Harden-Off
To gradually acclimatize a plant to a more harsh environment. A seedling must me hardened-off before planting outdoors.

H.I.D. (High Intensity Discharge) Lights
Lights that produce very intense light using an electric arc with metallic vapor inside a quartz arc tupe surrounded by a glass outer bulb that prevents ultraviolet rays from being radiated. These horticultural lights produce 5-6 times as much light compared to incandescent lights of the same wattage and last 10-30 times as long. These durable systems have a life span of 15+ years.

Hormone
Chemical substance that controls the growth and development of a plant. Root-inducing hormones help cuttings root.

Humidity (Relative)
The ratio of water vapor density (mass per unit volume) to the saturation water vapor density. Basically the ratio between the amount of moisture in the air and the amount of moisture the air could hold at that same temperature.

Humus
A brown or black organic substance consisting of partially or wholly decayed vegetable or animal matter that provides nutrients for plants and increases the ability of soil to retain water.

Hybrid
The offspring from two plants of different breeds, variety, or genetic make-up.

Hyrdrated Lime
Instantly soluble lime, used to raise or lower pH.

Hygrometer
An instrument for measuring relative humidity in the atmosphere.

Hydrogen Peroxide (H2O2)
A clear sharp smelling substance very similar in appearance to water. Like water it is made up of Hydrogen and Oxygen, however H2O2 has an extra Pxygen atom in an unstable arrangement. It is this extra atom that gives H2O2 it's useful properties. Hydrogen is useful in hydroponics for pretreating water.

Hydroponics
Cultivation of plants in nutrient solution rather than in soil.

Inbred (True Breed)
Offspring of plants of the same breed or ancestry

Inert
Chemically non-reactive; inert growing mediums make it easy to control the chemistry of the nutrient solution.

Inline Fans
Inline fans have the motor and blade inside a housing that's usually circular. They are useful for moving air from one room to another, or through walls, or ceilings to outside. They may be used with short lengths of ducting to control direction of air flow. These are the most commonly used fans for exhaust vents, at the top of the growing area. This draws out the hot, stale air. Inline fans have the advantage of being thin. They may be place directly into walls or ceilings. They may also be used to send air through short lengths of ducting.

Insects
There are several types of insects that attack plants. These include; aphids, caterpillars, cutworms, fungus gnats, leaf miners, nematodes, spider mites, thrips, and whiteflies.

Intensity
The magnitude of the light energy per unit; intensity diminishes the farther away from the source.

Iron (Fe)
This micronutrient acts as a catalyst in the photosynthesis/respiration process, and is essential for the formation of sugars and starches. Iron also activates certain other enzymes.

Necrosis
The dying of plant tissue, usually the result of serious nutrient deficiency or pest attack.

Nitrogen (N)
Nitrogen is used in various forms to promote rapid vegetative growth, leaf, flower, fruit, and seed development, and chlorophyll development; and to increase the protein content in plants.

Nitrification
The process by which soil organisms use oxygen to convert ammonium ions to nitrate ions.

Nitrogen (N)
Nitrogen is used in various forms to promote rapid vegetative growth, leaf, flower, fruit, and seed development, and chlorophyll development; and to increase the protein content in all plants.

Non-labile
A soil nutrient which is tied up in the soil and unavailable for plant uptake.

Nutrient Film Technique (NFT)
A hydroponic method in which nutrient is fed into grow tubes or trays in a thing film where roots draw it up. This "nutrient film" allows the roots to have constant contact with the nutrient and the air layer above at the same time.

Nutrient Solution
The mixture of water and water-soluble nutrients which is provided to the plants for nourishment in a hydroponic system.

Nutrients
Elements such as nitrogen and iron that are needed for plant growth and health.

Nutrients, Secondary
The elements other than N, P, and K which are considered nearly as important as N, P, and K but are not considered micronutrients. Calcium (Ca), magnesium (Mg), and sulphur (S) are the elements usually referred to by this term.

Parts Per Million (PPM)
The ratio figure that represents the amount of one substance that is in one million parts of another substance; often used to describe the relative concentrations of nutrient solutions.

PAR
Photosynthetically Active Radiation, the part of the sunlight spectrum that plants use

Passive
A hydroponic system without a pump. The plants take up nutrients through a fibrous wick from the pot to the nutrient solution.

Perlite
A white, grasslike volcanic mineral, about one-tenth the weight of sand, that is useful as a medium for rooting cuttings but lacks any nutrients.

pH
The pH scale is a way to measure the Acid or Akaline qualities of water. The offical definition of pH is: a unit of measure which describes the degree of acidity, or alkalinity of a solution. It is measured on a scale of 0-14. The term pH is dervied from "p" the mathematical symbol of the negative logarithm, and "H", the chemical symbol of Hydrogen. The folrmal definition of pH is the negative logarithm of the Hydrogen ion activity {pH=-log[H=]}. pH expresses the degree of an activity of an acid or base in terms of hydrogen ion activity. In general,plants grow best in a pH range of 6 to 6.8; 6.3 is considered ideal. If the pH is not within the acceptable range, nutrients may not be absorbed to maxium capacity.

Photoperiod
The duration of an organism's daily exposure to light, considered especially with regard to the effect of the exposure on growth and development.

Phosphorus (P)
Phosphorus promotes and stimulates early growth, blooming, and root growth. It hastens maturity and seed growth, and contributes to the general hardiness of plants. Phosphorus is a macronutrient.

Photosynthesis
The process in green plants and certain other organisms by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source. Most forms of photosynthesis release oxygen as a byproduct.

Pistil
The female, ovule-bearing organ of a flower, including the stigma, style, and ovary.

Pollen
The fine powderlike material, consisting of pollen grains, that is produced by the anthers of seed plants.

Pollination
The carrying of pollen grains to the female sex cells for fertilization.

Potassium (K)
Potassium promotes disease resistance and good development of carbohydrates, starches, and sugars. It also increases fruit production. Potassium is a macronutrient.

Propagate
1.Sexual: To produce seed by breeding different male and female flowers. 2. Asexual: To produce plantlets (also known as clones) by taking cuttings.

Pyrethrum
Natural insecticide made from the blossoms of various chrysanthemums.

Tap Root
The main or primary root that grows from the seed; lateral roots will branch off the tap root.

Transformer
A device in the ballast that transforms electric power from one voltage to another.

Transpire
Give off water vapor and by products via stomata and carbon dioxide intake at the leaves.

Trellis
Frame of small boards (lattice) that trains or supports plants.

Top-dressed
Uniform broadcast application of fertilizer to the soil surface after a crop or pasture has become established.

Tungsten
A heavy, hard metal with high melting point which conducts electricity well; tungsten is used for a filament in tungsten halogen and incandescent lamps.