I am going to grow in a greenhouse that loses light at about 3pm in the early summer. That gives the plants eight to nine hours of direct light.
The greenhouse is set up with propane but doesn't have electricity. Can I use a propane mantel to provide light for an additional four or five hours each day? I know it puts out lots of lumens and would supply CO2, but is it the right spectrum?
What would happen if I don't use the extra light?
I'm going to answer your last question first. The plants will grow fine vegetatively with no additional light. They will fill the greenhouse to bursting if they are started too early, and may have to be pruned.
Adding more light and CO2 to the mix will just exacerbate the problem. The mantels will also increase the heat load in the greenhouse.
The mantels will be more helpful in late summer and autumn. As summer progresses to autumn, the sun sinks lower in the sky, so garden space that is in full sun early in the season is in shadow during the fall. In addition, the sun's intensity diminishes significantly in September. The mantels can be used to supplement light during the day and to extend the daylight hours to 12 hours after Sept 22, when the night extends beyond 12 hours. The mantels produce a light high in the red spectrum, which is used efficiently for photosynthesis. The hot CO2-laden air from the mantels will rise and the CO2 won't get to the plants unless it is pushed down using a circulating fan regulated by a thermostat that turns it off when the air at the plant canopy level rises to 80degF (27degC).
During the summer the mantels' heat could be used to your advantage. The hot air rising from the mantels can be used to create a Venturi effect. The rising air creates a flow, drawing cool air from below. The combination of hot and used air is evacuated through roof vents. The airflow creates a breeze at garden level providing fresh CO2-laden air to the plants while evacuating the oxygenated air from the micro-environment at the surface of the leaf. The CO2 that the mantels produce never gets to the garden. It rises in the stream of hot air.
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