Greenhouse in Winter
Many thanks for your recent message enquiring what vegetables you could raise in your greenhouse during the winter.
Although a greenhouse normally bursts into life in late winter with early sowings of seeds destined for the vegetable garden, it's certainly possible to utilise the protection the structure offers to raise winter crops. You could grow almost anything throughout the winter but the limiting factors are light and heat - overcome these hurdles and a plethora of out of season crops could be yours!
Let's take the question of light first and examine it in some detail. Seedlings without supplementary winter growing lights generally grow spindly and weak - 'leggy' is the commonly used term. To germinate and raise young plants successfully in the winter months, Grow Lights (product code GPR-112) are really the only answer. The reason seedlings without the benefits of artificial light grow tall and weak is due to natural winter light; or more accurately the light quality and lack of it. Because the days are shorter this time of year and on the few occasions the sun does shine, the intensity of the light is not as it is in the spring and summer, so the plants grow tall and pale as they search for more light to enable them to photosynthesise and grow. It’s a little like what happens when you force rhubarb but nowhere near as extreme!
When using the Grow Lights, there is no strict lighting regime you need to adhere too (unlike the commercial practice of raising cut flower crops such as chrysanthemums) but plants will plants rarely benefit from receiving daylight over 18 hours. It’s also impossible to ‘overdose;’ the plants on light but given the previous statement, it wouldn’t make economical sense to keep the lights on constantly.
It’s good practice to use the Grow Lights in the morning and evening of bright sunny days to supplement the natural daylight, and the kit could be run off a timer to help illuminate overcast conditions. An 18 hour burst of light per day should be enough for propagation purposes and it’s probably worthwhile placing some plants out of the reach of the light to see how their growth contrasts of those under the bulbs. The bulbs we supply are 6400K rated, giving the correct spectrum of light to propagate and grow on young plants. A special bulb to promote flowering (2700K) is also available - the ‘K’ stands for Kelvin, a measurement of colour temperature.
I should point out that not all vegetables require additional lighting to grow - hardier crops such as lettuce, kale, carrots, cabbage and so on will still germinate and grow - but as we are looking at the possibility of growing temperate plants such as tomatoes and peppers, it's worth considering!
The other element required for winter germination and growth of greenhouse plants is heat. Again, the seeds of plenty of tougher vegetables like those mentioned above will germinate in a relatively cool greenhouse but providing bottom heat by way of heat mats, warming cables and propagators you can greatly increase the range of plants you can raise.
Keeping the germinated seedlings happy is another consideration; an electric fan heater will help heat the greenhouse and by using bubble insulation along the glass and in vertical curtains, you can create a mini 'hot section' for summer crops. A vitopod propagator also helps you keep smaller plants warm without having to heat the whole greenhouse but many vegetables once again will cope with the heater set on trusty 'frostwatch'.
As you can see, there is plenty to keep the greenhouse owner occupied during winter but striking a balance between the cost of running a hot greenhouse and the value of the crops is probably key. My advice would be to try some of the hardier vegetables - start with succession sowings of lettuce and other salad crops - and see how you progress. You might even want to start off some seed potatoes in containers and if you find that your germination success rate is low and seedlings are spindly, then you could look at a growing light.
Hopefully this information is of help and will assist you in making your greenhouse almost a year round growing venue. As always, please let me know of any other questions or queries which spring to mind and have a very Merry Christmas and a Happy New Year!