Lighting for houseplants – The complete guide

Just like us, our plants have certain basic needs that must be met in terms of quality and quantity to enable healthy growth. In addition to the right substrate, moisture and nutrients, this is especially the proper light. Plants are photoautotrophic organisms, i.e. they meet their energy requirements exclusively by capturing light. If there is not enough light, growth stagnates and the plant can survive for a while on stored starch, for example, until it finally starves to death.

So we have to make sure that the light available to our plants can permanently cover their energy requirements. Diffuse room lighting is generally not sufficient for this, even what appears bright to our eyes is usually not enough for plant growth.

This article will discuss how much we really need in the end, how we can save costs and what else to look out for in terms of the quality of the plant lamp.

Lamps should be adequately dimensioned, but also cost money to purchase and operate. So you will often find a compromise in between. The actual light requirement depends on the plants being cultivated and is most easily expressed in lux. Lux is made up of the Si units lumen/square meter and is therefore very easy to calculate. Lumen is always the total amount of light emitted, in this case by our artificial light sources, and is indicated on the packaging.

Example:
The shelf of a Milsbo display cabinet (66 x 36 cm) is to be illuminated with a lamp with 1400 lumens: 1400 lumens / 0.24 m2 = 5830 lux

But is that a lot or very little? A few examples:

Sun under a clear sky: > 100,000 lux
Overcast sky in summer: 10-20,000 lux
Overcast winter day: ~2-5,000 lux
Trop. Rainforest in the undergrowth: ~2,000 lux*
Office lighting: 500 lux
Moonlight: approx. 0.1 lux

*from Becek & Salim 2017

Unshaded solar radiation near the equator can reach up to 200k lux (own measurement). However, we will never have to imitate such extreme values, as even plants adapted to strong light cannot utilize more than ~50k lux. For most of our typical house plants such as Alocasia, Philodendron or Begonia, however, significantly lower values are sufficient. In fact, they would burn or fade if they were exposed to so much light for so long. The following illustration provides a practical overview.

The ~5000 lux from the example calculation is therefore in the middle range and is well suited for many well-known houseplants. For anthuriums or begonias, for example, it should be a little less, for candelabra flowers or desert roses more. Of course, this representation can only serve as a guideline and in many cases is intended as a minimum value. The amount of light required also depends on the spectrum and the duration of illumination; if the spectrum of the lamp is well matched, some reduction in lux can be achieved. A short 8h photoperiod, on the other hand, requires comparatively more light intensity than a 12h photoperiod.

The following illustration from our greenhouse shows that natural sunlight in our latitudes is often insufficient, not only in terms of intensity but also in terms of duration. The intensity of solar radiation on an average day in November 2023 is recorded behind double glass (similar to window glass) on a horizontal surface without shading. Significant lighting takes place from around 8 a.m. and sometimes reaches over 5000 lux at midday, but after 3 p.m. it is practically night for the plants. Plant lamps can supplement the missing intensity and/or duration effectively.

Fig. 2: Solar radiation in lux in our greenhouse in Herten, West Germany on a slightly cloudy day in November 2023. Measurement under double glass, horizontal surface without shading.

For millions of years, plants have only ever had one source of light – the sun. Accordingly, evolution has ensured that precisely this light can be used as efficiently as possible. So basically, you can do nothing wrong if you use artificial lighting that is as similar to sunlight as possible, i.e. white light. However, due to various effects of the absorption properties of the pigments in the plant tissue and of optoelectronics to imitate the desired light, there are also many possibilities for optimizing the spectrum.

Fig. 3: Comparison of the photosynthetic action spectrum according to McCree, K. J., 1972 (modified) with marking of PAR radiation and weighting of lumen

The graphical representation of the spectrum of the light source is necessary for assessment. It simply describes the brightness of the light source in the different colors, specified in nanometers of the wavelength of the electromagnetic radiation. Photosynthesis takes place in the range of approx. 400 – 700 nm (PAR), which actually corresponds pretty closely to our ability to see. Shorter-wave radiation, i.e. < 400 nm, is UV radiation that has no direct significance for photosynthesis, but can certainly support the formation of secondary metabolites. In plants, for example, these can be protective pigments (anthocyanins) that give leaves a reddish color. In private culture, however, this is usually ignored. After UV comes violet and then blue radiation up to approx. 500 nm. Blue light is essential for photosynthesis, chlorophyll has absorption maxima at approx. 430 and 450 nm. Between 500 and 600 nm is green to yellow, this range is also important for photosynthesis. Plants appear green because a slightly larger proportion of the green light is reflected, but the majority of the radiation is utilized by carotenoids and other so-called antenna pigments of the light-collecting complex of higher plants. Green radiation also penetrates deeper through the upper leaf layer (Bugbee, B., 2016). Above 600 nm wavelength comes orange and above 650 nm red, this radiation is also essential for photosynthesis, chlorophyll again has absorption maxima at approx. 640 and 660 nm. Above 700 nm, near-infrared radiation begins; we can no longer see this light. It is also no longer absorbed by chlorophyll, but this light still has important functions (more on this later).

Light-emitting diodes are currently superior to all other artificial light sources in terms of efficiency and light quality, so other technologies such as HID, fluorescent lamps etc. no longer need to be considered for hobby grow lights. LEDs initially only emit a monochromatic, i.e. single-color light with a bandwidth of approx. 10 nm, e.g. 660 nm +-5 nm. This is also used directly for blue and red in order to provide the most light at the strongest absorption peaks of the plant pigments.

If you choose to stick with it, you get the violet light familiar from many “plant lamps”. It looked very characteristic and was therefore a simple marketing argument that plants needed just such lamps. However, this has already been known better since at least the 1970s with McCree’s measurements. Nevertheless, the information persists.

White LEDs generate light by converting blue light of 450 nm into all other wavelengths using a yellow phosphor layer. The quality of the blue base LED and the phosphor mixture determines the values such as efficiency, color temperature or color rendering (CRI).

In addition to the quality of the light, we would of course also like to address the light intensity already mentioned. The more efficiently the lamp works, the less electricity we need to draw.

Depending on the manufacturer, different values may be specified. The best known is the lumen, which is also the basic unit for the lux standard. This is usually a good reference value, but it is a heavily weighted value, as shown in Fig. 3. Lumen is based on the sensitivity of the human eye, which is highest at 555 nm. Here, even low intensities produce high lumen values, although this radiation is actually somewhat less efficient for plants. If you compare similar light sources with each other, you can still make a good comparison with lumen/lux. However, manufacturers sometimes cheat when it comes to brightness by mixing in extra green for high lumen values.

The efficiency of the lamp is specified in lumen/watt.
Example: Jungle Lux 1400 Lumen / 9 Watt = 155 Lumen/W (Inkl. Netzteil)
If an external power supply unit is required for operation, you will have to add another ~10% to the watts.
(The consumption of Far Red or UV diodes should theoretically be subtracted from the watts, but that makes it a bit too complicated)

A more independent value is the PAR value, which is defined as µmol/s (total light as lumen) or µmol/Joule (efficiency as lumen/W), but is often not specified for many lamps. The units of the values may look rather abstract, but they are similar to the more familiar lumens, only from the perspective of plants. Unfortunately, this value also does not allow any conclusions to be derived about the quality of the spectrum. For example, a pure red lamp can have a very high PAR value, but is useless if the other spectra are missing. With a known, balanced spectrum, however, the PAR value is a useful indication.
The so-called PPFD value is rather difficult to interpret in terms of efficiency, as it depends on the measuring distance and, in particular, the housing of the lamp.

In the Jungle Lux example, the 1400 lumens correspond to around 21 µmol/s and the 155 lumens/W to around 2.2 µmol/J. Higher values are better, lower values are worse. Again, neither UV nor infrared values are taken into account.