How far should a new weed seedling be from the 100 watt light?

Light intensity is important to moving seedlings through germination to transplant quickly and consistently. How far lights should be from seedlings is one of the most common questions that we get from new and experienced growers alike.

Why your seedlings aren’t getting enough light？

Light is the number one underestimated factor of growing, even at the seedling stage. If seedlings don’t have enough light, they grow more slowly and can become leggy. Overall, growers who do not give seedlings enough light have longer crop timelines and higher rates of seedling mortality.

Why do growers mistake the level of seedling light?

  • They don’t have enough lighting.(100W LED grow light is not enough for big harvest)

  • They place lighting too far from seedlings (bad coverage).

  • They don’t leave lights on long enough.

How far away should I place seedling lights?

You’ve got to keep two goals in mind: the amount of light you’re giving seedlings, and the coverage of light that you’re giving seedlings.

1) Amount of light:

We can measure the useful light to a plant (PAR—photosynthetically active radiation, or the parts of light that the plant can use to photosynthesize) in micromoles per meter squared per second (micromoles/sec/m²). The goal here is to deliver 120–150 micromoles/sec/m² of PAR to your seedlings.

Some people will use a PAR meter to figure out how much PAR they’re getting to their seedlings and will adjust the distance of their lights based on that measurement. Don’t worry—you don’t need one of those if you’re using LED bars or fluorescent bars. Just remember the following distances.

LED bars (like Phillips): 8–12 inches away from seedlings

T5 fluorescents: 5–6 inches away from seedlings

2) Coverage of light

The next question is coverage. You can usually just eyeball this to figure out how far apart your lights need to be to deliver the same intensity of light across your entire seedling tray.

Again, we’ve outlined two simple rules to follow if you’re using the most common seedling lighting types:

LED bars: 8–10 inches apart from each other

T5 fluorescents: 4–5 inches apart from each other

Stick to these rules, and your seedlings will be ready to transplant in no time.

How To Measure Light for Plants？

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 light density a plant receives over time and is measured in micromoles [of photons] per square meter per second (1).

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

Adverse Effects of Too Much Light

When a plant like cannabis receives too much light at any stage, they often show symptoms of distress. Since LEDs do not give off much heat, the main issue that needs to be closely monitored is any indication of “light burn”. Other side-effects related to grow lights that have been placed too closely to the plant canopy could be discoloration or stunted/irregular growth. Both of which must be identified quickly and the grow light height adjusted accordingly.

Signs of light burn in cannabis plants include upward-pointing leaves and what is known as “bleaching”. Bleaching is the white or yellow discoloration on the leaves closest to the light. Light burn can also be identified when the plant veins stain green as the rest of the leaves turn yellow.

How to Recognize Light Burn On Your Plants？

Because LED lights give off so little heat, you don’t run much risk of causing heat damage by placing your fixture too close to the canopy. What you do risk is light burn. Unfortunately, the light burn is easily mistaken for nitrogen deficiency, and many growers wind up misdiagnosing their grow and addressing the wrong problem.

If your plants have light damage, you’ll see symptoms such as:

  • Leaves point upwards.

  • Bleaching, which takes the form of white or yellow discoloration, typically on the leaves closest to the light.

  • Veins remain green even as the rest of the leaves turn yellow.

A nitrogen deficiency has similar characteristics, but there are a few notable differences.

  • A plant with a nitrogen deficiency also develops yellow patches, but the damage starts from the bottom of the plant and works its way up, while light burn begins at the top, closest to the lights.

  • In the case of a nitrogen deficiency, leaves wilt. In the case of a light burn, leaves turn upward and sometimes even become brittle.

  • The leaves of a nitrogen deficient plant fall off on their own. This does not happen to plants suffering from a light burn.

If it sounds like your plant has a light burn, it’s time to reevaluate the placement of your LED lights.

Lighting Distance for Traditional Grow Lights

Before LEDs became popular for indoor growing applications, traditional High Intensity Discharge lamps (HID) like High Pressure Sodium (HPS) and metal halide, as well as fluorescent lamps, were widely used. These lamps historically have had a much lower first-cost than LED grow lights for seedlings, making them more affordable to purchase and therefore accessible for large-scale indoor growers.

The distance that HID or fluorescent grow lights should be from the plant canopy is greater than from LED. This is partially because they give off more forward heat than LEDs, but they also are only available in a few different wattages/light outputs. The distance that grow lights should be mounted varies for each stage of growth and depends on the wattage of grow lights used.

While traditional grow lights have a lower first cost (they are less expensive to buy) they require more maintenance over time – the bulbs need replacing and/or cleaning more frequently – and they have significant limitations related to lighting controls – many do not dim and can take a long time to turn on to full light output.

Fluorescent Grow Lights

There are three (3) basic types of fluorescent grow lights – T5, T12, and Compact Fluorescent Lamps. To achieve different light intensity, growers must adjust the height of the lamps from the plant canopy accordingly. One benefit to fluorescent is that it is very difficult to cause light burn because they don’t emit enough heat. Regardless, light intensity and heat should always be closely monitored.

For young crops, approximately 6-12 inches is a sensible height to begin since they need a higher intensity of light. As they mature from vegetative through flowering, it’s wise to double this distance to around 12-16 inches. As a general rule, when T5’s are used it’s wise to keep them as close as possible but to monitor overheating or drying out.

HID Grow Lights (Metal Halide and High Pressure Sodium – HPS)

Metal Halide (MH) lamps provide a large amount of blue light – the spectrum considered best for the vegetative stages of growth (3). High Pressure Sodium (HPS) lamps are ideal for both vegetative and flowering. Both of these produce significantly more heat than LEDs but are relatively inexpensive to buy.

One way you can test the heat of HIDs is with the back of your hand. This can help you gauge the right grow light distance. Simply place your hand just over the top of the canopy and hold it there for 30 seconds. Your hand should become hot, but not unbearable. If this happens, increase the grow light distance above the plants.

Many commercial growers use 1000W HID lighting, which normally would see a height of 19-26 inches used to begin. From here, they can gradually be moved closer.

However, since HIDs produce a large amount of heat it’s critical to avoid heat burn since this will damage any plant. Plus, indoor ventilation is very important with HID lamps due to the higher heat they give out.

With a 50% reduction in electricity consumption and up to 50% less heat produced compared with HPS, our SSL grow lights provide your plants with natural light for up to ten years. The BIOS Icarus® line of LEDs provide optimized light spectrums for superior results which also use plug and play configuration. Our grow lights are easy to set up with effortless maintenance, backed by our ongoing commitment to support our partners’ LED integration.

Final thoughts

The intensity or distance between your plants and LED lights is crucial to the health of your grow. Run the lights too high power or place the lights too close, and you can damage the plant. Underpower or hang them too high, and you miss out on the highest possible yields and potency.

Your best bet for getting the exact perfect height is to consult with the manufacturer of your specific full spectrum LED grow light. If you’re still on the hunt for an LED light, include customer service as a consideration when weighing your options. Choose an LED light manufacturer for your marijuana grow tent that offers easy access to expert insight.