Lars Lund

10 Mar 2021 15:41

Crack the code of light and find the right grow light

When you buy growth lights, it feels a bit like getting light in your eyes. There are many offers on the market and at many prices and what should one look for? Lars Lund tries to guide you through the secrets of growth light, so you can better make your choice.


It is as predictable as questions about the lawn. Every year, when people start sowing inside and make the small sprouts grow, people ask which growth light to buy.


The green shoots that show must get an extra light supplement to develop optimally, and it requires light, as the sun is spare in that time of the year. When you look online or at a retailer that sells growth light, you are often high and dry. Should one buy a lamp for 5 pounds or for 500 pounds? It would be nice and easy if those who sell or produce growth lights made a simple description of which plants their fluorescent tube or growth light, more precisely, could be used for, plus a few but necessary information for one to understand. What is the difference between expensive plant lights and cheap ones, and can’t you just use a neon lamp?


Although some players in the market are getting there and some, after strong criticism from consumers, have come a long way, there is a long way to go before they all have good consumer information. We must and can also do something ourselves to become wiser. When it comes to growth light, there are new concepts we need to understand in order to choose the right light at the right price for the task. There are countless of lamps and lights to help the little sprouts in the window ledge.


First one should understand the concepts behind the light in your home

Usually, we go for watts when we have to judge how strong light the human eye can see, and we use kelvin when we have to assess the colour of the light. At sunrise with a beautiful red morning sun, the sun is 1,800K. Later in the day, the sky turns blue, and the light gets a blue-white tone. The kelvin value increases, perhaps up to 16,000K. In a bulb for the home, we call it warm white when kelvin is approximately 3000K and cold when above 4000K. Another concept is also the term RA. It is an expression of how well the lighting reproduces the colours and is mostly used in shops. In a living room for cosiness, you should choose a lamp of approximately 3000K. In the kitchen we need a slightly brighter light and will go up to about 3,500K and in garages and office landscapes, the value is typically 4000-5000K.


Watt also indicates how much energy is used and thus how much power you use. The more watts the more light, and the higher the consumption. However, watts are not entirely reliable to use to indicate how much light a lamp now actually provides. Therefore, lumen is used more now. The lumen is a more accurate unit of measurement of the amount of light emitted. In order for us creatures of habit not to feel completely lost, a table has been made that shows how many watts a given light source provides in the lumen. For example, a 15W bulb provides 140 lm, a 40W 470 lm and a 60W 800 lm. Every time you want an old 60W bulbs, just remember that it is now an 800 lm. Finally, there is the scattered angle. If the light is scattered over an angle of 180 degrees, the bulb will be a good ceiling lamp and the smaller the angle, the more focused the light will be on one spot.


Plant lights love red and blue

When we move on to growth lights, we can still use knowledge about watts and the light scatter, but not much more either. Among other things, watts are important in relation to the cost. How many watts does the bulb, you use for your sprouts, use? It should preferably not cost a fortune. The scattering tells if the lamp at a given distance can scatter the light, so that it also covers the seed tray you have your sprouts in. Here it is important that you also know how high the lamp can or should be hung, to give enough light to your plants, for the higher, the more it covers, but the less light the sprouts also get. Therefore, you also need to know how much light your plants need.


All colours are white

You need to learn two new concepts namely wavelengths and µmol/m2/sec. To create some confusion, the latter is also called PPFD. Let's go outside in the sun. In the middle of the day, the sun sends 2,000 µmol per second down to the earth. In winter that is 1,200 µmol per second. The light that is emitted by the sun towards the earth, we see in several colours, during the day as a white light. White light is a mixture of all the colours of light that humans can see and also colours that humans cannot see. Rainbows, for example, are a phenomenon in which the white light from the sun is distributed so that one can see different colours from the white light. Light is a kind of wave and is called a nanometre (nm). The different colours each have their wavelength (nm). The white light from the sun is therefore an emission with a lot of different wavelengths. Red light is longwave and blue shortwave. The shortest wavelength humans can see is the violet of about 380 nm and the highest is 750nm. Wavelengths below 380 are called ultraviolet light and above infrared light.


The plants cannot do with the light we see as white light to form photosynthesis and growth. At different stages, they need the whole colour spectrum, but especially the red colours and the blue ones are important when we talk plant lights. The red colours in the growth lights go from 640-680 nanometres (nm) and the blue ones from 420-460 nm. The blue light gives the plants strong stems and leaves, whereas the red light is used to develop flowering and fruit. Therefore, it is not necessarily important that the plants get both the blue light and the red at the same time. Conversely, the latest research shows that when it comes to growth light, the plants use both the red and blue light, but they develop even better if they get the full colour spectrum, i.e., growth lights are typically from 400-700 nm. Especially when the plants grow in artificial light for a long time and become adult plants, it can be of great importance. The hemp growers know this.


So, when you buy a growth light, look at the colour of the light. If it is only blue light, it is probably 420-460 nm. If it is almost the whole spectrum it is from 400-700 nm. As a minimum, you should get a light that emits light in the spectrum 460 nm blue light and 650nm red light.



The price of your light depends, among other things, especially on how many diodes there are in the light. Diodes cost money and the quality of the diodes can be very different in terms of durability. Multi-spectrum light has several diodes, and then the price increases compared to lights that only emit blue and red light. On the other hand, the light in your living room, or where you have the plants, also becomes more natural. Watt partly indicates how much energy you use, but together with the number of diodes, it shows how large an area the light can cover at a given height.

Most used among us gardeners are lights that provide from 40 watts and up to 220 watts hung approximately 12 inches above the plants. In some cases, you can settle for 15 watts. It also depends on how much light they get from outside and from other light sources. If the light does not cover the entire area, you can buy several bulbs or buy an LED light fixture hung 47 inches or more and in extension of each other. Also note that some high wattage lamps have a motor for cooling the lamp and that it makes some noise.


Grandma's solution

My grandmother just used a regular fluorescent light. It's actually not that bad either, but here too you do not have to go for warm white with 2700k, if you want the red light and if it is blue, as it usually is when germinating, then you need 3000k or more.

In principle, you can also use an ordinary LED light, as long as you know which one. The LED lights we like best in a living room emit very little blue light. On the other hand, they are quite good when the plants need to use the red light. And then there is the distance to the plants. Let's say you hang the lamp 6 ft up, instead of 3 feet, the light will be 4 times dimmer. If you hang it 1.7 ft up instead of 3 ft, the light will be 4 times as strong.


Information you need:

How many watts does the light provide (cost and power)

Does the light have the full spectrum, i.e., all colours, or is it just the blue or red or both? (composition of diodes)

How long is the life and how long is the warranty on the light. (Life in hours and warranty this year)

Recommended time the light must be on (often 16 hours)

Which plants is the light suitable for and how many µmol does the plants need? It is very difficult to find similar data that describes how many µmol the plants we grow need. The range is very large. But here are some examples from different places on the internet and they should be used with great caution and used only as approximate. If your plants are in a window ledge they also get light from there.

PPFD = µmol/m2/sec = the number of photosynthetically active photons that fall on a given surface every second and is the amount needed to support the photosynthesis.


Citrus between 450-700

Olives 450-700

Peppers 250-450

Chillies 400-800

Tomatoes 450-700

Lettuce 200-600

Om Lars Lund

Lars Lund
Danish horticulturist and journalist
Lars Lund has for many years engaged in the garden and greenhouse. Lars has published many books about greenhouses, and he has participated in many Danish horticultural TV shows. He is a walking garden encyclopaedia, and he has answers for most basic cultivation questions – also the more ambitious ones.  

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