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heated propagator 24 hours or switched off overnight?

simple question... I have started tomatoes  and chillies off in a heated propagator and wondered if other users leave theirs on overnight or switch off? 
I think the seeds need fairly consistent heat, but also mimic natural conditions where it would be cooler overnight. 
Would love to know what fellow growers  would do.


  • Pete.8Pete.8 Posts: 11,311
    Keep the temps around 22-25c (ideal for chili and tomatoes) all the time if you can until germination.
    Once germinated I lower the temp to about 20c

    Billericay - Essex

    Knowledge is knowing that a tomato is a fruit.
    Wisdom is not putting it in a fruit salad.
  • WaterbutWaterbut Posts: 327
    Depends on where you keep them. My heated propagator is in the house on a time switch which switches off at night but I also have overhead lighting to ensure the seedling grow straight. The only reason I do not do what Pete suggests is that one of my heated propagators thermostat broke down in the past and fried my seedlings.
  • I start seeds indoors, as well, and I use a heat mat 24 hrs/day from sowing until I see that no new germination is occurring; usually about a week and a half to two weeks depending on the seeds sown.  My setup has grow lights on automatic timers that are on 14 hrs per day, but the heat mat is a basic type with no thermostat settings.
    New England, USA
    Metacomet soil with hints of Woodbridge and Pillsbury
  • Dobbin26Dobbin26 Posts: 59
    I have two hot boxes in the greenhouse which have under soil heat cables with digital thermostats. Each unit is 4' x 2' and 6" deep. I've had great success for over 20 years with one of them and built the second last year. They are on 24/7 whilst seeds germinate. I also have a bit of a horror story to tell too. 6 years ago,  I brought out an old electric propagator,  one that looks like a seed tray. Well it was working fine for about 8 week's and then one Friday evening in March 2018,  we awoke to the sound of breaking glass. Looking out of the bedroom window I was amazed to see my greenhouse was on fire !!!🔥 Flames licking 15 - 20 feet into the air. The propagator had developed a fault,  overheated and the molten plastic had dripped down onto the timber shelving which had by now caught fire.  Although I managed to extinguish the fire, the whole greenhouse was ruined.  A closer inspection the following morning showed the true devastation,  molten aluminium, shards of glass all around the garden. And to think, I'd been out in my bare feet and pyjamas😱 My greenhouse of 30 year's ruined.  So my advice, make sure you check anything electrical before putting it in your greenhouse. It's surprising the damage the sun can do to plastic. Ensure your supply is protected by an RCD. Oh, and finally, your insurance covers your greenhouse etc. 
  • A cautionary tale indeed @Dobbin26.
    I also used to use "hot beds" with a soil warming cable and found them extremely successful.
    Unfortunately ( or perhaps not ? ), I don't have an electricity  supply to my current GH.
  • most heated propagators are not meant to be used in a greenhouse
  • DovefromaboveDovefromabove Posts: 87,903
    edited 22 February
    “… If your home is cold or you have an unheated greenhouse, then heated propagators will heat just the seed trays and you won’t face a bill for heating the whole room...”

    Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.

  • Pete.8Pete.8 Posts: 11,311
    most heated propagators are not meant to be used in a greenhouse
    That's not good advice.
    A greenhouse has plenty of light but is very cold this time of year.
    I've used heated propagators in various greenhouses since the 1970's.
    Seed sown now will germinate quickly in a heated propagator and there's now just about good enough light to stop them growing leggy - as long as they're kept warm and in a greenhouse.

    Billericay - Essex

    Knowledge is knowing that a tomato is a fruit.
    Wisdom is not putting it in a fruit salad.
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