New Greenhouse, Seed Sowing and Propogator Advice

s_glover · 6 January 2020 at 12:08PM

I love growing fruit and veg but until now have never had a greenhouse.

This year, now we have a greenhouse I'd love to grow a wider range of things including chilies, aubergines, peppers and tomatoes.

I'm looking for some advice regarding buying a heater propagator / greenhouse heater. I want to avoid having seeds and seedlings around the house so ideally want to start everything off and grow on in the greenhouse. I'm therefore thinking a heated propagator is definitely needed and probably a heater. What temperature would I need to keep the Greenhouse at for such seedlings to grow?

The greenhouse is 7ft x 12ft.

Any advice would be gratefully received.

Justagardener · 6 January 2020 at 12:54PM

That's a nice size greenhouse. We keep ours above 10c when sowing in a propagator and then bringing them on. Use bubble insulation it really helps retain the warmth from the day. It's surprising how little it does cost to heat a greenhouse. A great place to potter too when it's a bit fresh outside.

Davesnave · 7 January 2020 at 10:00AM

I had a large greenhouse back in the days when electricity was cheaper relative to income, but I never aimed to sustain much of it above zero in very cold weather, except in the small part where the most delicate seedlings were, when a heat mat provided a high enough and steady temperature.

In other words, the main area inside never froze thanks to a 3kW heater calibrated to come on around 0c, while the inner sanctum for the chillies, toms or whatever was the only bit that had a higher sustained temperature. The warmer area was created with materials like bubble polythene, polystyrene and lots of sand. Once heated up, a large amount of sand holds the warmth and it can also be striking cuttings at the same time!

This worked for me. Tender plants, like pelargoniums for example, don't need anything like10c to survive.

The biggest problem with heating and insulating is that of providing enough control of ventilation as heat input from solar gain occurs, so think about that as much as you consider the frosts. Having the temperature yo-yoing wildly is often worse than having it on the low side. Everyone's personal circumstances and greenhouse location will be different, but what provision will you make for those warm spring days? Also a stagnant greenhouse encourages moulds and damping off.

Finally, when using electrical heat, a cheap thermostat set in a factory at 5c or whatever will waste plenty of money over the years, whereas a variable and accurate one will pay for itself very quickly.

crv1963 · 8 January 2020 at 2:39PM

If you have elecricity in the greenhouse then your option for a heated propagator is viable.

I have a 8x10 new greenhouse, I have an electric fan heater which has 10 settings each takes it up a notch at 2,6 degree intervals. I have it set at 5.2 degrees and I also have a Vitapod Heated Propagator I got for Christmas with lights.

I set it all up New Years Day- I have sown Onions and Leeks propagator set at 20 degrees. Turned on the power to the Fan Heater on the same day. The Temp in the greenhouse varied from -3.4 to 21 degrees prior to New Years Day, now we've had a bit of sunshine over the last week but the greenhouse has varied from 5.2 degrees (probably at night with the Fan heater turning itself on when it gets too cold) to 16 degrees.

So far as I can tell from our smart meter it has cost me a pound over the past 7 days to run both. Early days yet! We have had chilli nights but no frosts since I turned it all on. I do expect that to rise when a cold snap hits.

Edit- as the greenhouse didn't go up until November I kept it unheated. I plan to move the onion/ leek seedlings to unheated plastic propagators when a reasonable size, then to turn the heated one up to 29 degrees sow chillis then when they are through turn it down to 25, sow tomatoes, when they are through turn it down to 20 degrees grown them all on a bit until they can tolerate being in the greenhouse.

As warmer nights develop I'll turn the fan heater up to 10 degrees or so as it will hopefully not run all night every night!

New Greenhouse, Seed Sowing and Propogator Advice

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