An acidic solution has a pH below 7.


Supplying soil and roots with air or oxygen. Geolite is an example of a medium with excellent aeration. In some hydroponic systems, a nutrient solution is aerated by the output of an aquarium pump.


A system in which the roots of a plant are consistently or intermittently misted with fine droplets of nutrient solution.


any pH over 7 is considered alkaline.

Ampere (amp)

The unit used to measure the strength of an electric current.


The luminous discharge of electricity between two electrodes in HID lighting.

Arc Discharge

A transfer of electricity across two electrodes (anode and cathode), characterized by high electrode current densities and a low voltage drop at the electrode.

Arc Tube

The enclosure which contains the luminous gases and also houses the arc.


An auxiliary piece of equipment designed to start and to properly control the flow of power to gas discharge light sources such as fluorescent and high intensity discharge lamps. In metal halide systems, it is composed of the transformer, capacitor and connecting wiring; sodium systems require an ignitor in addition to the transformer and capacitor.

Bloom Booster, Blossom Booster

Fertilizer high in phosphorus (P) that increases flower yield.

Boron (B)

The function of this micronutrient is not well understood, but it is suspected that it might aid carbohydrate transport.


An industry code indicating that the bulb is to be operated only in a base up position.


The glass outer envelope component of an HID lamp which protects the arc tube.

Bulb Wall Temperature

The temperature at the bulb wall of a lamp, which effects lumen output and input wattage and which is important in lighting calculations.


Leaf tips that turn dark from excess fertilizer and salt burn.

Calcium (Ca)

Calcium is vital in all parts of plants to promote the translocation of carbohydrates, healthy cell wall structure, strong stems, membrane maintenance and root structure development. Calcium is a macronutrient.

Candela (CD)

A unit of luminous intensity in a given direction, equal to one lumen per steradian.

Candlepower (CP)

The luminous intensity of a light source, as expressed in candelas.

Candlepower Distribution Curve

A curve that represents the varying distribution of luminous intensity of a lamp or luminaire.


An electronic device that can store electrical charge. The capacitor is one of the main components of an HID lighting ballast. Because they can store a very strong electrical charge, capacitors can be very dangerous to someone who is unaware of this fact and opens a ballast in order to examine or repair it. If one does not know how to safely discharge the stored electricity, one should allow a trained technician to do any ballast repairs.

Carbon Dioxide (CO₂)

A colorless, odorless, tasteless gas in the air necessary for plant life. Occurs naturally in the atmosphere at .03%.

Chlorine (Cl)

This micronutrient is essential for photosynthesis, where it acts as an enzyme activator during the production of oxygen from water.


The condition of a sick plant with yellowing leaves due to inadequate formation of chlorophyll. Chlorosis is caused by a nutrient deficiency, usually iron or nitrogen; nutrient deficiencies are themselves often caused by a pH that is out of the acceptable range.


A plant produced through asexual reproduction including, but not limited to, cuttings, layering and tissue culture.


To soak new Rockwool in an acidic solution to lower the pH from 8.0 to 5.5.

Copper (Cu)

This micronutrient is an internal catalyst and acts as an electron carrier; it is also believed to play a role in nitrogen fixation.

Cold Start Time

The length of time required to bring an HID lamp to 90% light output from a cold condition.

Color Temperature Or Kelvin Temperature

The unit of measurement to express the color (spectrum) of light emitted by a lamp; the absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source (see correlated color temperature).

Conversion Bulb

A bulb of a certain spectrum type (e.g. sodium) specially designed to operate while used in the fixture/ballast of a different type (e.g. metal halide). The most popular conversion bulbs by far are sodium conversion bulbs, which allow one to have the sodium spectrum while still using a metal halide system.

Correlated Color Temperature (CCT)

A specification of the color appearance of a light source, relating its color to that of a blackbody radiator, as measured in Kelvins (K). CCT is a general measure of a lamp\’s “coolness” or “warmness.”

Damping-off Fungus

Disease that attacks young seedlings and cuttings, causing stems to rot at the base; overwatering is the main cause of damping-off.

Discharge Lamp

A lamp that produces light by discharging an electric arc through a mixture of gases and gaseous metals.

Dissolved Solids Or Total Dissolved Solids

The amount of dissolved solids, usually fertilizer salts, that are measured in water in parts per million.


The portion of an HID outer bulb located opposite base (the neck and threads).

Dome Support

The spring-like brackets which mount the arc tube within the outer envelope (bulb).

Drip Aeration

A hydroponic method wherein air pressure from a small air pump is used to percolate nutrient solution out through a ring of feeder tubing which encircles the plant.

Drip System (Drip Emitter System)

A very efficient watering system that employs a main hose with small water emitters. Water is metered out of the emitters, one drop at a time.

Ebb-and-flow (or Flood And Drain)

A hydroponic system in which the medium, usually aggregate pebbles, is periodically flooded with nutrient solution and then drained again, feeding and aerating the medium and root system.


Filaments located at either end of a discharge lamp that maintain an electrical arc between them. See arc discharge.


The electrical fitting used to contain the electric components of a lighting system.

Fluorescent Lamp

A discharge lamp in which a phosphor coating transforms ultraviolet energy into visible light. Fluorescent lamps are good for starting seedlings and rooting cuttings, but do not have enough intensity to sustain aggressive growth in plants in the later stages of life, and are not efficient enough in their conversion of electrical power to lumens of light output.


A standard measurement of light intensity, representing the amount of illuminance on a surface one foot square on which there is a uniformly distributed flux of one lumen. More simply, one footcandle of illuminance is equal to the light emitted by one candle at a distance of one foot.


The number of waves or cycles of electromagnetic radiation per second, usually measured in Hertz (Hz).

Foliar Feeding

Misting plants with fertilizer solution, which is absorbed by the foliage.


A product that destroys or inhibits fungus.


Any of a major group (Fungi) of saprophytic and parasitic spore-producing organisms usually classified as plants that lack chlorophyll and include molds, rusts, mildews, smuts, mushrooms, and yeasts. Common fungal diseases that attack plants are “damping-off,” Botrytis, and powdery mildew.


One of several brand names/varieties of clay aggregate medium (also known as LECA for light expanded clay aggregate). It is a lightweight, porous substrate with excellent aeration. Because it does not really wick water effectively, Geolite and other LECA mediums are favorites in ebb-and-flow and drip hydroponic systems.


The process of causing the initiation and development of a plant from seed.

Halogen Lamp

A short name for the tungsten-halogen lamp. Halogen lamps are high pressure incandescent lamps containing halogen gases such as iodine or bromine which allow the filaments to be operated at higher temperatures and higher efficacies. While excellent for home lighting and similar applications, halogen lamps are not effective or efficient as grow lights due to their limited spectrum and high operating temperatures.


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


The popular acronym for High Intensity Discharge.

High-Intensity Discharge (HID) Lamp

A general term for mercury, metal halide and high-pressure sodium lamps. HID lamps contain compact arc tubes which enclose various gases and metal salts operating at relatively high pressures and temperatures.

High-pressure Sodium Lamp

High-pressure sodium lamps operate by igniting sodium, mercury and xenon gases within a sealed ceramic arc tube. Sodium lamps emit light energy in the yellow/red/orange regions of the spectrum; the red spectrum stimulates flowering and fruit production. Many indoor gardeners switch to sodium lamps when it is time to induce flowering or fruiting of their plants.


The reflective cover used in conjunction with an HID lamp. The more reflectivity a hood can provide, the more effective it is.


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


An industry code indicating that the bulb is to be operated in a horizontal position.

Hot Spot

The area immediately under an HID lamp where the light intensity is strongest. Hot spots cause uneven growth, but can be remedied by using light movers.

Hot Start Time

The length of time required to bring an HID lamp to 90% light output after a short power interruption.


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

Hydrated Lime

Instantly soluble lime, used to raise or lower pH.


An instrument for measuring relative humidity in the atmosphere.


A component of the ballast necessary for the starting of the bulb in sodium systems.


The density of incident luminous flux on a surface; illuminance is the standard metric for lighting levels, and is measured in lux (lx) or footcandles (fc).


The act of illuminating or state of being illuminated. This term is often used incorrectly in place of the term illuminance to denote the density of luminous flux on a surface.

Incandescent Lamp

A light source which generates light utilizing a thin filament wire (usually of tungsten) heated to white heat by an electric current passing through it. Incandescent lamps are the most familiar type of light source, with countless application in homes, stores and other commercial settings. Light is produced by passing electric current through a thin wire filament, usually a tungsten. Incandescent lamps are totally ineffective as grow lights; they have very limited spectrum, are very inefficient in their conversion of electrical power to lumens of light output (lumen-to-watt ratio). They also put off far too much heat per watt to use in horticulture, even if the above-mentioned problems did not exist.


A term referring to the magnitude of light energy per unit; light intensity diminishes evenly as you get further 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.

Kelvin Temperature (K)

The unit of measurement to express the color (spectrum) of light emitted by a lamp; the absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source (see correlated color temperature). A standard clear metal halide HID lamp has an average Kelvin temperature rating of 4,000K.

Kilowatt (kW)

A unit of electric power usage equal to 1,000 watts.

Kilowatt Hour (kWh)

A measurement of electric energy. A kilowatt hour is equal to 1,000 watts of power used over a period of one hour.


An electrically energized source of light, commonly called a bulb or tube.

Lamp Life

A measure of lamp performance, as measured in median hours of burning time under ANSI test conditions.

Lamp Lumen Depreciation (LLD)

The decrease over time of lamp lumen output, caused by bulb wall blackening, phosphor exhaustion, filament depreciation, and other factors.

Lamp Starting

Generic term used to describe a discharge lamp’s starting characteristics in terms of time to come to full output, flicker, etc.


Radiant energy which can be sensed or seen by the human eye. The term generally applied to the visible energy from a source. Light is usually measured in lumens or candlepower. When light strikes a surface, it is either absorbed, reflected or transmitted. Visible light is measured in lumens.

Light Mover (e.g. Hydrofarm’s Light Track)

A motorized device which moves an HID lamp back and forth across the ceiling of a grow room to provide more even distribution of the light.


A measurement of light output; refers to the amount of light emitted by one candle that falls on one square foot of surface located at a distance of one foot from the candle.


A complete lighting unit, consisting of a lamp or lamps together with the components required to distribute the light, position the lamps, and connect the lamps to a power supply. Often referred to as a “fixture.”


A standard unit of illuminance. One lux is equal to one lumen per square meter.

Leaf Curl

Leaf malformation due to overwatering, over fertilization, lack of magnesium, insect or fungus damage or negative tropism.


The primary nutrients N-P-K or the secondary nutrients magnesium and calcium.

Manganese (Mn)

This micronutrient activates one or more enzymes in fatty acid synthesis; it also activates the enzymes responsible for DNA and RNA production. Closely associated with copper and zinc, manganese also participates directly in the photosynthetic creation of oxygen from water.


The substrate or soilless material which supports the plant and absorbs and releases the nutrient solution in hydroponic horticulture.

Mercury Vapor Lamps

The oldest member of the HID family, mercury vapor lamps work by arcing electricity through mercury vapor. While more efficient than incandescent, halogen and fluorescent lamps, mercury vapor lamps have the least efficient lumen-to-watt ratio of the entire HID family. This, combined with an improper color spectrum for horticultural applications, makes mercury vapor lamps a poor choice for a grow light.

Metal Halide Lamp

A high-intensity-discharge lamp in which the light is produced by arcing electricity through a mixture of metal halides. The light produced by metal halide lamps is in the white-blue spectrum, which encourages vegetative growth and “bushiness” while discouraging upward growth. This is the bulb to use in the first, vegetative phase of plant growth.


Also referred to a TRACE ELEMENTS, including S, Fe, Mn, B, Mb, An and Cu.

Molybdenum (Mo)

This micronutrient is essential for nitrogen fixation and nitrate reduction.


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

NFT (Nutrient Film Technique)

A hydroponic method in which nutrient is fed into grow tubes or trays in a thin film where the 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.

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.

Nutrient Solution

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


The elements needed by plants for normal growth and health. The major nutrients (MACRONUTRIENTS) are nitrogen (N), phosphorus (P), and potassium (K), but there are numerous MICRONUTRIENTS (also called TRACE ELEMENTS) which also have integral roles in maintaining plant health. A good quality hydroponic nutrient formula will contain all of the major nutrients and micronutrients needed by the vast majority of plants.

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.

Parabolic Reflector

A lighting distribution control device that is designed to redirect the light from an HID lamp in a specific direction. In most applications, the parabolic device directs light down and away from the direct glare zone.


Sand or volcanic glass which has been expanded by heat; perlite holds water and nutrients on its many irregular surfaces.Mineral soil amendment.


A scale from 1 to 14 that measures the acid to alkaline balance of a growing medium (or any other substance). 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 maximum capacity.

Phosphorus (P)

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


Day length; the relationship between the length of light and dark in a 24 hour period.


The process by which plants use light energy to collect carbon dioxide from the atmosphere and convert it to chemical energy in the form of sugar.


The gravitation of a plant part toward a light source.

Potassium (K)

Potassium promotes disease resistance and good development of carbohydrates, starches and sugars, and it increases fruit production. Potassium is a macronutrient.


Sexual propagation: to produce seed by breeding different male and female flowers.Asexual propagation: to produce plantlets (also known as CLONES) by taking cuttings.


Natural insecticide made from the blossoms of various chrysanthemums.


The measure of the reflective quality of a surface; the relative ability of a given surface to reflect light away from it without absorbing, diffusing or otherwise compromising the light’s quality, intensity and spectrum.


The term sometimes used to refer to the reflective hood of an HID lamp.


The container in a hydroponic system which holds nutrient solution in reserve for use.


Inert, soilless growing medium consisting of woven, thin strand-like fibers made from molten volcanic rock and limestone, which is heated to over 2900 degrees F, extruded, and formed into slabs, cubes and blocks.

Secondary Nutrients

Calcium (Ca) and magnesium (Mg) are considered to be the secondary nutrients.

Sodium Lamp (high-pressure Sodium Lamp)

High-pressure sodium lamps operate by igniting sodium, mercury and xenon gases within a sealed ceramic arc tube. Sodium lamps emit light energy in the yellow/red/orange regions of the spectrum; the red spectrum stimulates flowering and fruit production. Many indoor gardeners switch to sodium lamps when it is time to induce flowering or fruiting of their plants.

Specular Reflection

The redirection of incident light without diffusion at an angle that is equal to and in the same plane as the angle of incidence. The specular inserts included in Hydrofarm’s HID lighting systems work on this principle.


The component in the ballast that transforms electric current from one voltage to another.


Used in reference to a disease within the plant tissue, not initiated from the external cells. Also refers to materials and compounds which are taken up or absorbed by the plant and designed to fight disease (e.g. systemic fungicide).

Underwriters Laboratories (UL)

A private organization which tests and lists electrical (and other) equipment for electrical and fire safety according to recognized UL and other standards. A UL listing is not an indication of overall performance. Hydrofarm offers the only complete line of growlight systems that is UL-listed.


Mica which has been processed and expanded by heat. Vermiculite has excellent water-retention qualities and is a good soil amendment and medium for rooting cuttings.

Watt (W)

A unit used to measure electric power. One watt equals one joule/second.


Part of a passive hydroponic system using a wick suspended in the nutrient solution. The nutrients pass up the wick and are absorbed by the medium and roots.

Zinc (Zn)

Like copper and manganese, zinc is linked to chlorophyll synthesis.