Environment & Lighting

This page covers terms for the growing environment — humidity and vapor pressure measurements, temperature effects, air handling, and the units used to describe grow lighting.

abiotic stress

Strain on a plant caused by non-living environmental factors — temperature extremes, light intensity, water availability, humidity, salinity — as distinct from stress caused by pests, pathogens or other organisms (biotic stress). Multiple abiotic factors often act on a plant at the same time.

activated carbon

Carbon that has been processed to have a very high internal surface area full of microscopic pores, which lets it adsorb (bind onto its surface) gases and odor compounds as air passes through it. This is the material used inside carbon filters.

carbon filter

A filter, typically a cylinder packed with activated carbon, that air is pulled through to remove odor compounds before the air is exhausted from a growing space. Airflow rate and the amount of carbon inside both affect how completely odors get captured before venting.

Ceramic Metal Halide (CMH)

A type of gas-discharge lamp using a ceramic arc tube and a metal halide gas mixture, producing a broader, whiter light spectrum than HPS lamps of the same lighting family, while operating at similar wattages to comparable HPS fixtures used for the same growing area.

CO2 supplementation / enrichment

Raising the carbon dioxide concentration in a growing space above the ambient outdoor level, since CO2 is one of the inputs photosynthesis consumes. It is typically paired with adequate light intensity, since photosynthesis needs both to use the additional CO2.

cold stress

Physiological strain a plant shows when ambient temperature drops below its tolerable range, slowing metabolic processes and, in more severe cases, damaging cell membranes and tissue. How well a plant recovers depends on how low the temperature dropped and for how long it stayed there.

Daily Light Integral (DLI)

The total amount of light a growing area receives over a full day, measured as the sum of photosynthetic light delivered across all hours of the light period, rather than the light intensity at any single moment. Two growing areas can share the same peak intensity yet differ in DLI.

dark period

The uninterrupted span of darkness within a light cycle. For photoperiod-sensitive plants, an uninterrupted dark period of sufficient length is what allows the flowering response to proceed; a light leak during this window can disrupt it and delay or prevent that response.

DIF (day/night temperature differential)

The difference between daytime and nighttime air temperature. A positive DIF (warmer days than nights) or negative DIF (cooler days) influences stem elongation in many species, since some of that growth response is tied to the temperature swing rather than either temperature alone.

heat stress

Physiological strain a plant shows when ambient temperature exceeds its tolerable range, affecting processes like transpiration, stomatal function and enzyme activity. Symptoms depend on species and how far above the tolerable range the temperature rises, as well as how long the exposure lasts.

High Pressure Sodium (HPS)

A type of gas-discharge lamp that produces light by passing an electric arc through vaporized sodium, giving a light output weighted toward the yellow-orange part of the spectrum. It has been used in grow lighting for decades before LED became common.

HVAC

Heating, ventilation and air conditioning: the combined systems used to control a growing space's temperature, humidity and air exchange as a coordinated set rather than through separate, independent equipment, allowing conditions to be adjusted together rather than one at a time.

light cycle

The repeating pattern of light and dark periods a plant is exposed to over 24 hours, expressed as hours of light to hours of dark (e.g. 18/6). It is the schedule that sets the plant's photoperiod and, for sensitive species, its stage of growth.

Light Emitting Ceramic (LEC)

Another name used for ceramic metal halide (CMH) lighting technology; LEC and CMH describe the same underlying lamp type, built with a ceramic arc tube and metal halide fill, and manufacturers use the two names interchangeably when marketing otherwise identical fixtures.

Light Emitting Diode (LED)

A semiconductor light source that produces light when electric current passes through it, used in grow lighting because its output spectrum and intensity can be tuned by the diodes selected, and it produces relatively little heat for the light output it delivers.

light leak

Unintended light entering a growing space during what is meant to be an uninterrupted dark period, from gaps in enclosures, doors or equipment indicator lights. For photoperiod-sensitive plants this can interrupt the dark-period signal the plant relies on to time its flowering response.

light stress / light burn

Visible damage to leaf tissue — bleaching or scorching, usually nearest the light source — caused by light intensity exceeding what the plant's photosynthetic system can use or tolerate at that distance or temperature. Raising the light source or lowering its output reduces this exposure.

lumen

A unit of measurement for the total quantity of visible light a source emits, weighted according to human eye sensitivity rather than plant photosynthetic response. Because of that weighting, lumens are a less direct measure for plant lighting than PAR-based units like PPFD.

Photosynthetic Photon Flux Density (PPFD)

The amount of photosynthetically usable light landing on a given area each second, measured at a specific point and moment. It is an instantaneous intensity reading, while DLI is that same intensity accumulated across every hour of the light period over a full day.

Photosynthetically Active Radiation (PAR)

The range of light wavelengths, roughly 400–700 nanometers, that plants can use for photosynthesis. PPFD and DLI both measure light within this range specifically, rather than all light a lamp emits, since wavelengths outside it contribute little to plant growth.

Relative Humidity (RH)

The amount of water vapor present in the air expressed as a percentage of the maximum the air could hold at that temperature. Because the maximum changes with temperature, the same RH reading represents a different actual moisture level at different temperatures.

vapor pressure deficit (VPD)

The difference between the amount of moisture the air could hold at saturation and the amount it actually holds, at a given temperature. It reflects the drying pull the air exerts on a leaf's surface, which is why it's tracked alongside temperature and humidity rather than humidity alone.

ventilation

The exchange of air within a growing space, bringing in fresh air and removing stale air, which affects CO2 supply, temperature, humidity and airflow across the canopy. Poor ventilation can let localized pockets of humidity or heat build up around the plants.