Grow Light Bar Spacing: How Far Apart for Even Coverage
The short answer
Hold the bar gaps even across the whole growing area in both directions. Resource Innovation Institute lighting guidance suggests fixtures no more than 12 inches apart when they hang less than three feet above the canopy, and up to 24 inches apart when mounted higher. Then check light at the center and the edges and adjust.
Grow light bar spacing is the gap between LED bars on a fixture or frame. Bars set too far apart leave dim stripes, while closer bars spread light more evenly. Lighting guides note that fixture spacing and hanging height shape how uniform the light is. Start with the maker guide, then adjust gaps and height together.
Why does LED bar spacing matter?
Plants grow toward the light they get, and uneven light means uneven growth. Resource Innovation Institute guidance says uniform light levels are needed for crops to receive consistent light intensity, and that fixture spacing and mounting height contribute greatly to uniformity. It also notes that continuous runs of linear LED fixtures can spread light more evenly than grids of older HID fixtures.
Distance plays a part too. University of Minnesota Extension explains that the distance between a light source and a plant affects light intensity, and PPFD, the light reaching a leaf, goes down as plants sit farther from the source. For the wider picture, read optimal lighting for growing indoors.
How far apart should grow light bars be?
Follow the maker spacing first. Resource Innovation Institute advises consulting manufacturer specification sheets for recommended mounting height. Its working group suggests a general spacing rule tied to height, while noting each space differs.
| Factor | Effect | Notes |
|---|---|---|
| Bar gap | Even or striped light | No more than 12 inches when mounted under 3 feet (RII) |
| Mounting height | Light reaching the canopy | Gaps up to 24 inches when mounted higher (RII) |
| Distance to plants | Light intensity | PPFD drops as distance grows (UMN) |
| Even X and Y spacing | Uniform coverage | Hold spacing consistent in both directions (RII) |
| Edges | Weaker light at the sides | Consider how layout affects edge light levels (RII) |
Sources: Resource Innovation Institute, University of Minnesota Extension.
Height and heat go together, since lower fixtures sit closer to the leaves and lights give off heat. See grow light heat management for holding temperatures in range.
How do you check coverage?
Measure rather than guess. Resource Innovation Institute suggests looking past peak PPFD to point by point light level values at consistent spacing across the area, including the edges. Some makers model PPFD on six inch grids. A quantum meter reads the light plants can use at each point.
Under a full canopy, lower leaves often sit in shade. Extra bars placed lower can fill that gap, and under canopy lighting cannabis explains how.
How to: step by step
- Measure the growing area.
- Read the maker spacing and height guide.
- Space bars evenly across the area.
- Hang the fixture at the suggested height.
- Check light at the center and edges.
- Adjust gaps or height to even it out.
Troubleshooting
Dim edges: bars bunched in the middle. Spread them evenly to the sides.
Stripes of slow growth: gaps too wide. Move the bars closer together.
Hot spots: fixture too low. Raise it and check the canopy light again.
Even light, even growth
Grow light bar spacing comes down to even gaps, the right height and a quick check of the edges. The Kind Seed Co blog has more on indoor setups in the lights and climate section.
Sources
- Lighting for indoor plants and starting seeds, University of Minnesota Extension
- Lighting for controlled environment agriculture, Resource Innovation Institute
FAQ
Do bars need to run the full length of the area?
Covering the whole area holds light more even. Resource Innovation Institute guidance notes that continuous runs of linear LED fixtures can improve uniformity compared to grids of older fixtures. It also advises considering how layout affects light levels on the edges, where coverage often falls off first.
Is closer always better?
Not always. Closer bars and lower fixtures raise light at the canopy, but University of Minnesota Extension notes that a proper distance helps plants grow well even with LED lights. Resource Innovation Institute ties spacing to mounting height, so the right gap depends on how high the bars hang.
Can you add side lighting?
Yes. Resource Innovation Institute notes that intracanopy fixtures can be mounted in racks and that the purpose of the lighting system shapes the fixture choice. Side or under canopy bars can reach lower leaves that top lights miss. Hold spacing even so extra light fills gaps instead of creating bright spots.
Do light meters help?
Yes. Resource Innovation Institute suggests reviewing point by point light level values at consistent spacing, not just the peak reading, and mentions quantum meters for measuring light. Taking readings at the center and edges of the canopy shows where bars need to move closer or farther apart.
Does hanging height change bar spacing?
Yes. Resource Innovation Institute lighting guidance suggests fixtures no more than 12 inches apart when they hang less than three feet above the canopy and up to 24 inches apart when mounted higher.
