Once you understand what under canopy grow lights are and why uplighting from below works, the next question is simple: how much under canopy light do you actually need, and how should you space it? This article keeps the answer practical—no complicated formulas—so you can design a usable layout and adjust it with real crop feedback.

Start With the Job of Under Canopy Lighting

Under canopy LEDs are supplemental. They are not there to compete with your top lights or to turn the bottom of the room into a second sun. Their main jobs are:

• Keep shaded lower leaves photosynthetically active
• Support lower bud sites so they finish more like “A” material
• Reduce the quality gap between the top and bottom of the plant

When you think about intensity and spacing, always come back to this idea: under canopy lighting is about raising the floor, not trying to match the ceiling.

A Simple Way to Think About “Enough Light”

Instead of chasing perfect numbers, it helps to define “enough” in practical terms. For most indoor canopies:

• The top gets more than enough photons from your main fixtures.
• The middle is usually acceptable if top lighting is well designed.
• The lower third is where productivity starts to fall off.

Your goal with under canopy lighting is to: take that lower third from “barely contributing” to “reliably productive.” That means you want intensity that is clearly noticeable at the lower leaves, but not so strong that it drives excessive heat or stress at the base of the plant.

Approximate Intensity Ranges Without Overcomplicating It

Every facility, cultivar, and lighting stack is different, so there is no single magic PPFD value. But you can think in simple zones:

• Too weak: lower leaves still look flat and shaded; you barely see them reacting to light; you can turn the under canopy off and the plant almost looks the same.
• Reasonable supplemental: you can clearly see the lower canopy brighten; leaf undersides have a soft glow; temperatures remain manageable and plants do not curl away from the light.
• Too aggressive: lower leaves show stress or unusual curl; media near the stem dries noticeably faster; you feel hot radiant output at ankle level.

If you do have a PAR meter, a common approach is to target under canopy PPFD in a range that is dramatically lower than the top, but still meaningful. You might see, for example, a top canopy PPFD in the 700–900 µmol/m²/s range and under canopy supplemental levels closer to a few hundred µmol/m²/s, depending on your crop strategy. The exact numbers matter less than how well the plant responds over a full cycle.

Spacing Basics: Bars, Benches, and Racks

Spacing is where many layouts fail. You don’t want a few bright “light pipes” shooting between plants; you want a smooth, connected glow under the canopy. A few simple rules help:

• Follow the structure, not just the floor. On rolling benches or tables, align under canopy bars under the plant rows rather than under the aisle gaps.
• Keep gaps small enough that light overlaps. If you can see clearly dark strips between bars when standing back, your spacing is likely too wide.
• Match bar length to bench length. Short, scattered fixtures make coverage uneven; continuous or near-continuous runs give better uniformity.

For a quick overview of practical form factors and lengths, you can review your options in the under canopy grow light collection. This also helps you visualize how many bars are needed per bench or rack level.

How to Check Your Layout Without a Light Meter

Even without instruments, you can sanity-check intensity and spacing using the canopy itself. A few simple tests:

• Shadow check: turn under canopy LEDs on and top lights to a consistent level. Look at the underside of leaves and lower stems. You should see a soft fill of light, not deep black shadows.
• Distance check: step several feet back and look for dark “channels” between bars. If your eyes can see striping, the plants are feeling it too.
• Heat check: stand at plant height and feel the environment at the lower canopy. It should feel gently warmed but not harsh or uncomfortable.

If any area looks significantly darker or hotter than the rest, adjust spacing or dimming instead of assuming the entire room needs more power.

Using a PPFD Meter or Map When You Have One

If you have access to a PAR/PPFD meter, you can be more structured without turning it into a science project. Focus on:

• Mapping a small grid at lower canopy height (not the entire room)
• Comparing “under canopy off” vs “under canopy on” readings
• Checking that extremes (highest and lowest points) are not wildly different

You are looking for a reasonable lift in PPFD at the lower canopy and improved uniformity—not a perfect, flat heatmap. The real test will always be flower development and grade-out after harvest.

Run a Small A/B Test Instead of Guessing

Because every facility has its own combination of top fixtures, genetics, and HVAC, one of the best ways to answer “how much is enough?” is a small A/B test:

1. Pick one bench, table, or rack level as your “test zone.”
2. Install under canopy lighting with the spacing and intensity you believe is reasonable.
3. Leave a similar bench or level without under canopy lighting or with reduced intensity.
4. Keep all other variables as consistent as possible.

At harvest, compare:

• Top vs lower bud quality and size
• Total usable yield (not just wet weight)
• Trim time and how much material drops into lower grades

If the under canopy zone clearly delivers better lower-canopy development without new issues, you can scale that intensity and spacing pattern with more confidence.

Energy, Dimming, and Scheduling

Intensity is not just about how bright the bars are—it is also about how long they are on. Some facilities run under canopy lights for the full photoperiod; others only run them during certain hours or stages. Practical options include:

• Stage-based: use under canopy lighting mainly in mid-to-late flower, when shading is strongest.
• Time-based: run under canopy bars only during the most productive hours of the day rather than all day.
• Dimming-based: pair under canopy intensity with top light dimming schedules so the room feels balanced.

If you are already working within a utility rebate or energy-efficiency plan, it’s worth framing under canopy lighting as a tool that helps you get more marketable yield per kWh. For broader strategy and incentive context, you can review guidance at Grow Lights Rebate – Utility Energy Rebates Guide.

You can also look at how high-efficiency top fixtures, such as advanced LED rack lights, work together with under canopy systems to reach your production goals. For example: Griffin Advanced Grow Light shows how modern top lighting can provide strong overhead coverage while under canopy LEDs handle the shaded zones below.

Putting It All Together

Designing under canopy lighting does not have to be complicated. Start with a clear job description—support the shaded lower third of the canopy—then set intensity and spacing to match that job. Aim for a smooth, gentle glow from below, not harsh spotlights. Use your eyes, a simple heat check, and, if available, a small A/B test to refine your layout over a cycle or two.

As you tune your setup, keep your content and decision-making loop tight: refer back to the fundamentals in What Are Under Canopy Grow Lights? and Why Uplighting From Below Works, and then let your plants—and your harvest numbers—tell you when you have reached “enough.”