More and more growers are moving away from traditional HID and T5 fluorescent lights and installing LED lighting instead. Compared to HID light sources, LEDs emit less heat, which means LED grow lights have a different proximity to the tree canopy than traditional grow lights. It is also important to understand how the optimal growing light distance affects the different stages of plant growth.

This article outlines the different lighting needs of various plants, including cannabis, based on the plant’s growth stage. It also discusses how to determine the proper distance from the plant canopy for LED lighting to nourish plant growth, and the importance of PPFD (PAR) and other popular grow light terms.

How to measure the light used for plant growth

Let’s quickly define PAR and PPFD. PAR (Photosynthetically Active Radiation) describes the portion of the visible spectrum that plants “see” and use for photosynthesis (400nm-700nm). PPFD (Photosynthetic Photon Flux Density) measures the amount of light (PAR) a plant receives over time. PPFD represents the optical density received by a plant over time, expressed in micromoles [photons] per second per square meter.

One way to visualize PPFD is to imagine the sun “pouring” light onto the leaves of the plant. When sunlight hits plants, their leaves are collecting energy. PPFD is a measurement of the amount of light (photons) that the sun “pours” onto plants over time. PPFD is an important metric because it helps growers accurately measure light intensity for photosynthesis at the canopy level. This is also important as lights too close to the canopy can cause burning, fading, stunted growth or discoloration.

LED Grow Light Distance Chart

(M/inch)  (Lux) PPFD / PAR
(m² / ft2)*
2m / 79 inches 955lx 670 7.6m2 / 81.8ft2
1.5m / 59 inches 1692lx 1170 5.0 m2 / 53.8ft2
1m / 39 inches 3663lx 1670 3.0 m2 / 32.3ft2
0.5m / 20 inches 12,500lx 2170 1.5 m2 / 16.1ft2
0.2m / 8 inches 50,300lx 2670 0.5 m2 / 5.4ft2

The distance between the LED grow light and the plant canopy (600W LED grow light)*Coverage will vary depending on the grow lights used. The LED grow lights shown here do not use lenses to direct the distribution of light.

Table 1 shows the performance of 600W LED grow lights at different distances from the plant canopy. It provides an overview of light intensity (lux), provides PPFD (μmol/m-2/s-1 or micromoles per square meter per second), and also shows the “light footprint” or canopy coverage. Intensity, PPFD/PAR and “light footprint” all changed as the light increased or decreased in distance from the plant canopy.

Also highlighted is how changing the distance of the same 600-watt LED light changes the light intensity and “light footprint” or canopy cover the plants receive. When the light source is located closer to the canopy, the light intensity increases. In general, grow lights should be installed closer to the plant canopy for the vegetative stage of plant growth and higher (away from the plant canopy) during the flowering stage of the plant.

Where should grow lights be placed?

For seedlings, LED grow lights should typically be installed between 24-36 inches above the plant canopy – however, this depends on the wattage (wattage) of the light source. Place LED grow lights furthest away from seedlings (~36 inches) – this keeps heat and light levels low and helps prevent seedlings from drying out. Once the roots are established and budding begins, the lights can be moved closer (usually within the first 2-3 weeks).LED grow lights should be 12-24 inches from the top of the canopy during the plant growth stage. At this stage, photosynthesis requires more light, so the light source should be located near the plant.The need for bright light decreases as the plant enters the flowering stage. The top leaves of the canopy should be 18-24 inches from the light source to produce blooms. It is at this stage that the plant increases in height and bears fruit. Depending on the light and how you want your crops to grow, you don’t have to change the height of the light during bloom, especially if you don’t want tall plants.

How far should LED grow lights be from seedlings?

During the initial stages of growth, seedlings are fragile and require less light. This means that you don’t want to increase the strength too early, as the seedlings will multiply in a more gentle way. Depending on the amount of light, it is safe to keep growing light somewhere between 24-36 inches from the top of the soil.

How far should LED grow lights be from plant clones

Cannabis cloning is a process in which cuttings or cuttings from a mature plant are used to grow another plant of the same species. The LED grow light distance above the clone is not the same height as the seedling needs. For clones, they will need strong light to get started. Depending on the light intensity and maturity of the plant, the difference between 14-36 inches from the top of the plant canopy can be huge.

How far should LED grow lights be used

As plants grow, their needs change. After the vegetative stage is complete, the plant enters the flowering or “flowering” stage. For established factories, they have become a necessary place to thrive. During the flowering stage, LED grow lights should be located between 16-36 inches from the plant canopy. Moving the grow light closer will increase the light intensity, maximizing photosynthesis. However, if grow lights are placed too close to the top of the plant, they can cause the growth to expand, spread or even destroy the plant.

What about other growth stages?

Depending on each stage of growth, plants require different levels of light. To understand exactly how far into the light a plant should grow, it’s important to consider the different stages of growth. It is also important to consider the power output of the grow light – this is usually related to the power of the light source, which is the main factor in determining the optimal distance from the LED when starting the growing process.

The growth of plants can be roughly divided into three stages – nursery, vegetative and flowering stages. This means that once healthy roots are firmly established, it’s time to increase the intensity and lower the light.

seedling stage

In the initial stages as seedlings, LED grow lights should be placed from above the plants so that the soil does not dry out.Some growers may try to blast seedlings with high-intensity light to promote faster growth, however, this will not help until the plants are established. At this early stage, the seedlings are too fragile and require a more gentle approach. Once plants mature, higher light intensities are required to promote photosynthesis.

vegetative period

During the vegetative phase, plants respond well to bright light – it is during this period that plants mature and grow rapidly using photosynthesis. To increase light intensity, LED grow lights should be located closer to the plant canopy. Firm, healthy stems and roots are the key to successful repeatable yields.Increasing light intensity can promote growth during the vegetative period, but it is important to monitor plants closely, making sure to watch for adverse symptoms caused by too much or not enough light.

flowering period

Flowering is the final stage of the plant’s growth cycle – during this period, fruit production and stem growth are accelerated. It is good practice to “stage” the transition as plant growth transitions from vegetative to flowering or flowering. To do this, gradually increase the LED species

Height of plant lights so that they are well away from the canopy of the plants (use the height recommendations outlined earlier). Monitor distances closely and keep in mind the crop height and flowering needs required for a particular plant/crop.

Adverse effects of excessive light

When plants like cannabis receive too much light at any stage, they often show distressing symptoms. Since LEDs don’t dissipate much heat, the main issue to monitor closely is any signs of “light burn”. Other side effects associated with growing lights too close to the plant canopy can be discoloration or stunted/irregular growth. Both must be identified quickly and the height of the grow lights adjusted accordingly.

There are signs of mild burning in cannabis plants, including upward-facing leaves and so-called “bleaching.” Bleaching is the white or yellow discoloration on the leaves closest to the light. Mild burns can also be recognized when the veins of a plant turn green while the rest of the leaves turn yellow.

Increased Light Distance – Cannabis vs Other Plants

The use of LED grow lights for cannabis production is becoming more common as growers move crop production indoors. LEDs also bring benefits to growers, as they can use specific light spectra to provide more targeted lighting conditions for different crops. Other benefits when considering LED grow lights include lower power consumption, lower forward heat, and increased yield in a shorter period of time compared to traditional light sources.

The distance the light increases should reflect the grower’s desired height and plant uniformity with spread of foliage and healthy blooms. If we look at the crop demand for salad greens or lettuce compared to hemp, we see that salad greens and lettuce are better for short-term, wider growing, while hemp is better for taller, narrower growth.

No matter which crop you use, farmers and hemp growers want the highest quality yields in a short growing cycle.
Lighting distance of traditional plant lights.

Traditional high-intensity discharge lamps (HIDs) such as high pressure sodium (HPS) and metal halide lamps, as well as fluorescent lamps, were widely used before LEDs became popular for indoor growth applications. Historically, the first time these lights cost much less than LED grow lights, making them cheaper to buy and therefore available to large indoor growers.

The HID or fluorescent light should be placed at a greater distance from the plant canopy than the LED. That’s partly because they dissipate more forward heat than LEDs, but they’re also only available in a few different wattages/light outputs. The installation distance of grow lights varies for each stage of growth and depends on the wattage of the grow lights used.

While traditional grow lights have a lower first-use cost (lower purchase price), they require more maintenance over time – bulbs need to be replaced and/or cleaned more frequently – and they exist in lighting control Obvious limitations – many lights don’t dim and can take a long time to turn on full light output.

fluorescent lamp

There are three (3) basic types of fluorescent grow lights – T5, T12 and CFL. To obtain different light intensities, growers must adjust the height of the plant canopy lights accordingly. One of the advantages of fluorescent lamps is that they are difficult to light because they do not emit enough heat. Regardless, light intensity and heat should always be closely monitored.
For young plants, about 6-12 inches is a reasonable height to start with, as they require higher light levels. It is wise to double this distance to 12-16 inches as they mature from vegetative to flowering. In general, with T5s, it’s best to keep them as close as possible, but monitor for overheating or drying out.

HID Grow Lights (Metal Halide and High Pressure Sodium – HPS)
Metal halide (MH) lamps provide large amounts of blue light – this spectrum is considered best for the vegetative growth stage (3). High pressure sodium (HPS) lamps are ideal for plant growth and flowering. Both generate more heat than LEDs, but are relatively inexpensive to purchase.

One way to test HID heat is with the back of your hand. This can help you determine the correct grow light distance. Simply place your hand on top of the canopy and hold for 30 seconds. Your hands should get hot, but can’t handle it. If this happens, increase the grow light distance above the plant.

Many commercial growers use 1000W HID lighting and typically start out seeing 19-26 inches in height. From here, they can gradually approach.

However, since HIDs produce a lot of heat, it’s important to avoid calorie burn, which can damage any plants. Also, due to the high heat emitted by HID lamps, indoor ventilation is very important.

With over 10 years of collective experience and expertise, bbier Lighting is a leader in LED growth lighting solutions and continues to innovate and drive the industry forward. bbier Lighting’s biology-leading and research-based approach to LED grow lights leads the industry because we build brilliant full-spectrum LEDs that make plants stronger and more productive, from seed to maturity.