How warm is your cold greenhouse?

February arrives and all keen gardeners start to think about getting some seeds sown for early crops. This might be especially true if you like to try to produce some early vegetables ready for harvesting when shop prices are high. In my case this usually involves sowing seeds in my cold greenhouse, moving into my cold frame to grow on and harden, off before finally planting out into the plot.

From 2011 Weather Pictures 1

Through the winter the greenhouse has been to put to use for over wintering garlic and also as shelter from the weather to manufacture some new bird tables for the garden. It’s first use in the year is usually somewhere to pot on new arrivals that come bare rooted from nurseries and need a temporary home until conditions outside are suitable.

But just how soon can I start sowing seeds in my greenhouse. This mostly depends on the minimum temperature in the greenhouse over night as I don’t want to go to the expense of heating my greenhouse. On the 13th January the temperature in the greenhouse reached 17.1°C during the day, positively spring like, but the down side, it fell to -1.1°C on the 29th of the month.

Some research suggests that a cold greenhouse should be maintained at 7°C whilst if all that is required is just to keep away any frost the temperature shouldn’t be allowed to drop below 3°C allowing for some element of safety. So I’ve been monitoring the temperatures in my greenhouse over the last few months. Now that seed sowing time is approaching I’ve been taking much more notice of the daily temperatures and just how close to the 7°C or 3°C the temperature has been.

From 2011 Weather Charts

I’ve analysed these figure just to see how warm or cold it’s been in the greenhouse. Looking at the 7°C cold greenhouse levels then January’s results are pretty mixed, but clearly nowhere near the level required for classifying as a cold greenhouse level.

There were just 2 days in the month when day time temperatures didn’t manage to reach 7°C but more importantly there were only 3 days in the month when the night time temperature didn’t fall below this value. I think the lower values are far more important when it comes to germination than the high temperatures. There are 744 hours in the month and the greenhouse was below 7 °C for 464 hours and above for 280 hours. Obviously based on 3°C for a frost free greenhouse the figures are much better but there were still only 6 nights when the temperature remained above 3°C.

From 2011 Weather Pictures 1

The obvious conclusion to draw from this is that it’s still far too cold to start sowing seeds in my greenhouse during January. Seed is too expensive to waste and it’s much better to keep it in its packets and wait until the seed has more of a chance of germinating. I’m planning to get my seeds off to a better start this year using my new toy an indoor lighting system for some of my seeds. Especially the ones that benefit from higher temperatures for germination such as tomatoes and sweet peppers. As for the greenhouse I’m still not really planning to sow many seeds in there until the end of March.