When growing plants from seedlings indoors, providing the right amount of light is crucial for healthy growth. One important factor in setting up grow lights is determining the ideal height to hang them above the seedlings. Positioning the lights at the proper distance will ensure the seedlings receive adequate intensity without becoming stressed. This article provides tips on optimizing grow light height for various types of seedlings.
What factors determine optimal grow light height?
There are a few key variables to consider when deciding how far to elevate grow lights above seedlings:
- Type of light – Some lights are designed for closer proximity to plants than others. LED and T5 fluorescent lights can be kept quite close, while high-intensity discharge (HID) lights need more distance.
- Stage of growth – Young seedlings and newly germinated plants need less intense light than more established plants. The lights should be kept higher up at first and gradually lowered as the seedlings mature.
- Light manufacturer’s recommendations – Most grow light manufacturers provide guidelines for how far the light should be kept from the canopy of plants. This is a good starting point for height.
- Plant variety – Some plant species and varieties are adapted to higher or lower light levels. The height should be adjusted accordingly.
- Light wattage – More powerful lights putting out higher wattage need more height than lower wattage lights.
Getting the balance right between providing adequate intensity for growth while preventing light stress requires paying attention to all these variables and making adjustments to dial in the optimal range.
Recommended height for LED grow lights
LED grow lights have become popular for indoor gardening due to their energy efficiency, durability, and ability to produce a favorable spectrum for plant growth. Here are some general guidelines for LED light height above seedlings:
- Germination stage: 24-30 inches
- Seedling/young vegetative stage: 18-24 inches
- Established seedlings pre-flowering: 12-18 inches
However, you should also follow the manufacturer’s recommendations for the specific LED light model, as intensities vary. Some high-power LEDs may need over 3 feet of height while lower wattage models can be kept within 12 inches of seedlings. Monitor the seedlings for signs of light stress and make adjustments. Keep lowering the lights as the plants grow while making sure to avoid bleaching or overexposure.
Adjusting LED height for different plants
Some plants have higher or lower light needs which translates to lower or higher optimal LED hang heights:
| Plant Type | Height Range |
|---|---|
| Lettuce, kale, herbs | 14-20 inches |
| Tomatoes, peppers, cucumbers | 18-24 inches |
| Cannabis, coleus | 24-36 inches |
Recommended height for T5 fluorescent grow lights
T5 fluorescent tubes are long lasting, reasonably energy efficient, and don’t put out too much heat, making them a popular grow light choice. Here are some general height guidelines when using T5s for seedlings:
- Germination: 8-15 inches
- Young seedlings: 6-10 inches
- Established vigorous seedlings: 3-6 inches
T5s and other fluorescents are designed for close proximity to seedlings to provide moderate-intensity light. However, the tubes should be kept farther away and gradually moved closer as seedlings develop. Excessive light can stunt growth.
Adjusting T5 height for different plants
As with LEDs, some plants have lower or higher light needs, requiring lower or higher positioning of T5 lights:
| Plant Type | Height Range |
|---|---|
| Lettuce, herbs | 4-8 inches |
| Tomatoes, eggplant | 6-12 inches |
| Peppers, melons | 12-18 inches |
Recommended height for high-pressure sodium (HPS) and metal halide (MH) lights
HID grow lights like HPS and MH put out intense light and heat. They require substantially more distance from seedlings than LED or fluorescent lights. Here are approximate height ranges when using HPS or MH lights for seedlings:
- Germination: 3-4 feet
- Young seedlings: 2-3 feet
- Established seedlings: 1.5-2 feet
The intense light can easily overwhelm and bleach seedlings if the HID lights are positioned too close. Err on the side of more distance and gradually lower lights as seedlings acclimate and grow stronger. The hot illumination also requires proper ventilation at canopy level.
Adjusting HID height for different plants
HID lights demand greater height adjustments for plants with lower or higher light needs:
| Plant Type | Height Range |
|---|---|
| Lettuce, kale, herbs | 2.5-4 feet |
| Tomatoes, peppers | 2-3 feet |
| Cannabis, coleus | 1.5-2.5 feet |
Optimizing seedling light height step-by-step
Here is a summary of the key steps to dial in the optimal grow light height above seedlings:
- Refer to manufacturer guidelines for a starting height range based on the light type and wattage.
- Make adjustments based on the particular plant varieties you are growing.
- Elevate lights to the highest setting in the recommended range after seeding or transplanting.
- Observe seedlings daily for signs of excessive light such as bleaching, yellowing, wilting, or stunted growth.
- Gradually lower the lights over days or weeks as seedlings acclimate and grow stronger.
- Stop lowering once seedlings show signs of stress and raise lights back up slightly.
- Continue monitoring growth and adjust height further if needed.
- When flowering or fruiting starts, lights can be lowered to the minimum recommended height.
Tips for adjusting grow light height
Here are some additional pointers to get your grow light positioning dialed in just right:
- LED and fluorescent lights can be kept closer than HID lights. Start higher and work downward.
- Heat-sensitive plants need more distance. Prioritize airflow and ventilation.
- Young seedlings need more height, established plants need less.
- Raise lights up if leaves show purple, brown, yellow or white discoloration.
- Water-stress and intense heat call for increased light height.
- Record light measurements at plant level with a PAR meter to quantify intensity.
- Lift lights vertically without adjusting hanging accessories to hone in on ideal range.
Conclusion
Determining the optimal elevation for grow lights over seedlings depends on the light’s intensity and your plants’ requirements. But start with the manufacturer’s guidelines for height, monitor seedlings closely for stress signs, and make incremental adjustments until hitting the ideal positioning. Dialing in proper light exposure and intensity will give your seedlings the best start.
