Before You Buy a Grow Light: Basic Principles

A grow light is probably one of the most exciting purchases any new grower can make. Whether used as a complete replacement for the Sun, or to supplement natural light levels, there are several different kinds of grow light available so it’s important to choose the size and type best suited for your application: i.e. propagating young plants, promoting vegetative growth, and driving flowering and fruiting.

Heating and Cooling

Grow lights produce heat as well as light. If plants grow too close to your grow lights they can suffer from excess heat and localized low humidity. T5 fluorescent lights produce less heat and are less intense so they can be placed closer to plants than high intensity discharge (HID) lights. Many growers choose air-cooled reflectors to mitigate the heat issues that are sometimes associated with HID lights.

Lamp Depreciation / Maintenance

Lamps need to be replaced regularly (every 6-9 months) to maintain optimum growth rates in your indoor garden. Don’t be fooled: while your grow lights may appear no dimmer to the naked eye, they are dramatically less efficient. It’s also important to keep reflectors clean.

In order to make an informed decision when choosing a grow light it’s important to have a basic understanding of some fundamental lighting principles.


Light is made up of different wavelengths measured in nanometers (nm.) The band of the spectrum between 400 and 700 nm is called “Photosynthetically Active Radiation” (PAR) — this refers to the band of light that plants capture and use. Light rich in the “blue” part of the spectrum (around 440-490 nm) is ideal for vegetative growth, whereas “orange / red” light (around 580-700 nm) helps to promote flowers and fruit.


The right kind of light (PAR) needs to be sufficiently intense in order to “power” your plants. Different types of grow light produce a wide range of intensities. Correct positioning is therefore very important. For all grow lights, the further they are from your plants, the less usable energy reaches the leaves. Light intensity greatly affects plant growth patterns.


The electricity used to power grow lights is measured in watts (W). Power consumption can vary enormously between different types. For instance: a 2ft T5 fluorescent tube can use as little as 24W, where the most powerful high intensity discharge (HID) lights can use as much as 1500W.


The effective surface area covered by a grow mlight. If you hang your grow light higher, its footprint will increase but the intensity of light hitting the leaves (luminous flux) will decrease exponentially.


The amount of light in a 24-hour period. Many plants use photoperiod as a cue to start producing flowers and fruit. Therefore indoor gardeners can manipulate many plant species simply by altering how long they switch their grow lights on for each day, mimicking the natural seasons, by connecting their lights to 24-hour timers and relays.


Effective Illumination for Various Wattage HIDs

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