Supplying soil and roots with air or oxygen. Hydroton is an example of a medium with excellent aeration. In
some hydroponic systems, a nutrient solution is aerated by the output of a submersible pump or air stone.
A system in which the roots of a plant are suspended in air and are consistently or intermittently misted with
fine droplets of nutrient solution.
A solution with a low pH; an acidic solution has a pH below 7.
A solution with a high pH; any pH over 7 is considered alkaline.
Fertilizer with a balanced blend of N-P-K (i.e. 5-5-5)
Alternating Current (AC)
An electric current in which the movement, or flow, of electric charge periodically reverses direction.
Fortifying soil by adding organic or mineral substances in order to improve texture, nutrient content, or
The unit used to measure the strength of an electric current.
A plant that completes its entire life cycle in one year or less. Most plants fall under this category.
The luminous discharge of electricity (light) between two electrodes.
A transfer of electricity across two electrodes (anode and cathode), characterized by high electrode current
densities and a low voltage drop at the electrode.
A quartz container for luminous gases that also houses the arc in HID lights.
The classification of plant hormones that are responsible for foliage and root elongation.
Extremely small, one celled organisms. There are beneficial and non-beneficial bacteria.
An auxiliary piece of equipment designed to start and to properly control the flow of power to gas discharge in
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.
An insect that eats plant-eating insects.
Able to decompose or break down through natural bacterial or fungal action, substances made of organic
matter are bio-degradable.
A nutrient additive that is generally high in phosphorus (P), and sometimes Potassium (K), that increases
The term used to describe a plant that has gone to seed prematurely.
The function of this micronutrient is not well understood, but it is suspected that it might aid carbohydrate
Electrical circuit box having on/off switches rather than fuses.
An industry code indicating that the bulb is to be operated only in a base up position.
A withering condition that attacks flower buds, caused by the Tobacco Ringspot Virus (TRSV). The terminal
buds can be bent over to form a characteristic hook, and may become brown and fall off. The pith may develop
a brown discoloration, and leaflets may be smaller than normal. Pods may have brown patches and will
To reduce shock and provide a “cushion” against pH fluctuations.
By definition, the outer glass casing that protects the arc tube of an HID lamp.
Bulb Wall Temperature
The temperature at the bulb wall of a lamp, which effects lumen output and input wattage and is important in
A rounded, underground, storage organ that contains the shoot of a new plant. A bulb consists of a short stem
surrounded by fleshy scales (modified leaves) that store nutrients for the new plant. Tulips, lilies, and onions
grow from bulbs.
Leaf tips that turn yellow or brown from excess fertilizer and/or salts.
Calcium is a macronutrient. 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.
The pod containing the female ovule and two protruding pistils; the sepals.
A unit of luminous intensity in a given direction, equal to one lumen per steradian.
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
Neutral compound of carbon, hydrogen, and oxygen. Sugar, starch, and cellulose are carbohydrates.
Capable of killing, eating away, or otherwise destroying something by chemical activity.
The base structural unit that plants are made of. Cells contain a nucleus which houses its DNA.
A complex carbohydrate that stiffens a plants tissue.
Cubic Feet per Minute
Combining nutrients in an atomic ring that is easy for plants to absorb. Beneficial bacteria and fungi do this
naturally by consuming nutrients, digesting them and excreting them back out as a new chelated nutrient.
As a concentrate, it is a chemical used to purify water. As a micronutrient, chlorine is essential for
photosynthesis, where it acts as an enzyme activator during the production of oxygen from water.
A plastid (a small, double-membrane organelle of plant cells) containing chlorophyll.
The condition of a sick plant with yellowing leaves due to inadequate formation of chlorophyll. Chlorosis is
caused by nutrient deficiency, usually iron or nitrogen, often due to imbalanced pH.
A compact soil made of very fine organic mineral particles. Clay is not suitable for container gardening.
The average condition of the weather in a garden room or outdoors.
A plant produced through asexual reproduction including, but not limited to, cuttings, layering, and tissue
Cold Start Time
The length of time required to bring an HID lamp to 90% light output from a cold condition.
The band of colors (measured in NM) emitted by a light source.
The relative whiteness of a piece of tungsten steel heated to that temperature in degrees Kelvin.
Soil condition that results from tightly packing soil. Compacted soil allows for only marginal aeration and root
Planting garlic, marigolds, etc. along with other plants to discourage insect infestation.
A mixture of decayed organic matter, such as beneficial bacteria and fungi.
A “tea” brewed from compost that is full of live beneficial bacteria and fungi. Compost tea is great for foliar
feeding and application to the root zone, as beneficial organisms will fight out unwanted pathogens and assist
in nutrient uptake.
To soak new Rockwool in an acidic solution to lower the pH from 8.0 to 5.5.
A bulb of a certain spectrum type (i.e. Metal Halide) specially designed to operate while used in the
fixture/ballast of a different type (i.e. High Pressure Sodium). The most popular conversion bulbs are Metal
Halide conversion bulbs, which allow one to have the halide spectrum while using an HPS ballast.
This micronutrient is an internal catalyst and acts as an electron carrier; it is also believed to play a role in
The transformer in the ballast is referred to as the core in HID lighting systems.
Corms, Rhizomes, and Tubers
Dormant stems planted in the fall for spring blooms, or forced indoors for winter blooms. Common varieties are
Dahlias and Irises.
Energy storage components of a seed that feeds the plant before the emergence of its first true leaves.
Pollinating two plants that have different genetics.
A measurement of volume, in feet. L’ x W’ x H’ = Cubic feet
A growing tip cut from a parent plant for asexual propagation, also called a clone.
Disease that attacks young seedlings and cuttings, causing stems to rot at the base; overwatering is the main
cause of damping-off.
To exhaust soil of nutrients, making it infertile.
To cause to dry up. Insecticidal soap desiccates insects.
Having distinct male and female organs on different plants within the same species.
Direct Current (DC)
An electric current that flows in only one direction.
The part of an HID outer bulb that is opposite the neck and threads.
The spring-like brackets that mount the arc tube within the outer envelope of an HID lamp.
A way to empty grow medium of excess water. With good drainage, water passes through media evenly.
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.
If you drew a line around a plant directly under its outermost branch tips, that would be the drip line. Roots
seldom grow beyond the drip line.
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.
A cold, white substance that is formed when carbon dioxide is compressed and cooled. Dry ice changes into
CO2 gas at room temperature.
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.
A conductor used to establish an electrical arc or contact with the non-metallic part of a circuit.
Outer protective bulb or jacket of a lamp.
The point at which the sun crosses the equator and day and night are each 12 hours long. The equinox occurs
twice a year, in spring and fall.
To give nutrients to the plant via roots or foliage.
Pistillate, ovule, seed-producing.
To fertilize and irrigate at the same time.
Over fertilization. First leaf tips turn yellow and brown, and then curl.
Flood and Drain
See EBB AND FLOW.
Electric lamp using a tube filled with fluorescent material, which has a low heat output.
The leaves or more generally, the green part of a plant.
Misting plants with nutrient solution, which is absorbed by the foliage.
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.
The number of waves or cycles of electromagnetic radiation per second, usually measured in Hertz (Hz).
Fungicide or Fungistat
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.
Electrical safety device consisting of a metal that melts and interrupts the circuit when the circuit is overloaded.
A box containing fuses that control the electric circuits.
The part of a chromosome that influences the development of a plant. Genes are inherited through sexual
Genetic Make Up
The set of genes inherited from parent plants.
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.
Gallons Per Hour
Gallons Per minute
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
One plant having both male and female organs. The breeding of these types of plants is hard to control.
A unit of frequency that cycles one time each second. Example: A home with 60 Hertz AC current cycles 60
times per second.
High Intensity Discharge
High Intensity Discharge Lamp (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 (HPS 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 for their plants.
A sticky, honey like substance secreted into foliage by aphids, scale, and mealy bugs.
Reflective cover of an HID lamp.
An industry code indicating that the bulb is to be operated in a horizontal position.
Chemical substance that controls the growth and development of a plant. Root-inducing hormones help
Hot Start Time
The length of time required to bring an HID lamp to 90% light output after a short power interruption.
Humidity (Relative Humidity)
The ratio between the amount of moisture in the air and the greatest amount of moisture the air could hold at
the same temperature.
Dark, fertile, partially decomposed plant or animal matter. Humus forms the organic portion of the soil.
The offspring from two plants of different breeds, variety or genetic make-up.
Instantly soluble lime, used to raise pH or sweeten soil.
Light or colorless, odorless gas. Hydrogen combines with oxygen to form water.
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 foot-candles (fc).
Inbred (True Breed)
The offspring of plants of the same breed or ancestry.
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. 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-towatt
ratio). They also put off far too much heat per watt to use in horticulture, even if the above-mentioned
problems did not exist.
Chemically non-reactive. Inert growing mediums make it easy to control the chemistry of the nutrient solution.
The magnitude of the light energy per unit. Intensity diminishes the farther away you get from the light source.
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.
The protective outer bulb or envelope of a lamp.
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. A standard clear metal halide
HID lamp has an average Kelvin temperature rating of 6,000K.
A unit of electric power usage equal to 1,000 watts. 1kW = 1,000w
A measure of electricity used per hour. A 1000-watt HID bulb uses one kilowatt in one hour.
A beneficial insect that preys on aphids.
An electrically energized source of light, commonly called a bulb or tube.
Lamp Lumen Depreciation (LLD)
The decrease over time of lamp lumen output, caused by bulb wall blackening, phosphor exhaustion, filament
depreciation, and other factors.
To dissolve or wash out the soluble components of soil by heavy watering.
Leaf malformation due to overwatering, over fertilization, lack of magnesium, insect or fungus damage or
A small immature leaf.
The external part of a plant attached to branches and stems for the purpose of taking in light from the sun’s
energy. They do this with chloroplasts in the cells which contain chlorophyll.
Abnormally tall internode space, with sparse foliage. “Leggyness “ is usually caused by lack of blue light or
CO2. This can also be caused by too much nitrogen.
A series of growth stages through which a plant must pass in its natural lifetime. The stages for an annual plant
are seed, seedling, vegetative, and floral.
A motorized device that moves a lamp back and forth or in a circle across the ceiling of a garden room to
provide more even distribution of light.
Used in the form of dolomite or hydrated lime to raise and/or stabilize pH.
Chemically sensitive paper used to test pH.
Organic soil mixture of crumbly clay, silt, and sand.
A measurement of light output. One lumen is equal to one foot-candle (see Foot-Candle).
A standard unit of Illuminance. One lux is equal to one lumen per square meter.
The primary macronutrients are Nitrogen (N), Phosphorous (P), and Potassium (K), with the secondary
macronutrients being Calcium (Ca) and Magnesium (Mg).
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.
In regard to HID lighting, the average throughout life. Example: HID’s are rated in mean lumens.
The substrate or soilless material which supports the plant and absorbs and releases the nutrient solution in
The tip of a plants growth.
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.
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.
Also referred to as trace elements, micronutrients include Sulfur (S), Iron (Fe), Manganese (Mn), Boron (B),
Molybdenum (Mo), Zinc (Zn), Copper (Cu), and Chlorine (Cl).
One Thousandth of a meter. Approximately one twenty-fifth of an inch.
An electronic device that measures the exact moisture content of soil at any given point.
This micronutrient is essential for nitrogen fixation and nitrate reduction.
Producing only one color. LP Sodium lamps are monochromatic.
A protective covering of organic compost, leaves, etc. Indoors, mulch keeps soil too moist and could possibly
result in fungus.
One billionth or .000000001 of a meter. NM is used as a scale to measure wave lengths of light. Color and light
spectrums are expressed in Nanometers (NM).
The dying of plant tissue, usually the result of serious nutrient deficiency or pest attack.
Tubular glass end of an HID bulb, attached to the threads.
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.
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
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.
The mixture of water and water-soluble nutrients which is provided to the plants for nourishment in a
Ohm’s Power Law
A law that expresses the strength of an electric current. Watts equals Volts times Amps. Which can also be
mathematically switched to find the value of either of the three measurements. Volts equals Amps divided by
Watts, and Amps equals Watts divided by Volts.
Made of, derived from, or related to living organisms. Organic means “a molecule or substance that contains
carbon”. Natural refers to something not man-made, but not carbon based. Rocks are natural, dead plant matter
would be organic.
A plant’s egg found within the calyx, it contains all of the female genes. When fertilized, an ovule will grow into
Tasteless and colorless element, necessary in soil to sustain plant life as well as animal life.
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.
An organism that lives on or in another host organism. Fungus is a parasite.
Partially decomposed vegetation (usually moss) with slow decay due to extreme moisture and cold.
A plant, such as a tree or shrub, which completes its life cycle over several years.
A sand or volcanic glass which has been expanded by heat. Perlite holds water and nutrients on its many
irregular surfaces. Perlite is also used as a soil amendment, providing better aeration and drainage.
pH (Potential Hydrogen)
A scale from 1 to 14 that measures the acid to alkaline balance of a growing medium (or any other substance).
In soil, plants generally grow best in a pH range of 6.3 to 6.8; in hydroponics plants generally grow best in a pH
range of 5.8 to 6.3. If the pH is not within the acceptable range, nutrients may not be absorbed to maximum
capacity or can be locked out completely.
pH Meter or Tester
Electronic instrument or chemical used to find where soil or water is on the pH scale.
Internal bulb coating that diffuses light and is responsible for variations in color outputs.
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.
The study of light, especially in regard to color.
The relationship between the length of light and dark in a 24 hour period.
The building of chemical compounds (carbohydrates) from light energy, water, and carbon dioxide.
The specific movement of a plant part towards a light source.
The substance in a paint or anything that absorbs light, producing (reflecting) the same color.
Fine, dust like microspores containing male genes.
Potassium promotes disease resistance and good development of carbohydrates, starches and sugars, and it
increases fruit production. Potassium is a macronutrient.
Interruption or change in intensity of electricity.
Sexual propagation: to produce seeds by breeding different male and female flowers.
Asexual propagation: to produce plantlets (also known as CLONES) by taking cuttings.
To alter the shape and growth pattern of a plant by cutting stems and shoots.
PVC Pipe (PolyVinyl Chloride)
Plastic pipe that is easy to work with, readily available and used to pipe water into a garden room.
Natural insecticide made from the blossoms of various chrysanthemums.
The term sometimes used to refer to the reflective hood of an HID lamp.
To restore youth. A mature plant that has completed its life cycle (flowering), can be stimulated to rejuvenate or
produce new vegetative growth by a starting a new 18 hour photoperiod.
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, spun like “cotton candy”, and formed into slabs, cubes and
Roots stifled or inhibited from normal growth, by the confines of a container.
Their purpose is to anchor a plant and provide a means in which to feed and hydrate the plant.
Crystalline compound that results from improper pH or toxic buildup of fertilizer. Salt will burn plants,
preventing them from absorbing nutrients.
Calcium (Ca) and Magnesium (Mg) are referred to as secondary macronutrients.
A dry calyx containing a mature or maturing seed.
Condition that results when wires cross and form a circuit. A short circuit will blow fuses.
Threaded, wired receptacle for a bulb.
Able to be dissolved in water.
Seed like offspring of a fungus.
A recently germinated seed, or small new growth of a leaf or stem.
Square Feet (Sq. Ft.)
Length (in feet) times width equals square feet.
Male, pollen producing.
Complex carbohydrate. Starch is manufactured and stored in food.
A unit solid angle on the surface of a sphere equal to the square of the sphere’s radius.
To make sterile by completely removing dirt, germs, and bacteria.
Small mouth like or nose like openings (pores) on a leaf’s underside, responsible for transpiration and many
other life functions. The millions of stomata must be kept very clean to function properly.
A physical or chemical factor that causes extra exertion by plants. A stressed plant will not grow as well as a
non stressed plant.
A quick pulsating or flashing of a lamp.
Food product of a plant. Carbohydrates that contain a hydrocarbon chain.
Production of a substance, such as chlorophyll, by uniting light energy and elements or chemical compounds.
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.
The main or primary root that grows from the seed. Lateral roots will branch off the tap root.
Warm, 70 to 80 degrees Fahrenheit. Always use tepid water around plants to facilitate chemical processes and
ease of shock.
Bud at the growing end of the main stem.
Cull or weed out weak, slow growing seedlings. Also referred to as “thinning out”.
A device in the ballast that transforms electric power from one voltage to another.
To give off water vapor and by products via stomata and carbon dioxide intake at the leaves.
Frame or netting (lattice) that trains or supports plants.
A heavy, hard metal with a high melting point which conducts electricity well. Tungsten is used for a filament in
tungsten halogen and incandescent lamps.
Stands for ‘Universal’. An industry code indicating that the bulb can be operated in any position: horizontal,
vertical, base up, or any other.
UL (Underwriters Laboratories)
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.
Light with very short wave lengths, out of the visible spectrum, past the blue-violet.
Opening such as a window or door that allows the circulation of fresh air.
Circulation of fresh air, fundamental to a healthy indoor garden. An exhaust fan creates excellent ventilation.
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.
A unit used to measure electric power. One watt equals one joule/second.
Compound that reduces the droplet size and lowers the surface tension of the water, making it wetter.
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.
Like copper and manganese, zinc is linked to chlorophyll synthesis.