The Nursery (Part 2: Let there be light!)

Before going into detail about artificial lighting I should probably first comment on why I went with that solution instead of natural light. Most people sow using natural light, and have perfectly good success with it. I have too, of course, and I probably would have decided on natural light if I had had a greenhouse to keep the plants over winter. When I gave in to the urge to buy seeds I was living in a tiny east-facing apartment, in which there was simply no space and not enough suitable natural light for sowing cacti. And when we moved to our current apartment later that year, it was July already and suddenly a little late in the year to sow. The solution became to use artificial lighting which I had already contemplated using for quite some time.

I won’t go into a very detailed analysis of the various artificial lighting options in this post. Maybe I will expand on this at some later date though. There are a lot of things to consider such as the available space or how much space you need, cost of electricity, whether you want to grow the plants beyond the seedling stage, and how much money you’re willing to invest.

I pondered for months about what kind of artificial lighting to use. I initially did not foresee having so much space available for seed raising and planned for perhaps 0,5 m2 at most. I researched quite a bit about various options and the choice stood for a long time between fluorescent tubes or LED lights. The former was cheaper and there were several tubes on the market offering a spectrum of light simulating natural daylight. There were also tubes offering specific parts of the spectrum, such as UV, which was interesting considering UV is probably an important factor in cacti for developing the most robust spines or certain attractive grey or blue blooms on the epidermis (the outer skin layer of the plant).

LED was also an interesting option, though more expensive than fluorescent tubes and requiring a bit more effort to install. It has the advantage of being able to create a bespoke spectrum as it were, seeing as you can install as many diodes as you like which emit light in a specific part of the spectrum only.

In the end it turned out that I did have quite a bit more space available than first thought, so I decided to instead use a 400 W ballast and reflector for HPS (high-pressure sodium) and MH (metal-halide) bulbs I had bought a couple of years before to raise cacti through the winter months in Kristiansand at my father’s house. The ballast is an electrical ballast to limit the current running through the bulb as otherwise it might overload. The reflector is a relatively thin sheet of aluminium polished on the inside to a high sheen and which reflects downwards the light which the bulb emits upwards and sideways and which would obviously otherwise be of no use to the plants.

MH bulb with reflector.

HPS and MH bulbs are more efficient than fluorescent tubes and LED lights, i.e. their PPFD (photosynthetic photon flux density) output per watt is greater. The downside is that a 400 W HPS/MH bulb produces quite a lot of heat, so unless one installs a fan or two the temperature will quickly become much too high in a small or closed environment. In a spare bedroom with a window it is not much of a problem, though.

Generally HPS bulbs emit relatively more light in the red part of the spectrum while MH bulbs emit relatively more light in the blue part of the spectrum. For the purpose of growing plants, very simply put the former is slightly better for producing flowers and germination, while the latter is slightly better for vegetative growth. I had both kinds but I was not particularly interested in the “flowering-aspect” of the HPS bulb, and I suspected the effect of the HPS bulb on germination would be slight. I therefore went with the MH bulb and had good germination rates with it (I have not tested the HPS bulb, so I do not know if it would actually increase germination rates).

Not living in Kristiansand it was difficult to create ideal conditions in terms of heat, watering, and day-length, and in the end I disconnected the lamp. This time around I would able to control these parameters, however. The seeds germinated and grew so well that I quickly decided I had to expand. After all, I had bought just a few too many seeds. One bulb and one shelf would never be enough!

Two months after I started sowing I therefore promptly went and bought another 400 W ballast and reflector. I also bought two MH bulbs with a so-called daylight spectrum with a temperature of around 5600 K (Kelvin is in this case a measure of colour temperatures of light sources). I had no information about the spectrum on my present MH bulb and I also did not know for how long it had run (bulbs do, after all, have a limited life), so I thought it best to install two new bulbs of the same type.

The two reflectors hanging from the top shelf of the racks.
At the back on the left you can just make out an Adenium multiflorum in bloom.

I keep the light on for between 12-14 hours per day (I’ve landed on 12), which seems like a good amount. This may immediately sound like a lot but, after all, artificial light cannot compare with natural sunlight (unless you have a lot of it), so you have to increase the time the plants receive artificial light to make up for the reduced quality of it. I have installed electrical switches to regulate when the lights turn on and off.

Temperatures range between 18 C at night up to 35 C during the day (depending on the outside temperature and whether or not I keep the window wide open). For vegetative growth this range is quite ideal for most cacti, though there are some notable exceptions that prefer lower night temperatures. In terms of germination it can be on the high side considering that pots under plastic or inside plastic bags are exposed to an additional greenhouse effect. It is necessary with some additional shielding from the light as well as not placing them directly beneath the bulbs. I don’t know if the temperature has really adversely affected germination to any particular degree, though. The germination rates seem quite similar to the data I have collected on germination rates in previous years when I have sown in the spring or summer with the help of natural (shaded) sunlight and temperature variations normal in a greenhouse.

Now to the next part of this series: Seeds!

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