Light

Light refers to the energy source that drives photosynthesis, which converts light, water, and CO2 into the sugars that plants need to grow. The amount, quality, and duration of light a plant receives directly affects its size, structure, and yield.

Much like other variables involved in growing plants, light can

Sunlight

The sun provides a full spectrum of light that no artificial source can perfectly replicate. Outdoor growers benefit from free, powerful light that naturally shifts in spectrum and duration with the seasons — bluer in summer to promote vegetative growth, redder in autumn to support flowering.

On a clear summer day, direct sunlight delivers roughly 800–1000+ µmol/m²/s of photosynthetically active radiation (PAR), far more than most indoor setups.

Grow Lights

Indoor growers rely on artificial lighting. The most common types are:

LED (Light Emitting Diode)

The current standard for most indoor grows. For most growers, this is it. Modern LED panels are energy-efficient, run relatively cool, and are available in full-spectrum configurations.

• Low heat output compared to HID
• Long lifespan (50,000+ hours)
• Higher upfront cost, lower running cost
• Available in a wide range of sizes and price points

HID (High Intensity Discharge)

An older technology that is still widely used. HID lights come in two main types:

• High light output
• Significant heat — requires good ventilation and often air-cooled hoods
• Higher electricity cost than LED
• Bulbs need periodic replacement

Some growers use MH during veg and switch to HPS for flower. Dual-arc or ceramic metal halide (CMH/LEC) bulbs combine both spectrums.

Fluorescent (CFL / T5)

Low-power lights suited for seedlings, clones, and small-scale grows.

• Low heat, low cost
• Limited penetration — not ideal for flowering full-sized plants
• Useful as supplemental lighting or for early growth stages

Advanced Concepts

PAR, PPFD, and DLI

These terms describe how much usable light a plant actually receives:

PPFD targets vary by growth stage:

Light Spectrum

Plants respond to different wavelengths of light in different ways:

• Blue light (400–500nm) — Promotes compact, bushy vegetative growth
• Red light (600–700nm) — Drives flowering and bud development
• Far-red (700–750nm) — Influences stretch and flowering response
• UV (280–400nm) — Some evidence suggests UV-A and UV-B can increase trichome production
• Green (500–600nm) — Once considered useless to plants, but research shows it penetrates the canopy and contributes to photosynthesis in lower leaves

Full-spectrum LEDs aim to cover the entire PAR range and are the simplest option for growers who do not want to manage spectrum shifts.

Light Distance and Coverage

The distance between the light and the canopy affects both intensity and coverage:

• Too close — Light burn, bleaching, or heat stress on the top of the plant
• Too far — Stretchy, weak growth as the plant reaches for the light
• Uneven coverage — Hot spots in the center and dim edges

Most LED manufacturers provide recommended hanging heights. A general starting point is 45–60 cm (18–24 inches) above the canopy during vegetative growth, adjusting based on the plant’s response.

Light Schedules

The duration of light per day is critical, especially for photoperiod plants. See the photoperiod article for a full breakdown of light schedules and their effects on flowering.

Common schedules:

See Also

• Get Started Growing — Beginner’s overview of cannabis cultivation
• Photoperiod — How light duration controls flowering
• Autoflower — Varieties that flower regardless of light schedule
• Nutrients — Feeding plants to match their growth under light