Propagation – light vs heat

There are certain times of the year when propagation can be a little tricky. One is the height of summer when certain crops – lettuce is a notable example – do not like to germinate because of high temperatures, and when it is so easy for young seedlings to become scorched or otherwise keel over from just a minor lapse in attention. The other, of course, is early in the year when temperatures are low and light levels are poor.

Early in the year is also the time when certain of the warmth loving plants need to be started, things like peppers, chillies, and aubergines. These need a long growing season; started too late, they will struggle to ripen their crop before the season closes. They are a rather different proposition from many of the fairly hardy sorts of plants that have been grown here for hundreds of years and need only moderate warmth for germination. We have already sown onions, for example, but these germinate reasonably above 10°C and very well above 15°C. Peppers and the like, however, really want a temperature of around 25°C and that can certainly be difficult to achieve outdoors, at least without throwing a lot of electricity at the problem.

For some years now, we have started our peppers, chillies, and aubergines in our polytunnel in a couple of large heated propagators. One might be forgiven for thinking that a heated propagator in a polytunnel would provide the perfect environment, but propagators are limited in their ability to raise the temperature far above ambient. The ones we have can, according to the specifications, lift the temperature by about 12 degrees. During the recent cold spell we would have struggled to produce a temperature of 15°C, which is fine for onion seeds and other fairly hardy sorts, but not the for exotic peppers, chillies, and aubergines. Our germination times in the past have been a little longer than they would be under ideal conditions and germination has been occasionally patchy. Nonetheless the seedlings we produced did better outdoors than when we raised them indoors.

Good temperatures may be easier to achieve indoors, but the light levels, even in the brightest parts of the house, are not as good as outdoors. Outdoors, the light is still rather poor at this time of year, but the seedlings we produced in the polytunnel were less leggy than those we have raised indoors. I am inclined to think that the quality of the light, being diffuse due to the nature of the plastic covering, helped in this regard. This year, with our polytunnel frame now repurposed as the new fruit cage we have set up propagators in a couple of the greenhouses, but because the temperature is not really high enough for germinating chillies and such crops we have decided to move one or two of them indoors. This will allow us to achieve the ideal conditions for germination, but to avoid producing leggy seedlings due to poor light we will move them back the greenhouse as soon as there are signs of green shoots. Hopefully we will enjoy the best of both worlds – the warmth of the indoors and the light of the outdoors.

Of course, gardeners with grow lights, which seem to be increasingly popular, can avoid this problem entirely, propagating indoors with artificial light. However, like most gardeners, we do not have grow lights. We do have plenty of heated propagators, though, and with these we should be able to raise some good seedlings. Grow lights might be something of a luxury but heated propagators are, in my view, a most valuable aid to propagation and I would not want to be without them. Whilst we could, and indeed did for some years, raise seedling indoors in unheated propagators, without the heated propagators, or then without some other form of heating, we would be unable to move the seedlings outdoors and this would inevitably lead to somewhat leggy specimens. It is difficult indeed to manage poor light levels and especially light that is directional and encourages a plant to stretch towards it, as is the case when placing seedlings in front of windows or patio doors.

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