The Danish landscape architect, Jørgen Larsen, is a lucky man. He has an oil-fired boiler in one of his greenhouses. Here he is checking the newly sown lettuce. The lemons are already ready for harvest.

Photo: Grøn Kommunikation
Written by Lars Lund 
Edited by Louise Curley 

Spring can be a tricky time in the greenhouse. On sunny days the temperature inside can rise quickly, reaching well into the high 20s and even as far as 30C. However, on a night, particularly if there are clear skies, the temperature can fall dramatically, getting close to freezing. Most plants like an even temperature and young plants in particular will struggle to cope with such fluctuations, so it’s important to try to manage the temperature extremes in a greenhouse.

During the day it’s important to open vents and the door as the temperature starts to rise above 20C. Automatic vents are a great option to include on your greenhouse as they will automatically open vents when the air inside the greenhouse gets too warm – this is invaluable if you are out during the day.

Invest in a thermometer for your greenhouse so that you know how hot and cold it’s getting in there, and this will help you manage the conditions.

Make sure you close the vents and the door by late afternoon so that you keep as much warm air inside for overnight. If temperatures are predicted to get very cold then you can cover vulnerable plants with several layers of horticultural fleece.

A frost-breaker heater is a good option if you have a lot of tender plants in your greenhouse. It doesn’t heat the greenhouse all the time, but it will kick in when the thermostat notices the temperature dropping close to freezing and help to keep the temperature just warm enough to protect vulnerable plants.

A traditional idea to keep the temperature above freezing is to have a dipping pond inside the greenhouse. This could be a large metal tank or barrel that is filled to the top with water, which will absorb the warmth from the sun and hold onto it for longer than the air. They’re known as dipping ponds because they were used to dip and fill a watering can.