Lars Lund

08 Sep 2020 14:34

The greenhouse in January – light but not enough for it all

Finally, the new year has started and last year is done. Christmas in the greenhouse was really nice, with lots of elves, fire in the stove, Danish pancake puffs and glögg. After all, some days it's nice to be outside but still inside. 

January, what does the month offer? It is the month where everything green turns white and almost everything lies still. And yet. It's also the month when potted onions in the greenhouse sprout, and it's the month where you can enjoy snipping the fresh herbs that are cosy under the warm glass. You can snip every month, but the further we go into winter, the fresher they taste, partly because there is not so much of it. Cabbage grows slowly but surely in the winter.

These tomatoes are from the time when neon lights with a special colour were used. It works, but the plants are hardly as green as they should have been if they had had enough light. With LED light, the colour is more accurate, at least that is the experience of many greenhouse enthusiasts. 

One of my greenhouses was in December used for both Christmas and New Year's cheers in champagne. From there we could toast on New Year's Eve in shelter and warmth without missing the fireworks. During the Christmas days, it was used for warm stays and with nice hot Danish pancake puffs and glögg.

Summer starts now

In January, the most enthusiastic start sowing many of the earlier summer flowers for later transplanting or sowing e.g., chilli. You can of course do this outside in your greenhouse, but it still requires some energy for the heating, so it makes more sense to start inside in a bright window ledge and then put up extra light. In recent years, there has been a great development of LED lights that are particularly suitable for plants. Prices have fallen, although they are still clearly in the expensive end. The average price for an LED bulb suitable for plants is around 60 pounds or 1/3 of the price a few years ago. The power consumption and their durability make them a good investment, at least for you who cannot wait to get started. Later you can use them e.g., tomato seeds.

Lots of light

Even though we are moving towards brighter times, the problem is to provide enough light for the sprouts in the window ledge. Neither in March nor April is there as much light as plants could well use. Therefore, artificial light is needed, and it may be difficult to find the perfect combination, or perhaps the right light.

Plants need light for photosynthesis, and they use most light in the red and blue spectrum. Plants are green because they reflect some of the green light, while the other colours are absorbed by the plant.


Cheap and easy

The simplest method to get more light is to buy a fluorescent lamp with the colour numbers 83 or 31. You can, for example, put the sprouting plants in a large aquarium and put a light in the aquarium lid. The light should be on for about 12-16 hours.

Incandescent lamps that are on their way out of the market give the wrong light and way too much heat. Only 7% is light, while the rest of the energy is used for heat. The energy-saving bulbs do not provide enough light nor light in the right colour scale, so the plants can grow satisfactorily

High pressure sodium lamps are the yellowish light we know from streetlights and large greenhouses. They are quite effective, but not enough in themselves, and therefore the nursery gardeners use two kinds of lamps: one for growth and one for flowering. The sodium lamps are also expensive, and the efficiency decreases the longer they are used.

LED lights, not least for plants, are so new that only a few have tested the light as a grow light. In Denmark, it is used in some greenhouses on an experimental basis, but the development within LED light for plants is fast. It is expected that LED lighting will be the future of the nurseries that are hit financially hard in these times.

If you want to read more about LED lighting, just Google "LED for plants". Here you can find both companies that sell LED lights for plants, as well as greenhouse amateurs who experiment with LED lights as if they were professionals. The great thing is that they are sharing their experiences. Look for the dates as you read about opinions and experiences. What was true 5 years ago is not necessarily true today. Here is a website where you can read about grow lights.

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.  

Get to know Lars Lund