In the good old days, it was easier for gardeners to talk about the artificial light plants required because most gardeners used fluorescent bulbs and most manufacturers produced about the same quality of bulbs. We could easily say that, for example, African violets should be grown 6 inches below a two-bulb fixture, and that would be good advice that anyone could follow.
That has all changed with LED lights. Both the quality and intensity of light from fixtures vary a lot. Telling someone that you grow African violets 6 inches below LED grow lights does not tell them very much. That height under your lights could be too high, or so low that it burns the leaves.
It’s time gardeners start using other units of measure, such as the Daily Light Integral (DLI).
Key Takeaways
- Gardeners need to understand the Daily Light Integral (DLI) and PPFD to grow better plants indoors.
- The quality of LED lights is measured differently than old fluorescent lights.
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What is the Daily Light Integral?
The daily light integral, or DLI, is a measure of the total amount of usable light a plant receives in a day. It is an excellent way for gardeners to compare light systems and growing conditions.
To understand a plant’s need for light, think of light and water as being similar. A bucket of water will wet the soil around the roots and a bucket of light allows the plant to get the energy it needs to make a day’s worth of food. The DLI is equivalent to a full bucket of light.
If a plant only gets half a bucket of light per day, it won’t grow properly. You have seen that happen with seedlings when they get very leggy.
Shade-tolerant plants, like ferns, have smaller buckets. Sun-loving plants, like tomatoes, have larger buckets. The same plant can also have different-sized light buckets at different times of its life. For example, a growing pepper seedling has a smaller light bucket than a pepper plant, which is ripening fruit.
On the other hand, if the plant gets more than a bucket of light, it can’t use the excess, and if there is too much excess it can harm the plant. Your job as a gardener is to select lights and set them up so that they will give your plants a bucket of light each day.
The amount of light a plant gets depends on four factors:
- Quality of light.
- The intensity of the lamp.
- Height of the lamp above the plant.
- Duration of exposure.
Let’s have a closer look at each of these.
Quality of Light
Light is made up of different wavelengths which we see as different colors. Plants use some colors more efficiently than others. They prefer blue and red light but also need a bit of green and yellow light.
With older fluorescent bulbs we used to measure this as the color of light and refer to units such as 2700K for warm white light and 4000K for cool white. These kelvin values describe the light as humans see it so it is not that useful when we want to measure the light plants use. A better way to measure light for plants is with PAR, the Photosynthetically Active Radiation that plants use for photosynthesis. PAR measures light based on how plants use light.
Any LED light sold as a “grow light” should provide the spectra of light to show that it is suitable for plant growth. Unfortunately, many so-called grow lights don’t do this, which means you have no idea if the light produces the right wavelengths for plants. Such lights may produce some plant growth but I don’t consider them to be “real” grow lights.
Intensity of the Lamp
Gardeners have traditionally measured light intensity in lux or foot-candle units and several fairly inexpensive meters can be used to get these values. The problem with these devices is that they measure the light as we see it and not as plants use it. A better way to measure light intensity for plants is to use PPFD values.
A high PPFD provides a lot of light in the wavelengths plants can use. A low PPFD is a low level of light for plants.
There are several ways to get the PPFD values for LED lights: manufacturer information, a PAR meter, a PAR app for your phone, and a standard light meter.
The manufacturer of a true LED grow light will provide a PPFD Map, which is a diagram showing the values under the light at different heights. A light that does not have this is not a grow light.
You can measure your own lights using a PAR meter. These tend to be a bit expensive for home gardeners, but if you plan to grow a lot of plants under lights it might be worth the expense.
There are several PAR meter apps for your phone. These have been tested by Migro who found the one from Photone to be very good.
The third option is to use a regular light meter that gives either lux or foot-candle values. You can convert these values to PPFD values. The conversion factors are approximate and depend on the type of light, but the formulas below are close enough for home gardeners.
Height of the Lamp above the Plant
As a light is moved up and down above a plant, the intensity of light changes. A light that is closer to the plant provides much more light than one that is higher up.
If you use either a PAR meter or a lux meter to measure light intensity you can simply hold the meter at the surface of the plant and you will automatically get a value that is corrected for the height of the light and you can ignore the height of the light in calculations.
Duration of Exposure
Think back to our bucket of water analogy. You can fill a bucket with water slowly or quickly. The same goes for light. The bucket full of light can be supplied with a lower intensity of light over a longer period of time, or with a higher intensity over a shorter time.
One way to compensate for a cheaper light with lower intensity is to leave it on longer.
There is a limit to this. Firstly, the bucket has to be filled in a 24-hour period. Secondly, plants also need a dark rest of at least 8 hours each day. Within these limits, you can set the duration of light to provide the required total light needed.
Calculating DLI Values
The DLI is easily calculated for artificial lights using this equation:
DLI (mol/m²/day) = 0.0036 x PPFD x duration (in hours) (example: a PPFD of 800 for 12 hours = DLI of 34.6)
If you don’t have a PAR meter you can convert lux and foot-candle values to PPFD using the following conversion factors:
PPFD value = Foot-candles X 0.15 (cool white light factor) (example: 700 fc = 105 PPFD)
PPFD value = Lux X 0.015 (example: 7,000 lux = 105 PPFD)
The conversion factors are approximate and depend on the type of light. The above factors are based on cool white light and are close enough for home gardeners. Other factors can be found online.
What DLI Does Your Plant Need?
The best DLI value depends very much on your plants. Low-light plants need a smaller value while high-light plants need larger values. Some plants grow well at lower values but require higher values to flower.
Plants are quite adaptable to light and many will grow well in a range of values. The following table will give you some guidelines.
| Plant | Suggested DLI (mol/m²/d) |
|---|---|
| African violet | 4 – 12 |
| Basil | 15 – 25 |
| Ferns | 5 – 15 |
| Lavender | 15 – 25 |
| Lettuce | 12 – 20 |
| Orchids – Phal | 4 – 8 |
| Peppers | 25 – 35 |
| Pothos | 4 – 14 |
| Snake plant | 4 – 14 |
| Spider plant | 4 – 14 |
| Streptocarpus | 7 – 12 |
| Succulents | 30 – 50 |
| Tomato | 22 – 30 |
| ZZ plant | 2 – 10 |
The best DLI value also changes as the plant goes through its growth cycle. Seedlings can usually grow well with less light. As they get older they can take more light. However, it’s been my experience that seedlings do just fine under the same light as adult plants of the same type.
Plants that go dormant and slow their growth can also use a lower value. The extreme case is a dormant bulb or tuber which requires no light.
Some seeds need light to germinate, but normal room light is adequate. They should be moved to higher light as soon as you see growth above the soil level.
Proper Light for Seedlings
Seedlings should get a good light supply as soon as they germinate. As a general rule give them a DLI of 6 to 10 with a 12-hour light cycle, or treat them as mature plants. High-light seedlings like peppers and tomatoes will do well with a DLI of 20-25.
