Many discussions about plant lighting for hydroponic systems are based on the lamp output required per unit of area, which is a good place to start, but it doesn’t tell the full story when it comes to accurate plant illumination. A better way to determine your garden’s lighting needs is to sort out the daily light integral (DLI) requirements of what you’re growing.

DLI is not a complex idea, it simply refers to the total amount of light plants receive during a 24-hour period. It is a combination of not only the light intensity given off by the lamps, but also the duration of the light period. In an indoor garden, unlike many greenhouses, the amount, intensity and duration of lighting can be controlled and manipulated, and there are tools indoor growers can use to maximize plant quality using DLI measurements and recommendations for different crops.

Understanding Daily Light Integral

A DLI measurement tells us how much plant-usable light the crop receives during a 24-hour period. It is expressed as photosynthetically active radiation (PAR) in the 400-700-nm wavelength range. If you compare this to the optimal DLI value for the crop being grown, it’s possible to ensure there's no light deficit that restricts growth and yields.

DLI is expressed as mol/m-2/d-1, which means moles of light (mol) per square meter (m-2) per day (d-1). A mole is a standard scientific unit used to measure a massive number of small entities, such as atoms—6.022 x 1023 to be exact. In other words, DLI refers to the number of light particles or photons plants receive in 1 sq. m (10.8 sq. ft.) of growing space in a day.

Many greenhouse growers use automated systems with sensors and computer controllers that calculate an area’s DLI and graph the results. However, smaller-scale growers can make use of portable DLI monitors, which can be moved around the indoor garden to get an approximate reading for the DLI at the end of each 24-hour period. The DLI can also be calculated using hand-held light meters with some basic conversion factors for different types of lamps.

To get a handle on exactly what DLI refers to and how it relates to light intensity as we see it, it helps to visualize the natural light outdoors. For example, the DLI in the United States can vary from 2-60 mol/m-2/day, depending on factors such as location and season. Outdoor winter light levels in temperate zones are typically much lower than is required for optimal greenhouse production of many common crops, while excessive light can cause problems in the summer.

A dull winter’s day with heavy cloud cover and a short day-length may see a DLI inside a greenhouse as low as 3 mol/m-2/day. On the other hand, a bright, clear, sunny mid-summer’s day with a long day length of 18 hours can create an average DLI of 35 mol/m-2/day inside a greenhouse. Lower-light crops like lettuce require a DLI of 12-14 mol/m-2/day for maximum growth rates, and higher-light crops such as tomatoes require at least an average DLI of 22 mol/m-2/day or up to 30 mol/m-2/day to reach light saturation at maturity.

Greenhouse growers often aim for a target minimum DLI of 10-12 mol/m-2/day for maximum year-round production, with those in lower light climates often making use of supplementary lighting to increase the natural light up to these levels. In an indoor garden, where artificial lighting is the only source of illumination, light can be adjusted as required for the different stages of growth.

Light Measurement

Measuring light in an indoor garden over a 24-hour period may seem like a relatively simple process. After all, lamps provide the same output each hour of the day they are switched on. This is unlike natural light, which varies considerably over the course of a day and is also weather-dependant. However, while the lamp output remains the same, at any one time there are usually light variations within an indoor garden based on factors such as plant distance from the lighting source, the impact of reflectors and reflective surfaces, the overlapping of adjoining lamps, and the shading and density of plants.

When measuring light indoors, the most accurate data can be obtained by taking readings in a number of locations around the system. This process not only allows growers to check where the lower light positions are, but it also helps them know where to place lamps so lighting is as uniform as possible within the indoor garden. Light should always be measured at canopy height, which is at or slightly above the top leaves of the plants. Obtaining a few other readings just inside the canopy is also worth doing to see how much light is reaching the foliage further down in the crop or where smaller plants might be sited.

How to Calculate Daily Light Integral

The DLI for an indoor growroom can be calculated based on readings from a hand-held light meter that records PAR in micromole/m-2, or in foot candles, the more old-fashioned unit.

Steps for using a foot candle meter:

  1. Take and record the foot candle light reading in a single position at canopy height. Repeat this process for different positions around the indoor garden if light is not uniform.
  1. Multiply the foot candle reading by the number of hours the lights are on in each 24-hour period. For example: 1,200 foot candles x 14 hours = 16,800. Divide this by 24 hours =

700 foot candles per hour. Using the equation 700 x 0.15* converts this to 105 micromoles PAR. Finally, 105 x 0.0864** gives a DLI of 9 mol/m-2/day.

* The metal-halide (MH) conversion factor used in this calculation varies for different light sources. Sunlight has a conversion factor of 0.2, high-pressure sodium (HPS) lamps have a conversion factor of 0.13, MH lamps have a conversion factor of 0.15, LEDs have a conversion factor of 0.13 and cool white fluorescent lamps have a conversion factor of 0.13.

** The 0.0864 factor is the total number of seconds in a day divided by 1,000,000.

Once the DLI for an indoor garden has been calculated, it is possible to compare this to the optimal level for the types of plants being grown and adjust light levels as required. For plants that have specific day-length requirements for flower initiation, it may not be possible to simply extend the number of hours the lights are run to increase the DLI to more optimal levels. In this case, the only option is to increase light intensity by installing more lamps if the DLI needs to be increased to maximize growth.

For other plants that are not day-length sensitive, a low DLI can be improved by increasing the number of hours the lamps are run in each 24-hour period. Longer hours means more photons of light are received by the plants for growth and development. In a mixed-crop system, where high- and low-light plants are growing in the same system, providing the ideal DLI becomes more of a compromise. In this case, positioning the higher-light plants directly under the lamps and the lower-light plants further away from the light source can help maximize photosynthesis across the indoor garden.

Light Requirements for Different Crops

The DLI has a significant impact on a number of plant variables, including root and shoot growth, stem thickness, plant height, branching, flower amount and flowering timing. All these factors have a big impact on yields and overall crop quality.

For propagating seedlings and many young cuttings, a low DLI of 6-8 mol/m-2/day is recommended, which should increase to 10-12 mol/m-2/day for older transplants, flowering annuals and small herbs.

  • Many shade-loving indoor plants and ornamentals require a relatively low DLI. African violets and phalaenopsis orchids prefer an average DLI of 4-6 mol/m-2/day.
  • Many ferns perform best at a DLI of 4-6, cyclamen at 6-8, fuchsias at 10-12, chrysanthemums at 10-14, petunias at 16-18 and cut-flower rose plants at 18-22 mol/m-2/day.
  • For butterhead lettuce production, plants need a DLI of approximately 14-16 mol/m-2/day for high-quality head formation, while iceberg lettuce requires even higher levels.
  • Larger, warm-season plants such as tomatoes, cucumbers, capsicums and eggplants require DLIs of 20-30 mol/m-2/day for maximum production. The actual optimal light levels depend on density.
  • Higher-density crops produce more inter-plant shading and require a higher DLI to completely penetrate the thick canopy.

The Effects of Low DLI

Low light has a number of negative effects on many plants, the most significant one being slow growth and lower production levels. However, a low DLI for high-light vine crops such as tomatoes also restricts fruit quality in terms of sugar content, dry weight and flavor. If the DLI falls particularly low, flower and fruitlet drop may occur, as the plants can’t produce enough assimilate for reproductive growth. Plants under low light may also become unbalanced, with tall, weak, thin or excessive vegetative growth, delayed flowering and minimal fruit set and growth.

If a low DLI is combined with warm growing conditions, many plants such as lettuce, herbs and many other vegetative plants may develop physiological disorders like bolting. This is characterized by elongated growth and the development of a flowering stem while plants are still relatively young. Low light can also weaken foliage, making plants more susceptible to a number of disease pathogens.

Knowing an indoor garden’s DLI gives growers the flexibility of strategically placing different types of plants within their hydro system, according to their different lighting requirements. The well-thought-out placement of plants ensures that sufficient light—delivered through a combination of illumination intensity and day length—is provided for each crop and each stage of growth to achieve maximum yields and quality.