How Does Light Affect Plant Growth?

Light can be used to optimize your grow light environment to grow healthy, uniform crops, and benefit from year-round production, increased yields and shorter harvest cycles.

There are three characteristics of light you should consider when designing the ideal light environment for your crops:

  • Light quality
  • Light quantity
  • Light duration.

But how do they work together?  Let’s start with defining light quality.

Venn Diagram showing light quality, quantity and duration
Optimizing your LED grow light environment requires multiple factors

What is Light Quality?

Light quality or light spectrum are the wavelengths of light reaching your crop. Light for crop production comes primarily from the photosynthetically active radiation (PAR) waveband of the light spectrum. PAR includes wavelengths of between 400 and 700 nanometers (nm). These wavelengths drive photosynthesis, the chemical process that drives plant growth. LED technology allows you to provide your plants with the wavelengths of light that are most useful to them.

The electromagnetic spectrum showing UV, PAR, Far-red, and visible wavebands
Different wavelengths of UV, PAR, and far-red light drive photosynthesis.

What is Light Quantity?

Light quantity, also known as light intensity, is the total amount of light that hits your crop. Indoor agriculture measures light quantity in PPFD. PPFD is the number of micromoles of light that hit any given square meter of your crop each second.

LED Grow Light Scientist
Can plants use other wavelengths of light for photosynthesis?

While we currently focus on wavelengths in PAR (400-700 nm) for photosynthesis, there is emerging evidence that wavelengths outside of this waveband can contribute to photosynthesis. As LED research continues, we may see a change to the boundaries of the PAR region in the future.

What is the Ideal PPFD?

Every crop has an ideal PPFD that it requires for optimal growth. This light requirement reflects the plants’ natural habitats. For example, plants that grow on forest floors, like orchids, have lower PPFD needs than tomatoes that grow in open valleys.

An ideal light environment achieves your crop’s target PPFD without exceeding it and distributes light uniformly across the plant canopy. This ensures that your crop’s growth is similarly uniform.

PPFD is the number of micromoles of light that hit any given square meter of your crop each second.

Overlighting your crop can cause plant damage and wastes energy. For example, overlighting lettuce can cause tip burn, which makes the damaged plants more difficult to sell.

What is Light Duration?

Light duration is the number of hours of light that your crop receives. Light duration is also called photoperiod. Photoperiod controls flowering, dormancy and other biological responses in many plants.

What is Photoperiodism?

Photoperiodism is how plants respond to the number of hours of light they receive each day. Photoperiodism is triggered by a critical period of darkness.

Long-day plants require a shorter period of darkness relative to light to trigger flowering. Short-day plants require a longer period of darkness relative to light to trigger flowering. Day-neutral plants will flower regardless of the ratio of darkness to light each day. Instead, day-neutral plants flower upon reaching a particular developmental stage or if other environmental cues are present.

A plant’s reliance on photoperiod for flowering can be further categorized as either necessary (obligate) or helpful (facultative).
Photoperiod is the amount of time a plant receives light.

LumiGrow grower
Here’s a Photoperiodism Terms Cheat Sheet:
  • Short-day: (Long-night) Plants that flower when the darkness period is longer than a critical duration
  • Long-day: (Short-night) Plants that flower when the darkness period is shorter than a critical duration
  • Day-neutral: Photoperiod does not affect flowering, dormancy or other biological processes
  • Obligate: Plants that require a darkness period of a critical length to flower
  • Facultative: Plants that accelerate flowering when given a darkness period of a critical length

What is Daily Light Integral (DLI)?

DLI is the total amount of light (PPFD) provided to the crop over a 24-hour period. In other words, DLI is a measurement of both light intensity and duration. DLI is measured in moles per square meter per day (mol·m-2·d-1). An ideal lighting environment will ensure that the crop receives the same total amount of light each day so that the crop can grow predictably and consistently.

How do I use DLI?

DLI can be used as a tool for maximizing crop growth. As seen in our Research department’s study, greenhouse-grown baby lettuce yields can be increased by 40% maintaining DLI over an extended photoperiod.

Integrated smart lighting technology, such as the LumiGrow Light Sensor, can automate your light delivery to ensure that you’re reaching your crop’s DLI target. This takes the guesswork out of reaching your crop’s DLI, which helps you avoid overshooting your target DLI and wasting energy.

Now that you understand the factors that go into calibrating your lighting, you’re ready to maximize your crop’s growth.

LumiGrow grower
Here are some tips for calculating your facility’s ideal light levels:
  • Include ambient solar PPFD when calculating your target PPFD in your greenhouse production so that you don’t overlight your crops.
  • Use recommended target DLI and PPFD ranges as a starting point to establish the ideal metrics for your specific varieties and site conditions.