Grow lights for hydroponics: PAR, PPFD and DLI explained, light hours and distance

TUREFF• •0

Indoors, light is usually what limits growth. A windowsill that seems bright to us often gives plants far less than they would get outdoors, especially in winter. That is why many hydroponic growers add a grow light. Choosing and setting one up is easier once you know three terms that appear on every specification sheet.

PAR: the light plants use

PAR stands for photosynthetically active radiation: light in the wavelength range of roughly 400 to 700 nanometres, the range plants use for photosynthesis. It describes a type of light, not an amount. Lumens and lux, the units on household bulbs, measure brightness as seen by the human eye, which is most sensitive to green and yellow. They are a poor guide for plants.

PPFD: how much reaches the leaves

PPFD (photosynthetic photon flux density) is the amount of PAR light landing on a surface each second, measured in micromoles per square metre per second (µmol/m²/s). This is the number that matters at the plant.

PPFD falls as you move the light further away, and it is usually lower at the edges of the light's footprint than in the centre.
Good manufacturers publish a PPFD map at a stated hanging height. Treat figures without a height and area as marketing.

DLI: the total for the day

DLI (daily light integral) is the total amount of PAR a plant receives over a whole day, measured in moles per square metre per day (mol/m²/day). It combines intensity and duration, which is why it is the most useful single target. You can estimate it as: DLI = PPFD × hours of light × 0.0036. The 0.0036 simply converts seconds to hours and micromoles to moles. For example, a steady PPFD of 250 µmol/m²/s for 16 hours gives a DLI of about 14.4 mol/m²/day. As a rough guide, lettuce is often grown at a DLI in the low to mid teens; published targets commonly fall between about 12 and 17. Fruiting crops such as tomatoes and peppers generally need considerably more, which is why they are harder to grow well under a modest indoor light. Herbs sit somewhere in between, depending on the species.

Light hours (photoperiod)

Leafy greens and herbs are commonly given around 14 to 16 hours of light a day.
Plants also need a dark period. Running lights 24 hours a day is not a shortcut and can stress some plants.
Some plants are triggered to flower or bolt by day length, so long days can push lettuce, coriander or spinach to run to seed sooner, especially when warm.
A plug-in timer is the easiest way to keep the schedule regular.

Because DLI is intensity multiplied by time, a less intense light run for longer can reach the same daily total as a stronger one run for fewer hours, within limits.

Distance

Closer means more intense light, but also more risk of bleaching or scorching leaves, and more heat on the plants from some fittings.
Follow the manufacturer's recommended hanging height for your crop and growth stage as a starting point, then watch the plants.
Too far away: long, pale, stretched stems reaching up (often called legginess).
Too close: bleached or yellow patches at the top of the plant, leaf edges curling up or crisping.
Seedlings generally want gentler light than mature plants. Raise the light as plants grow so the tops keep roughly the same distance.

Choosing a light

LEDs are now the usual choice for home growers: efficient, cool-running and available as bars, panels and small clip-on lights.
Full-spectrum white lights are easier to live with than purple/pink red-and-blue ones, and let you see the true colour of leaves, which helps when diagnosing problems.
Match the size of the light to the size of the growing area. A light that is too small leaves the edges dim.
If you want to measure, a PAR meter gives real PPFD readings. Phone light-meter apps can give a rough idea but are not reliable for precise numbers.

Safety

Use lights rated for the conditions, keep plugs and cables away from water, use a drip loop on cables and do not look directly into bright LEDs for long. Ask about your light, its height and what your plants are doing in the comments, and share any set-up that has worked well.