Cannabis is a short-day plant that naturally has its vegetative growth period during the long days of summer and then initiate flowering as the days are getting shorter in autumn . When grown in an indoor cultivation, it is therefore important to adjust the photoperiod to these growth stages and provide longer photoperiods of 16-18 hours during the vegetative phase and shorter periods of about 12 hours during the flowering phase  . In an indoor cultivation it is possible to produce up to six harvests per year , but in difference to other horticultural plants it is not only the amount of harvest that is of importance, but also the chemical composition of the end product . A homogenous yield and uniform production of cannabinoids between plants and growth cycles are of outmost importance , and consequently a constant and optimized light environment, regarding both quantity and quality, is required to achieve this.
Cannabis is an extremely light demanding plant that thrives under very high light intensities around 1000-1500 μmol/m²/s during flowering, or even higher if the surrounding conditions are ideal. A common mistake is to supply the plant with too low levels of carbon dioxide for it to be able to utilize all the light, making carbon dioxide the limiting factor that restricts the growth of the plant. Cannabis plants prefer day temperatures up to 30 ℃ depending on growth stage, but requires cooler conditions during the night  . Regarding the optimal quality of the light, a full spectrum with a relatively high portion of blue light during the vegetative phase gives compact, bushy plants with thick leaves, that are able to carry more flowers during the reproductive stage  .
If you plan to exchange a current HPS system with LEDs for cannabis cultivation, there’s a few things to have in mind to make the transition as successful as possible. While it is widely known that switching to LEDs entails major energy savings for lighting, many growers are worried that the loss of excess heat from the HPS fixtures will have a negative impact on their growing environment or yield. In most cases, an exchange of HPS for LEDs will indeed require some additional heating during winter, but using a dedicated heat system is actually more cost efficient and also offers greater control of the growing environment than using inefficient heating from HPS lamps . For example, it gives the opportunity to increase light intensity even when ambient temperatures are high in summer, without overheating the crops or increasing costs for ventilation . In short, decoupling lighting from heat can result in great benefits and more precise control of the growing environment, but remember that small adjustments of other surrounding factors might be necessary, like decreasing humidity and possibly also irrigation .
Growers who have taken the step from HPS to LED lighting systems have reported not only up to doubled yields, but also greater control of plant development as well as more compact and higher quality crops . Depending on what kind of replacement you choose one can either achieve extensive energy-savings or increased light output for the same energy consumption.