What Is the McCree Curve? Understanding How Plants Use Light

When I first started using a light meter and reading about plant lighting, I kept encountering something called the McCree curve. At the time, it just sounded like a scientific chart that didn’t have much to do with what I was trying to do in my backyard garden.

But after weeks of measuring light under different conditions, watching how plants responded, and comparing light sources, I realized that the McCree curve actually explains something I had struggled to put into words:

Plants don’t use all light equally.

This article is about what I observed from real measurements, how my plants behaved over time, and why understanding the McCree curve helped me make better decisions in my garden.

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What Confused Me at First

When I learned the basics of plant light, I understood that plants need usable light for photosynthesis. But why did some light sources with similar numbers on my meter produce very different plant results?

When two light fixtures had similar PAR readings but very different plant growth outcomes, I felt puzzled. I started doubting my measurements, thinking I must be doing something wrong.

Eventually I discovered the McCree curve, which helped me understand that not all wavelengths within the PAR range are equally effective at driving photosynthesis.

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What the McCree Curve Tells Us

The McCree curve was developed based on experiments measuring the relative effectiveness of different wavelengths of light for plant photosynthesis. In practical terms, it shows how efficiently plants use light across the range of wavelengths we measure with PAR.

From what I learned and observed:

• Light in some parts of the PAR range contributes more to photosynthesis than light in other parts
• Blue and red wavelengths are generally more effective than green in driving photosynthesis
• A light source that produces many photons in the effective parts of the spectrum can support better plant growth than one that emits the same total number of photons but concentrated in less effective regions

This matched what I saw in my garden. For example, two lights with similar PAR values but different spectral distributions produced different results in leaf thickness, stem strength, and overall growth vigor.

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How I Tested This in My Garden

I ran a simple comparison with two lights that had similar total PAR values but very different spectral distributions. I measured both lights at canopy height over identical plants for a week.

Here’s a summary of what I recorded:

Light Type	PAR (µmol/m²/s)	Dominant Wavelengths	Observed Plant Growth
Balanced Spectrum LED	600	Blue, Green, Red mix	Compact plants with full leaves
Red-Biased LED	590	Mostly Red	Taller stems and thinner leaves

Although the PAR numbers were nearly the same, the plant responses were noticeably different. Under the balanced spectrum light, plants formed thicker leaves and sturdier stems. Under the light with mostly red wavelengths, plants stretched taller and looked less robust.

This observation made me realize that PAR value alone does not tell the full story of how plants use light.

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Why the McCree Curve Matters for Everyday Gardening

In practical gardening terms, the McCree curve helped me understand why:

• Two lights with similar PAR numbers can produce different plant results
• A balanced spectrum often works better than a light heavily weighted toward one color
• Plants may look healthy under a light that measures high PAR but may not develop fully if the spectrum is not effective

Before understanding this curve, I would pick a light based on its PAR number alone. After comparing plant responses under different lights and noticing actual growth over time, I changed my approach.

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What I Do Now When Choosing Light

Over several growing seasons, I developed a simple habit that helped me make better lighting decisions:

First, I look at the PAR value to ensure there is enough usable light for my plants. Then I pay attention to how the light is distributed across wavelengths. I try to choose lights with a more balanced output in the effective parts of the PAR range.

I do not rely on spectrum charts alone; instead, I use measurements along with observing how plants actually grow. For example, if seedlings under a new light are stretching rather than forming compact, healthy foliage, I revisit the light choice or placement.

This practical approach consistently led to stronger growth in herbs, leaf vegetables, and flowering plants in my garden.

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A Simple Way to Think About It

Understanding the McCree curve helped me stop thinking of PAR as a single number that tells the whole story. Instead, I began to see light in terms of quality and effectiveness.

Here is how I now think about it:

Plants need usable photons in the wavelengths they can convert into energy. The McCree curve shows which parts of the PAR range are most useful. If most of the photons are in those effective regions, plant growth tends to be healthier and more vigorous.

This helped me interpret my light meter values more meaningfully and judge whether a light setup was truly supporting plant growth.

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Final Reflection

At first, the McCree curve seemed like an extra technical concept I did not need. In practice, it became one of the most useful ideas for understanding why plants behave the way they do under different lights.

By combining what the curve shows with actual measurements and real plant observation, I could make more informed choices about lighting in my garden.

Plants do not respond to light numbers alone. They respond to the kind of light they can use. Understanding that difference changed my approach to plant lighting for the better, and it can help other gardeners make better decisions too.