Glossary
Hydroponics Glossary

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Click on a term for the definition.

- A -
Acid
An acid or sour substance has a pH below 7.0.
Aeration
Supplying soil and roots with air or oxygen.
Aeroponics
Growing plants by misting roots suspended in air.
Alkaline
Refers to soil with high ph; any pH over 7.0 is considered alkaline.
Amendment
Fortifying soil by adding organic or mineral substances in order to improve texture, nutrient content, or biological activity.
Ampere (Amp)
The unit used to measure the strength of an electric current.
Annual
A plant which normally completes it entire life cycle in one year or less. Tomatoes are examples of annuals plants.
Arc
Luminous discharge of electricity (light) between two electrodes.
Arc tube
A quartz container for luminous gases also houses the arc in hid lights.
Auxin
Classification of plant hormones; auxins are responsible for foliage and root elongation.
- B -
Ballast
A device which maintains the current through a fluorescent, mercury, MH, or HPS lamp at the desired constant value, sometimes also providing the necessary starting voltage and current.
Bloom booster
A fertilizer high in phosphorus (P) which increases flower yield.
Bolt
Term used to describe a plant which has gone to seed prematurely.
Boron (B)
The function of this micronutrient is not well understood, but it is suspected that it might aid carbohydrate transport.
Breathe
Roots draw in or breathe oxygen, stomata draw in or breathe carbon dioxide
BU
An industry code indicating that the bulb is to be operated only in a base up position.
Bud blight
A withering condition which attacks flower buds.
Buffering
The ability of a substance to reduce shock and cushion against pH fluctuations.
Burn
Leaf tips which turn dark from excess fertilizer and salt.
- C -
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.
CALYX
The pod harboring female ovule and two protruding pistils, seed pod.
Carbon dioxide (CO2)
A colorless, odorless, tasteless gas in the air necessary for plant life and biomass accumulation.
Cell
The base structural unit of which plants are made; cells contain a nucleus, which houses its DNA.
Cellulose
A complex carbohydrate which stiffens a plant tissue: tough stems contain cellulose.
CFM
Cubic Feet per Minute.
Chelate
Combining nutrients in an atomic ring which is easy for plants to absorb.
Chlorine (Cl)
This micronutrient is essential for photosynthesis, where it acts as an enzyme activator during the production of oxygen from water.
Chlorophyll
Green coloring matter of leaves and plants, essential to the production of carbohydrates by photosynthesis.
Chloroplast
Containing chlorophyll
Chlorosis
The condition of a sick plant with yellowing leaves due to inadequate formation of chlorophyll chlorosis is caused by nutrient deficiency, usually iron or imbalanced ph.
Clone
A plant produced through asexual reproduction including, but not limited to, cuttings, layering, and tissue culture.
Cold start time
The length of time required to bring a high intensity discharge (HID) lamp to 90% light output from a cold condition.
Color spectrum
The band of colors (measured in nm) emitted by a light source.
Color 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. Also called Kelvin temperature. (See correlated color temperature.)
Color tracer
A coloring agent added to many commercial fertilizers, so the horticulturist knows there is fertilizer in the solution.
Conditioning
The soaking of new Rockwool in an acidic solution to lower the pH from 8.0 to 5.5.
Conversion bulb
A bulb of a certain spectrum type (e.g., sodium) specially designed to operate while used in the fixture or 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.
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.
Core
The transformer in the ballast is referred to as the core in hid lighting systems.
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."
Cotyledon
Energy storage components of a seed which feed the plant before the emergence of its first true leaves.
Cross-pollinate
Pollinating two plants having different ancestry.
Cutting
(1) Growing tip cut from a parent plant for asexual propagation (2) Clone.
- D -
Damping-off
Disease which attacks young seedlings and cuttings causing the stem to rot at the base.
Disharge lamp
A lamp which produces light by discharging an electric arc through a mixture of gases and gaseous metals.
Dissolved solids
The amount of dissolved solids, usually fertilizer salts, which is measured in water in parts per million.
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
A very efficient watering system which employs a main hose with small water emitters. Water is metered out of the emitters, one drop at a time.
- E -
Ebb and flow subirrigation
In its simplest form, there is a tray above a reservoir of nutrient solution. The tray is either filled with growing medium (clay granules being the most common) and planted directly, or pots of medium stand in the tray. At regular intervals, a simple timer causes a pump to fill the upper tray with nutrient solution, after which the solution drains back down into the reservoir. This keeps the medium regularly flushed with nutrients and air. Also called flood and drain.
Elongate
Grow in length. Envelope outer protective bulb or jacket of a lamp.
- F -
Feed
Deliver nutrient to the plant via roots or foliage.
Female
Pistillate, ovule, seed-producing.
Fertilizer burn
Over fertilization: first leaf tips burn (turn brown) then the leaves curl.
Fixture
The electrical fitting used to contain the electric components of a lighting system.
Flat
Shallow (three inch deep container, often 18 by 24 or 10 by 20 inches with good drainage, used to start seedlings or cuttings.
Flood and drain subirrigation
In its simplest form, there is a tray above a reservoir of nutrient solution. The tray is either filled with growing medium (clay granules being the most common) and planted directly, or pots of medium stand in the tray. At regular intervals, a simple timer causes a pump to fill the upper tray with nutrient solution, after which the solution drains back down into the reservoir. This keeps the medium regularly flushed with nutrients and air. Also called ebb and flow.
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.
Foliage
The leaves or more generally, the green part of a plant.
Foilar feeding
Misting fertilizer solution which is absorbed by lilt foliage. Best to do when first turning on your lights.
Fungicide
A product which destroys or inhibits fungus.
Fungistat
A product which inhibits fungus, keeping it in check.
Fungus
A lower plant lacking chlorophyll which may attack green plants; mold, rust, mildew.
- G -
GPM
Gallons per minute.
Gene
Part of a chromosome which influences the development of plant; genes are inherited through sexual propagation.
Geolite
One of several brand names or 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.
Germination
The process of causing the initiation and development of a plant from seed.
Growing medium
The substrate or soilless material which supports the plant and absorbs and releases the nutrient solution in hydroponic horticulture. Also referred to simply as medium.
- H -
Halide
Bu compound of a (halogens) with an electropositive elements
Harden-off
To gradually acclimatize a plant to a more harsh environment. A seedling must be hardened-off before planting outdoors.
Hermaphrodite
One plant having both male and female organs; the breeding of hermaphrodites is hard to control
HID
High Intensity Discharge.
High-intesity 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 (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 of their plants.
Hood
The reflective cover used in conjunction with an HID lamp. The more reflectivity a hood can provide, the more effective it is. Sometimes it is called a reflector.
Hor
The abbreviation stamped on some hid bulbs meaning they must be burned in a horizontal position.
Hormone
Chemical substance which controls the growth and development of a plant. Root-inducing hormones help cuttings root.
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.
HPS
High Pressure Sodium
Humidity (relative)
Ratio between the amount of moisture in the air and the greatest amount of moisture the air could hold at the same temperature.
Hybrid
An offspring from two plants of different breeds, variety or genetic makeup.
Hydrated lime
Instantly soluble lime, used to raise pH or sweeten soil.
Hydrogen
Light or colorless, odorless gas; hydrogen combines with oxygen to form water.
Hygrometer
Instrument for measuring relative humidity in the atmosphere.
- I -
Ignitor
A component of the ballast necessary for the starting of the bulb in sodium systems.
Inbred
(true breed) offspring of plants ofthe same breed or ancestry.
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.
Inert
Chemically non reactive; inert growing mediums make it easy to control the chemistry of the nutrient solution.
Iron (Fe)
This micronutrient acts as a catalyst in the photosynthesis and respiration process, and is essential for the formation of sugars and starches. Iron also activates certain other enzymes.
Intensity
The magnitude of the light energy per u intensity diminishes the farther away from the source.
- J -
Jacket
Protective outer bulb or envelope of lamp.
- K -
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. A standard clear metal halide HID lamp has an average Kelvin temperature rating of 4,000K. Also called color temperature. (See correlated color temperature.)
- L -
Lamp
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.
Leaf curl
Leaf malformation due to over-watering over fertilization lack of magnesium, insect or fungus o or negative tropism.
Leaves
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.
Leggy
Abnormally tall internode space, with sparse foliage. Leggyness of a plant is usually caused by lack of blue light or CO2 too much nitrogen can also cause this.
Life cycle
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.
Light mover
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.
Lime
Used in the form of dolomite or hydrated lime to raise and stabilize soil ph.
Lumen
A measurement of light output; refers to the amount of light emitted by one candle which falls on one square foot of surface located at a distance of one foot from the candle.
Lumenaire
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.
- M -
Macronutrient
One or all of 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.
Medium
The substrate or soilless material which supports the plant and absorbs and releases the nutrient solution in hydroponic horticulture. Also known as growing medium.
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.
Meristem
Tip of plants growth.
Metal halide (MH) 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 a bulb to use in the first, vegetative phase of plant growth.
MH
Metal Halide
Micronutrient
Also referred to as trace elements, include S, Fe, Mn, B, Mo, Zn, and Cu.
Molybdenum (Mo)
This micronutrient is essential for nitrogen fixation and nitrate reduction.
- N -
Nanometer
.000000001 meter, nm is used as a scale to measure wave lengths of light; color and light spectrums are expressed in nanometers (nm).
Necrosis
Localized death of a plant part.
Nitrogen (N)
Nitrogen is used in various forms to promote rapid vegetative growth, leaf, flower, fruit, and seed development, and chlorophyll development. It is also needed to increase the protein content in all plants.
Nutrient film technique (NFT)
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.
Nutrient solution
The mixture of water and water-soluble nutrients which is provided to the plants for nourishment in a hydroponic system.
Nutrients
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.
- O -
Organic
Made of, or derived from or related to living organisms. In agriculture organic means "natural", in chemistry organic means "a molecule or substance which contains carbon."
Ovule
A plant's egg found within the calyx, it contains all the female genes; when fertilized, an ovule will grow into a seed.
- P -
Parabolic reflector
A lighting distribution control device which 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.
Parasite
Organism which lives on or in another host organism; fungus is a parasite
PerennialL
A plant, such as a tree or shrub, which completes its life cycle over several years.
Perlite
1. Sand or volcanic glass which has been expanded by heat; perlite holds water and nutrients on its many irregular surfaces.2. Mineral soil amendment.
pH
A scale from 1 to 14 which measures the acid to alkaline balance of a growing medium (or any other substance). In general, plants grow best in a pH range of 5.8 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, blooming, and root growth. It hastens maturity and seed growth, and contributes to the general hardiness of plants. Phosphorus is a macronutrient.
Phosphor coating
Internal bulb coating which diffuses light and is responsible for variations in color outputs.
Photoperiod
The relationship between the length of light and dark in a 24 hour period.
Photosynthesis
The building of chemical compounds (carbohydrates) from light energy, water and carbon dioxide.
Phototropism
The specific movement of a plant part towards a light source.
pH tester
Electronic instrument or chemical used to find where soil or water is on the pH scale.
Pigment
The substance in paint or anything which absorbs light, producing (reflecting) the same color.
Pollen
Fine, dust like micro- spores containing male genes.
Potassium (K)
Potassium promotes disease resistance and good development of carbohydrates, starches, and sugars, and it increases fruit production. Potassium is a macronutrient.
Primary nutrients
N-P-K (Nitrogen-Phosphorus-Potassium)
Propogate
(1) Sexual: produce a seed by breeding different male and female flowers (2) Asexual: to produce a plant by taking cu1tings.
Prune
Alter the shape and growth pattern of a plant by cutting stems and shoots.
Pyrethrum
Natural insecticide made from the blossoms of various chrysanthemums.
- Q -
- R -
Reflector
The term sometimes used to refer to the reflective hood of an HID lamp.
Rejuvenate
Restore youth; a mature plant, having completed its life cycle (flowering), may be stimulated by a new 18 hour photo period, to rejuvenate or produce new vegetative growth.
Reservoir
The container in a hydroponic system which holds nutrient solution in reserve for use.
Rockwool
An 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.
Root bound
Roots stifled or inhibited from normal growth, by the confines of a container.
Roots
Their purpose is to anchor a plant and provide a means in which to feed and hydrate a plant.
- S -
Salt
Crystalline compound which results from improper pH or toxic buildup of fertilizer. Salt will burn plants, preventing them from absorbing nutrients.
Secondary nutrients
The elements other than Nitrogen, Phosphorus, and Potassium which are considered nearly as important but are not considered micronutrients. Calcium (Ca), magnesium (Mg), and sulphur (S) are the elements usually referred to by this term.
Seed pod
A dry calyx containing a mature or maturing seed.
Sodium (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 of their plants.
Soluble
Able to be dissolved in water.
Spore
Seed-like offspring of a fungus.
Sprout
(1) A recently germinated seed
(2) Small new growth of a leaf or stem.
Stamen
The male, pollen-producing part of a plant.
Stress
A physical or chemical factor which causes extra exertion by plants; a plant under stress will not grow as well as one without stress.
Stomata
Small mouth-like or nose-like openings (pores) on leaf's underside, responsible for transpiration and many other life functions; the millions of stomata, must be kept very clean to function properly.
Sugar
The food product of a plant. Carbohydrates which contain hydrocarbon chains.
Synthesis
Production of a substance, such as chlorophyll, by uniting light energy and elements or chemical compounds.
Systemic
Used in reference to a disease within the plant tissue, not initiated from the external cells. This term also refers to materials and compounds which are taken up or absorbed by the plant and designed to fight disease (e.g., systemic fungicide).
- T -
Tap root
The main or primary root which grows from the seed; lateral roots will branch off from the tap root.
Tepid
Warm 70 to 80 degrees F (21 to 27 degrees C); always use tepid water around plants to facilitate chemical processes and ease shock.
Terminal bud
Bud at the growing end of the main stem.
TDS
Total Dissolved Solids
Thin
To cull or weed out very slow growing seedlings.
Transpire
Give off water vapor and by products via stomata and carbon dioxide intake at the leaves.
- U -
U (for universal)
An industry code indicating that the bulb can be operated in any position: horizontal, vertical (base up), or any other.
Ultraviolet (UV) light
Light with very short wavelengths, out of the visible spectrum.
- V -
Vermiculite
Mica which has been processed and expanded by heat. Vermiculite has excellent water-retention properties and is a good soil amendment and medium for rooting cuttings.
- W -
Wetting agent
Compound which reduces the droplet size and lowers the surface tension of the water, making it wetter.
Wick
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.
- X -
- Y -
- Z -
Zinc (Zn)
Like copper and manganese, zinc is linked to chlorophyll synthesis.
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