IAA - indoleacidic acid (IAA) is one of the plant hormones called auxins. IAA is most commonly used to stimulate the growth of roots on cuttings. It is often applied as a powder to the cut end of the fresh cutting.
IBA - indolebutyric acid (IBA) is one of the plant hormones called auxins. IBA is most commonly used to stimulate the growth of roots on cuttings. It is often applied as a powder to the cut end of the fresh cutting.
imbricate - the overlapping in a sequence such as the scales in a pine cone.
inarching - method of grafting in which a plant which is still attached to its own roots, is grafted to another another plant. 
incubation period - the time between infection by a pathogen and appearance of symptoms on the plant. It may be affected by the ambient temperature and moisture levels in the area. Most fungal infections are encouraged by warm, wet weather.
indehiscent - applies to fruit which does not split open to release its seeds upon ripening. See dehiscent.
indeterminate - this term most often refers to vines that just keep on growing indefinitely. Some plants will grow to a certain size and, when the flowers on the terminal end open, growth stops.  See determinate.
indexing - this is a technique used to determine the presence of a transmissible disease, especially a virus, in a plant. Plants sold as disease free are often the result of indexing.
indicator plant - plant that reacts to a pathogen (such as a virus) or to an environmental factor with specific symptoms and is used to identify pathogens or determine the effects of environmental factors
inert ingredients - the active ingredients of a pesticide or the actual amount of nitrogen, phosphorus and potassium in a bag of fertilizer is only a fraction of the volume (and weight) in the container. The remainder of the liquid or granular material is made up of "inert ingredients." These ingredients such as clay particles may be added to provide enough volume in fertilizers so that they may be spread more evenly around the garden.

In pesticides, the inert ingredients often fall into that cloudy area called "trade secrets". They may be chemicals that help a spray stick to the leaf or spread over a surface better or water or... well, that is a secret. At times, there have been human health concerns raised about certain so-called inert ingredients in pesticides.

infection - process in which a pathogen enters, invades, or penetrates and establishes a parasitic relationship with a host plant.
inflorescence - includes the flower stem (peduncle) and its branches (pedicels) and the flower or flowers. There are many different ways that flowers are arranged in an inflorescence and that arrangement can help identify the species of the plant. See flower.
inflorescence leaves - in addition to flowers, some inflorescences also have leaves along the stem. In hostas, certain species or cultivars have flat, leaf-like structures on the flower stem.
informal garden - this is a garden or flower beds and borders that are meant to reflect a natural arrangement of plants. Informal gardens consist of wide, sweeping curves and flows of plants in irregular or rounded sweeps. See formal garden.
informal landscape plant - plants which generally fit best in an informal garden often have flower types and colors that reflect those found in natural settings. The may also be more likely to have a medium to coarse texture. See formal landscape plant.
inoculate - (n: inoculation) to expose a plant to a micro-organism i.e. inoculum. This may be an infectious organism such as a fungal spore or virus that will cause a disease or may be beneficial such as the bacteria that helps members of the Legume family extract nitrogen from the air.

See Mycorrhizae

inoculum - See above.
inorganic - generally something that is not part of a living organism. We are all carbon based organisms so this generally refers to mineral elements or compounds. See organic.
insect - refers to those six-legged creatures that crawl or fly around our gardens. Spider mites have eight legs and, therefore, are not insects.
insect growth regulator (IGR)

- some insecticides kill insects by blocking their ability to change forms in their life cycles. Many insects must lose their external skeleton (molt) as they grow to replace it with a larger one. Growth regulators may prevent this and the insect dies.

insecticidal soap

- one of the first "organic" pesticides used to combat insect problems in the garden. These are soap products that are sprayed onto the body of insects causing the waxy coating to melt and the insect to die of dehydration. As stated, they must be applied onto the insect. There is no carry over if they are just sprayed onto leaves. Obviously, they are not toxic to other organisms.

Care should be taken with soap products since they will also melt the waxy cuticle on plants if over applied. Also, plants with natural waxy coatings such as Colorado blue spruce or blue leaved hostas will lose their color if soaps are applied to them.

insecticide - any word with the suffix, icide, is a killer of something. insecticides are substances either natural or synthetic in origin which are meant to cause the death of an insect.
instar - as many insects go through the different stages of their life, they need to sluff off (molt) their current skin (exoskeleton) to be able to move up to the next bigger size or to a new form. Each of these stages is called an instar. Every insect has a specific number of instars that it will go through between egg and adult.
Integrated Pest Management (IPM) - a system of plant management that uses a variety of strategies to maintain plants in a healthy and vigorous state. For pest control, cultural, biological, physical and chemical techniques are always considered and not just "spray and pray". Generally, the least toxic approach that will provide acceptable control is used.

More on IPM

internode - the part of a stem between two adjacent nodes. The length of the internode on woody plants is often an indicator of the amount of growth that occurred a particular growing season.
interstem - normally, grafting involves uniting a scion (stem or bud) with a rootstock (or stock) to form a new plant. Sometimes the scion and the rootstock are not compatible with each other and will result in a failed graft. In these cases, success may be accomplished by putting a stem of a third plant between the scion and the root stock. This interstem needs to be compatible with both the scion on top and the rootstock below.
interveinal - the area between the veins of a leaf.
invasive - Invasive plants are those that tend to move around freely and soon are found in places that are undesirable from the gardener's point of view. They may move by rhizomes beneath the ground or they scatter a lot of seed around.

There is a thin line between a groundcover and an invasive plant. Both have similar characteristics but, depending on the site, they may be viewed very differently.

invertebrates - these are animals that have no spine such as earthworms, slugs, nematodes.
in vitro - literally means "in glass" but may also refer to any artificial environment. Tissue culture is, perhaps, the most common form of in vitro plant production.
involucre - a ring of bracts at the base of an inflorescence in such plants as members of the Asteracea (formerly Compositae) Family of plants.
- this term relates to flower petals or leaves whose margins are rolled, bent or curled inward. See revolute.
ion - an electrically charged atom or group of atoms that may be positive, as in cation (+) or negative, anion (-)


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