How light relates to nutrition
During photosynthesis, plants use light energy to form organic compounds from carbon dioxide and water. Mineral nutrition supports plant function and development, but it does not replace that energy source. Increasing fertilizer concentration cannot compensate for darkness.
Check the light before changing the feed recipe. Lighting and feeding need to suit the plant's stage. If growth slows, inspect those conditions and record the readings first. That gives the next adjustment a reason and leaves you with a starting condition to compare against.
Read useful fixture specifications
Power consumption lets you estimate electricity use. PPF describes the photon output of a fixture, commonly in µmol/s. PPFD describes the photon flux per unit area at a measurement location, in µmol/(m²·s). Light at leaf height matters to the plant, so a PPFD map is useful when it states the distance and area measured.
Check whether advertised wattage is actual consumption or an “equivalent” figure. Compare the lit area with your reservoir footprint. A high reading immediately below the centre does not mean the edges receive the same amount. The conditions behind a map, including reflective walls, matter too.
Lux measures brightness as seen by humans. Converting lux to PPFD depends on the spectrum, so one conversion factor for every fixture can be misleading. A phone app may help compare the centre and edge under the same light, but its accuracy and suitability need to be checked separately.
Spectrum without choosing by colour alone
A white LED fixture with suitable spectrum and intensity can grow plants. Purple light is not proof of quality. Its visible colour does not tell you about coverage, heating, service life or photon delivery at the leaves.
For a first setup, choose a fixture with clear specifications and operating instructions. Being able to inspect leaves under reasonable colour rendering is useful too: it is harder to judge yellowing when everything looks purple. Establish adequate light delivery before treating spectral adjustments as your first experiment.
PPFD and DLI: intensity over a day
DLI is the amount of light received during a day, expressed in mol/(m²·day). Under constant intensity, calculate it as DLI = PPFD × hours × 0.0036. Daily light quantity is useful alongside an instantaneous intensity reading.
For an illustrative example, PPFD of 200 µmol/(m²·s) over 16 hours gives DLI of 11.52 mol/(m²·day). This is a calculation, not a universal crop target. At PPFD 100, the same schedule gives 5.76. Identical timers can therefore produce very different conditions.
Where intensity varies, as near a window, a single midday measurement cannot give the entire DLI. You need readings over time or an estimate of total light received. Plants on the same shelf may also receive different daily totals because of position and shading.
Set the photoperiod for the crop
Day length affects both light quantity and the development of some crops. Choose a schedule for your plant and use a timer. Do not automatically switch to continuous lighting to make up for a weak fixture. Plant responses differ, and the schedule should follow your growing program.
When comparing schedules, record on and off times, fixture power and hanging height. Changing duration and intensity together leaves the cause of any result uncertain. For a first observation, keep one of those conditions constant.
Hanging distance changes as plants grow
Start with the manufacturer's guidance for your fixture and growth stage. “Every light belongs 30 centimetres above the plant” ignores power, optics and fixture size. Raising or lowering a fixture changes both intensity and the area it covers.
As the canopy grows, upper leaves move closer to the lamp. Recheck the gap and inspect the middle and edges of the planting. Do not judge the whole shelf from the tallest shoot; shorter neighbours may remain shaded.
A sudden increase after a weak-light regime calls for attention to plant response. Record the change and follow new growth. Dry margins, pale areas and wilting have several possible causes, so check water, temperature and feeding as well.
Coverage and heat
Space similar plants so their leaves do not overlap excessively. A fixture with a strong centre and weak edges may suit a smaller planting area. Moving one pot can sometimes address the problem more directly than increasing power across the entire shelf.
LED fixtures also release heat. Check conditions near the plants and the solution temperature, especially when a reservoir sits near a warm surface. Air movement should support exchange without directing a constant strong stream at one young seedling.
Calculate running cost
A light drawing 60 W for 16 hours a day over 30 days consumes 0.060 × 16 × 30 = 28.8 kWh. Multiply that by your electricity rate. The example covers one lamp and excludes the air pump, water pump and other equipment.
Compare consumption for the area you need to illuminate as well as the fixture price. A low-power lamp can still be inconvenient if it leaves half the planting in shade. Record actual settings when using a dimmer, since full-power specifications may not describe your operating conditions.
Check lighting before changing the feed
- Confirm the timer follows the intended schedule.
- Check the gap above the growing leaves and shading from neighbours.
- Compare centre and edge while accounting for your meter's limits.
- Record air and solution temperature.
- Change one setting and assess new growth at the next observation.