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Heat or no heat for seedlings

Morning everyone!

Very excitingly my new greenhouse arrives tomorrow, which will be a heated greenhouse with power. My current one will remain unheated, and there is no possibility of electricity to this one.

I’d be grateful for your advice on seedlings. Thanks to those who have already cautioned me against paraffin heating in the new greenhouse! It will now have an electric heater.

I’ve always germinated my seeds on heat mats, located in the house in front of a large window. As soon as they’ve germinated I’ve moved them into the (unheated) greenhouse, and have just fleeced them if there’s been any particularly cold nights. This has always worked fine for me.

The heat mats will now move into the new heated greenhouse. Once the seeds have started, should I keep them in this greenhouse (just off the mats) or move them into the unheated greenhouse?

Thank you,



  • ObelixxObelixx Posts: 29,846
    It depends!  On which seedlings and how warm or cold is your greenhouse and whether or not you are using extra cloches against changes in temperature from night to day.

    Hardy annuals will take far colder temperatures than, say, chillies or tomatoes but you wouldn't be wanting to waste heat mat resources on hardy annuals anyway when they'll get away just as fast on their own when light levels are better in March/April.

    Half hardy annuals and tender plants, whether ornamental or edible, will need some warmth till after the last frosts in May.

    I suggest you use the RHS website or the info on the seed packets or their websites to check which seeds and seedlings need heat.
    Vendée - 20kms from Atlantic coast.
    "We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
  • FairygirlFairygirl Posts: 54,353
    If you're happy to spend money heating a greenhouse, you can certainly grow plenty of seeds, but sowing early also means you have to pot them on and keep them warm and sheltered until they can go outside. As @Obelixx says - that depends on the seeds/plants you want to grow, as some will need a lot of attention, and others won't.
    Anything hardy is better left until it's warmer for sowing outdoors, or in a cold greenhouse/cold frame in March or so, and that makes it easier when it comes to finding room for them as they can hardened off very quickly and put outside until big enough to plant out permanently  :)
    It's a place where beautiful isn't enough of a word....

    I live in west central Scotland - not where that photo is...
  • Sorry, I clearly haven’t explained well. I’m a pretty experienced gardener and have sown one or two seeds before. Just looking for a suggestion of whether a heated or cold greenhouse is preferable once the seeds have germinated. Obviously anything sown after May is a moot point.
  • ObelixxObelixx Posts: 29,846
    It still depends on what you're sowing, regardless of experience.  I have never had a heated greenhouse so wait until the weather and light conditions are favourable for after care once germinated.   I have used a heated propagator to help with germination but mine have both now died so this year I'm trying a small heat mat for chillies and toms but otherwise waiting for better natural heat and light.
    Vendée - 20kms from Atlantic coast.
    "We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
  • Busy-LizzieBusy-Lizzie Posts: 23,141
    If the seeds you are sowing are half hardy annuals, tender perennials or frost tender veg, such as tomatoes and chillies, then a heated greenhouse would be good for growing them on. You could turn off the heating on mild days and turn it on at night. But don't forget to take light levels into account as well. If you sow your seeds too early they can become weedy because of lack of daylight hours. Tomatoes can grow tall and take up space before you are ready to plant them out, outside too cold and inside the GH I used to find I hadn't enough room as it was full of baby plants!

    Some half hardy annuals, such as impatiens, rudbeckia and salvia farinacea can be slow to start off so sowing early in a heated GH can be an advantage but others will grow too fast and be wanting to be planted outside before the weather is warm enough. Also they will fizzle out earlier at the end of the summer season.

    Even after I had heating in my GH I learnt not to sow many seeds before March.
    Dordogne and Norfolk. Clay in Dordogne, sandy in Norfolk.
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