"Bought a 'full spectrum' LED with purple light — plants are growing but look odd: stretched, pale, large leaves and thin stems."
A combination of red (660 nm) and blue (440 nm) does not constitute a true full spectrum. Plants also use green and yellow light for normal growth regulation.
Quick Glossary
- Blue range (400–500 nm): stimulates compact growth, thick stem formation, regulates stomatal opening, serves as a day signal for photoperiodic responses
- Red range (600–700 nm): primary for photosynthesis with the highest chlorophyll absorption; peak at 660–680 nm
- Green range (500–600 nm): partially reflected, but penetrates deeper into leaf tissue
What the Plant Actually Absorbs and Why
Chlorophylls have two main absorption peaks: blue (430–450 nm) and red (640–680 nm). However, photosynthetic efficiency remains nearly uniform across the entire PAR range from 400 to 700 nm.
Green light penetrates deeper into leaf tissue and reaches lower canopy layers in dense plantings, where blue and red have already been absorbed by upper leaves.
Phytochrome responds to the ratio of red (660 nm) to far-red (730 nm), regulating flowering and stem elongation. Cryptochrome responds to blue, regulating circadian rhythms and stomatal opening.
How Different Spectra Affect Morphology
Excess blue: compact plant with short internodes, small thick leaves, dark green colour. Useful for microgreens and dense herbs.
Excess red: elongated plants with long internodes, thin stems, large leaves. Resembles a "plant searching for light."
Balanced white spectrum: broad PAR coverage with natural morphology and typical crop development.
Far-red (700–750 nm): influences the phytochrome system, enabling manipulation of flowering and morphology parameters.
Spectrum and Growth Stage: What Is Needed When
Germination and early vegetative: higher blue percentage (20–30%) for compact habit and thick stem.
Active vegetative: balance of blue and red with broad green for balanced growth.
Flowering and fruiting: shift toward red with optional channel adjustment and far-red component for photoperiod-sensitive crops.
Practical Comparison: Purple LED vs White LED
Purple LED: high PAR fraction of total consumption and high efficiency (PPE), but uncomfortable to work under, distorts colours, complicates diagnosis. Lack of green and yellow can cause abnormal morphology.
White broadband LED: includes green and yellow, comfortable to work under, natural morphology. Modern LEDs deliver PPE 2.8–3.2 µmol/J with a broad spectrum.
For commercial production, white LED is the more practical choice.
Three Mistakes That Cost the Most
- Choosing a spectrum based on a "full spectrum" label without checking the actual spectral power distribution (SPD graph)
- Ignoring spectrum when diagnosing diseases and deficiencies. Under purple lighting, symptoms are nearly invisible to the human eye
- Locking in a single spectrum for the entire cycle without adapting to the needs of different growth stages
Signs That the Spectrum Is Suitable
- The plant has typical morphology: natural height-to-width ratio, stem thickness, and leaf size match the crop standard
- No excessive elongation at adequate PPFD levels
- Leaf colours are natural and uniform, allowing disease diagnosis without moving plants to natural light