January 9, 2020
Quick Guide: How Mounting Height, Light Footprint and Intensity Affect Your Crop
A critical aspect of your indoor grow setup is how high you mount your LED fixtures. Experienced indoor growers know it plays an important role in your light’s ability to provide uniform coverage, and it directly impacts your plants’ survival. Whether you’re setting up your lights for the first time, or you’re wondering how to improve plant health, review this important guide to mounting height.
Impacts of Improper Light Coverage
Improper uniformity and intensity not only impact cannabis plants’ ability to photosynthesize, but they might also lead to lower cannabinoid content, weaker terpene profiles and may cause the plants to consume more nutrients than normal. They may fail to reach full maturity, or flower yields may be particularly low. In order to prevent serious loss of profits, indoor growers will need to ensure each plant receives the optimal amount of light.
At the same time, you don’t want to hang your lights too close to your plant canopy in an effort to maximize output. Despite LED’s lower operating temperature, they can still cause light bleaching in your plants, which inhibits plant health. It’s more a matter of finding an ideal distance – not too close or too far away – that provides an even balance of light in accordance to the plant’s maturity.
How Distance Affects Light Footprint and Intensity
Uniformity is most accurately measured through photosynthetic photon flux density (PPFD). This measurement shows how much photosynthetically active radiation (PAR) is actually reaching the plant during any given moment and illustrates how effective your fixtures are. PPFD is measured in umol/m2/s, and growers should seek an even measurement across their canopy. The amount of space receiving PAR is called a light footprint. The amount of PPFD is considered the intensity.
Importantly, PPFD can decline if your light is hung too far away from the plant canopy, as photons may disperse around the plant, rather than directly at it. On the other hand, your light footprint declines the closer your plants are to your fixture, potentially resulting in an imbalance.
Let’s break it down:
- Shorter distance = high PPFD (intensity) + small light footprint (coverage)
- Longer distance = low PPFD (intensity) + big light footprint (coverage)
Lighting According to Growth Cycle
Cannabis needs more light intensity (PPFD) as it matures and enters its reproductive phase, so growers will need to adjust the height according to the cannabis plants’ stage of growth. Your team should have an effective way to adjust this distance. For example, some growers use a pulley system to bring the lights closer or farther away through the plants’ lifecycle.
A general guide to hanging height is as follows:
- Seedling stage: 30” – 36” away from the plant canopy
- Vegetative stage: 24” – 30” away from the plant canopy
- Bloom stage: 12” – 24” away from the plant canopy
These distances may be slightly different based on grower preference, goals and the individual strain’s biological needs.
Of course, as with any changes at your indoor facility, gradual is better than sudden. You can prevent your plants from experiencing shock by slowly introducing more intensity as they mature. Plant behavior is usually a pretty accurate indicator for if your lights are hung at a proper height. If the lights are too far away, you may see some stretching. If they’re too close, you could see bleaching or burning. But, once you find that optimal distance, they’ll truly thrive. Be thorough in documenting the distance from fixture to canopy, across the life cycle of the plants, towards optimization particular to given strains and results.