, mechanical, and chemical controls, while taking into account the impact of control methods on the environment. |
| intensive gardening | The practice of maximizing use of garden space, for example by using trellises, intercropping, succession planting, and raised beds. |
| intercalary meristem | A meristem located between nondividing tissues such as at the base of a leaf. |
| intercropping | The practice of mixing plants to break up pure stands of a single crop. |
| internode | The portion of a stem between two nodes. See interstem (1) – below. |
| interplanting | See intercropping (above). |
| interstem | (1) The portion of a stem between two nodes. (2) The middle piece of a graft combination made up of more than two parts, i.e., the piece between the scion and the root stock. |
| interstock | The middle piece of a graft combination made up of more than two parts, i.e., the piece between the scion and the root stock. See interstem (2) – above. |
| invasive | Growing vigorously and outcompeting other plants in the same area; difficult to control. |
| ion | An atom or molecule with either positive or negative charges. See anion, cation. |
| isoptera | An insect family made up of species having soft bodies, strong mandibles (mouth parts), and well-developed claws. Workers and soldiers are wingless and sterile. Termites are an example. |
