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Germinating Vegetable Seeds in a Heated Propagator

Close up of 2 week old chard seedlingsIf you’re staring off your vegetable plants in early Spring you’ll need to give them some heat as temperatures will be too cold for the seeds to germinate. If you’re growing in a greenhouse or polytunnel you can use a heat mat, a heat cable buried in a tray of sand or an enclosed heated propagator. A greenhouse heater can be handy for frost protection (green houses or tunnels are not immune to frost) but I find horticultural fleece perfectly good and a lot cheaper to buy and run!

Can’t be bothered reading the text? You can watch it all on a video instead! Here’s 2 recent videos which cover most of the text and will help you use your propagator to grow some perfect baby plants. All the products mentioned in the text are included in the text below with links through to the shop if you’d like to order any of them.



French bean seedling plug plantSowing the Seeds
For most vegetables with the exception of root crops like carrots and parsnips sowing indoors in modular trays is the best way to start off healthy vegetable plants. Sowing indoors also allows you to get ahead of the season so you’ll be ready with established 4-6 week old plants when the soil warms enough to plant out in late April or early may.

We use 84 cell seedling trays for most crops as the module size is about perfect for the 4 weeks of growth normally needed before our seedlings are ready to plant out. For plants that need to spend longer in the pot use a 6 cell tray or a larger pot.

The advantages of sowing in modular seed trays are:

  • You use a fine seed compost perfect for germination instead of sowing in heavier garden soil.
  • You control the early stages of the plant life.
  • Emerging plants are protected from pests, particularly slugs.
  • Weeding is easier as plants are over 4 weeks ahead of the weeds.
  • You’ll have a more uniform garden as you won’t have empty spots where seeds didn’t germinate.
  • Your garden will be more productive as valuable space isn’t taken up germinating seeds, this means more space for growing.

Seed compostFrom our trials we’ve found Klasmann organic seed compost to be the best, with consistently good results every time. We use this for our seedling production business and have large 70 litre bags available on our website. Seed compost is lower in nutrients than ordinary multipurpose compost as a high level of soluble nitrogen can inhibit germination and burn fragile emerging roots.

I also like to use ‘Grochar’ peat free seed compost which has a very fine consistency and has the added benefit of charcoal, seaweed meal, wormcast and mychorrizal fungi to aid seedling growth. The nutrients in ‘Grochar’ are slow release so gentle enough not to damage roots. A little word of caution with Grochar is the coir peat free base holds water very well so water a little less than you normally would.

Sifting compostSowing in modular trays
Fill your tray to the top and give it a couple of sharp bangs on the bench to settle the compost and then make small indentations with your fingers. When filling the seed tray rub the compost through your hands to break up any lumps.

As a rule of thumb seeds should be sown at a depth 3 times their diameter but I sow everything about a fingernails depth which seems to work perfectly. The exceptions would be large seeds like peas and beans which obviously need to be a little deeper and lettuce and celery/celeriac which need light to germinate so shouldn’t have any covering at all. When sowing seeds which require light (not covered with compost) keep an eye on the surface of the compost as it needs to be kept moist.

Sowing seeds in modular traySowing small seeds can be tricky so here’s a neat trick that will cost you nothing and make fiddly seed sowing a doddle. Fold a piece of stiff card to make a nice sharp groove and sprinkle your seeds inside. You’ll see they all obediently line up in the fold ready to be pushed out to the seed trays below. Use a pencil to push them out as small seeds can stick to a pen.

Cover the tray with another layer of compost, again rubbing it through your hands to break up any large lumps. Don’t press the compost down but clear it off with a ruler or piece of scrap wood, this leaves a nice light, un-compacted layer of compost for the seedling to easily push its way through.

Don’t saturate the compost
Take a bit of care watering as a strong spray from hose or watering can can easily wash small seeds out of their positions. I love the inexpensive bottle top waterer which screws onto any empty plastic bottle to create a mini watering can with a fine and gentle spray that’s perfect for this job. The compost should be moist but not saturated or the seeds may rot.

Wooden plants lables for marking seedsLabel your seeds
Label your seeds with the variety and sowing date so you know what’s what later on, the sowing date is very handy to see quickly whether germination is on track or whether you have a problem. I like to use wooden plant marker labels and a pencil as I can rub out the pencil and use them again.

Using the Propagator
Temperatures vary for seed germination with the average being between 18 and 20ºC. We’re using a propagator because the outside soil temperature in early Spring is too low so we need to provide some extra heat. I’m using a vitopod electric propagator because they are definitely the best (and we sell them!) but the following instructions apply to any good electric propagator.

A good electric propagator will come with a temperature sensor that is inserted into the soil of one of your pots or trays. The sensor tells the thermostat what temperature the compost is so it can turn itself on or off as required to keep the compost at the heat you’ve set. The temperature that matters with seed germination is the soil rather than the air temp which is why the sensor must be buried in the compost.

Vitopod heated electric propagatorThe Vitopod control panel has two readings, the current and desired soil temperatures. Choose your desired settings with the simple + and – buttons, as we’ve already said between 18 and 20ºC is average. Higher temperatures are more suitable for heat loving plants like tomatoes and peppers, up to 30ºC for peppers. Lettuce, on the other hand, won’t germinate above 25ºC so a quick look at our germination temperature guide is worthwhile.

Different plants take different lengths of time to germinate so don’t be disheartened if some things appear and others don’t. For example at optimum temperature a turnip seedling should show above the compost in 3 days whereas beetroot, celery or onions will take 6 or 7 days to appear.

Leggy elongated vegetable plantsHeat and Light
Now, here’s the important bit when you’re growing early in the year when light levels are low:
Seedlings need moisture and heat to germinate (most don’t need light which is why you can germinate them in the airing cupboard) but once the first shoot emerges they need light to photosynthesise. If the new plant has plenty of heat and not enough light it will try to find it fast by growing a long stem quickly to try to search out a better source. This is what’s known as a leggy seedling which will be weak and prone to disease and damage.

Most cold climate crops can be removed from the propagator altogether which will slow growth in proportion to the available natural daylight, warmer climate crops will need to stay on the propagator but at a lower temperature of 16-18ºC.

Protecting with Horticultural Fleece
Plants which have been removed from the propagator need to be protected from freezing night time temperatures and a  layer of horticultural fleece is perfect for this job as it protects down to -2 or 3 degrees and is very inexpensive to buy. Lay 1 or 2 layers of fleece over the seedlings before dark and remove once the greenhouse or tunnel warms again the next day.

When it comes to watering – treat em mean! Under rather than over water as this encourages the plant to produce a stronger root system looking for the water and helps avoid damping off disease which thrives in cold, damp conditions. Also remember roots need oxygen as well as moisture so will literally drown in water logged compost.

Don’t Worry Be Happy!
Your seeds want to germinate and grow, that’s what they are designed to do. We’re just trying to give them the most suitable conditions to get the strongest, healthiest plants. With the right tools and a bit of care growing seedlings early in the year is easy, satisfying and fun and will get your years growing off to a flying start.

  1. g. jackson

    thanks, for your information about sowing seeds and what to do after germination. also about it being better to under water than give them too much. do you put water in the groves which are in the bottom of the heated propagator.

    1. admin

      Hi Joyce. I am glad you found the information useful. It is a good idea to water plants from below as it encourages deeper root growth but I’m not sure it will work adding to the base of the propagator. Can I ask which model propagator you are using? Andrew

  2. J M Baker

    My seeds have now germinated. when should I turn of the eclectic heat as they don’t seem to be doing too good now. They don’t seem big enough to re plant yet.

    1. admin

      Hi there. Thank you for getting in touch. You only need heat to germinate the seeds, once they have come up you can turn the heat down. Please let me know where the propagator is located and what seeds you have germinated and I can give you a much more accurate (and hopefully helpful!) answer. Andrew

      1. Steve Flitton

        In your instructions about the temperature sensor you state that it must be placed into the compost. However, I’ve just purchased a Geopod propagator that, in the enclosed instructions clearly states it must NOT be placed into the compost but about an inch from the base and sides. So, which is correct?

        1. Andrew

          Hi Steve
          Thank you for getting in touch. I have checked with the manufacturers and the temp probe needs to be outside the compost. The older version of the probe was used in the compost hence the confusion. I will change this information in the article.

  3. Martin Creasser

    Hi there, I have just sown my first seeds in a new vitopod propogator. In your instructions above you said that the thermostat sensor should be pushed into soil in a pot but the makers of the the vitopod advised me not to this so I’m a bit unsure what to do. Please could you advise. Many thanks.

    1. admin

      Hi Martin
      I have always used the thermostat in the compost as compost temp is more important than air temp when germinating seeds. I have always had good results this way. If vitpod are saying it should be in the air I guess we must defer to them. I would see which suits best, when the thermostat is in the compost it doesn’t do it any harm.

      I hope this helps


      1. Martin Creasser

        Thanks Andrew, yes it does make more sense for the sensor to be in the compost. I’ve tried it and the temperature seems only to increase by 1 degree than when the probe is just left in the air. I’ll see what results I get from both. Thanks for the help. Martin.

  4. Martin Creasser

    Hi Andrew, could you offer some advise regarding temperature setting. Many of the seeds I have bought have a recommended germination temperature which seems quite wide eg 15 – 20 degrees. Will germination be pretty much the same anywhere within this range or have you found in your experience that setting the Vitapod thermostat in a specific place within the range eg in the middle or at the top of the range gives better results. I would be grateful for any advise on this.
    Thanks again for your help.

    1. admin

      Hi Martin. As a rule of thumb most outdoor planted vegetables will germinate happily at 18 degrees which is the temperature I set my benches at. Warm climate crops like tomatoes, peppers, courgettes and cucumbers will be happiest at a temperature of 22-24 degrees. If you have a mix in the Vitopod I would suggest setting it at 20, I would be very surprised if everything doesn’t come up as you want it. It is worth remembering that different vegetables have an optimum germination temp as shown on the packets but will be perfectly happy if this is broadly observed rather than accurately adhered to. I hope this helps.

  5. John Burson


    i am using a vitapod and my lettuce and some other veg seeds have now come through. I also have lights above the vitapod but should I now take the pots and trays out or reduce the heat at present set at 20c. thanks John

    1. admin

      Hi John. Thank you for your question. Yes, I would reduce the heat at this stage to 10 degrees or you will get very fast growth and outgrow the propagator when it is still too early to plant out. I hope this helps. Andrew

  6. Hussaam

    Hi Andrew,I was wondering when growing tomatoe seeds just after sewing them can you put them in the propagator same with the chilli

    1. admin

      Hi Hussaam. Yes, you can put tomato seeds in a heated propagator with the chilis, the germination temperatures will be about the same. I hope this helps.

    1. admin

      Hi there. Yes, this will be fine, just make sure you get the seedlings into the light as soon as they come up. Thanks for getting in touch! Andrew

  7. Christine Barnes

    Hi Andrew
    I am so pleased I have found this information as at the moment I am wishing I had not bought my vitapod! I am a beginner so learning so much the hard way!

    Could you advise what sort of plants you would overwinter in the vitapod?

    Also where is the best place to keep the vitapod? I have mine in the greenhouse but the greenhouse has reached temperatures over 25 degrees in a very warm April. I have turned the vitapod off during the day. Is that the right thing to do?

    Sorry for so many questions.

    1. admin

      Hi Christine

      Thank you for getting in touch. The Vitopod is thermostatically controlled so there is no need to turn t off in the day as it will turn itself off. When the temperature in the tunnel gets above the temperature you set on the thermostat it will turn the heat off. I also have the Vitpod in the greenhouse as the plants will need as much light as possible. It depends what height your vitpod is but they can be used to protect frost sensitive plants like chilies.

      I hope this helps


  8. Caroline Stewart

    I have an electric propagator and seeds for next year, one thing I would like to know is can i put the plastic seed container in the propagator until they are big enough to transfer to a larger container, you see I am growing flowers rather than veg, also new to gardening? Thanks

    1. admin

      Hi Caroline

      Yes, you can start seeds off in the propagator before transferring to a larger container. Be aware, however, that there probably isn’t enough light available in the Winter for the flowers you want to grow (I am presuming you are thinking of growing now). My advice would be to wait until the Spring.

  9. Tim Holland

    I’m using a thermostatic controlled propogator for sweet peppers, and theres quite a variability in the germination; one is about an inch tall, 4 are just showing and the rest nothing. No sign of leaning to the light yet. When can I safely re pot and get them into better light?

    1. Andrew

      Hi Tim
      Once the plants are growing you need to reduce the heat and get them into an area with plenty of sunlight. You probably have your propagator set at around 20 degrees C and while this is fine for germination of pepper seeds it will be too warm for the light levels available at this time of year. Depending on what propagator you are using they can be warmer towards the center which can cause variability in germination. I would be inclined to move any peppers that have come up into the brightest are you have, at least in the daytime. It will still be too cold to leave them in an unheated tunnel or greenhouse overnight but they should be fine in the day which will give them a good fix of light. They will grow slower but will produce a stronger seedling than if you left them on the heat.

      I hope this helps


  10. Lotti

    Hi Andrew
    I have a heated propagator with lights over. My seeds have germinated very quickly and I have transplanted soon as secondary leaves begin to show, removed from propagator and grown on in modules in a very light top floor living room windows floor to ceiling, hardly any leaning to the light , however religiously turn twice daily but Still my seedlings are long spindly stemmed! Sprouts, tomatoes, French marigolds, cosmos and zinnias. Compost wyvevale general purpose! Regular watering also. What else can I do to improve seedlings! I also planted stems deep.
    Despondent Lotti

    1. Andrew

      Hi Lotti. If the stems are leggy they need more light. You need a balance of heat and light so the room they are in may be too warm relative to the light they are getting. Bear in mind that light intensity at this time of year is low so even on a sunny day they will not be getting as much as you think. I would use the propagation lights on the plants and leave them on 24 hrs a day. Germinating seeds do not need light, only heat and moisture so once they come up take them off the heat and put them under the lights only.
      I hope this helps

  11. Jax

    I really don’t know what I am doing!!

    I’m very confused about watering, but after reading the comments above I understand that I don’t understand the concept at all.

    I have 2 vitapods – one with lights. I help out an older person who has one in a greenhouse and I have one in a garden shed – mine has lights.

    The one in the greenhouse has tomatoes which are doing very well- 21 degrees put with them I planted Statice and parsley, as my friend loves them. The two later have done poorly 12 Statice have germinated out of 80 seeds and the parsley about the same.

    Is this because it is too warm?

    They told me to leave the tomatoes in the propagator – they are about 6 inches high, at this temperature.

    The Statice that germinated I have potted on but put back into the propagator?? To keep warm them warm as the greenhouse has broken glass and is a bit drafty.

    Also I have ‘condensation’ within both propagators – I thought this was good as it gave them a ‘sealed’ environment but it looks like I have too much? They did look like they were drying out to me so i watered them.

    So is the principal once germinated take out or turn off heat until they can be potted on?? And then once potted on they come out and cover with fleece if chilli

    I am sorry I do feel such an idiot!!

    1. Andrew

      Hi Jax. Easy there tiger, don’t be so hard on yourself, no one is an expert straight out of the box.

      Basically seeds need a certain germination temperature to trick them into thinking it is spring (in the case of a tomato that is Spring in South America rather than here). We need temp in the low twenties to achieve this but once they come up they will need light levels to match the heat or they will grow tall (and leggy) looking for more light. As far as the plant is concerned it is ‘thinking’ if there is 21 degree heat there must be bright strong sunlight around here somewhere, I need to grow fast and find it.

      You are right in thinking that a draughty greenhouse will be too cold but we need a happy medium. Ideally you need to keep them in the propagator but turn the heat down to about 12 degrees to slow growth until the light levels rise.

      I hope this helps


  12. Greg

    Hello there, helpful site as always …
    I have just started using an electric propagator so get a headstart with Chillies and Bell Peppers. It’s the Garland Big 3 model which has a fixed temperature. The seeds have germinated nicely (75% – 85%) and I have started leaving the lids off during the day (it’s in my lounge near the french doors to the conservatory). The seedlings are 3 cm tall on average and so I have just turned off the heat figuring that it is not required now. I have the seedlings in the supplied seed trays but would like to prick them out, (I know how to do that), but would you;
    a) Prick them out into reasonable cell trays, or
    b) Three inch pots
    Also, I was going to move them into the (unheated) conservatory during the day for extra light levels but maybe bring them back inside at night as I was thinking that it might be too cold for them being so tender?

    I am inclined to use cell trays as shifting circa 120 x 3″ pots in and out just might be a game that I would quickly tire of.

    A little guidance please, good Sir?


    1. Andrew

      Hi Greg

      I am not being lazy here but you are bang on in everything you say, continue to do what you are doing and yes, cell tray make much more sense for the reasons you describe. It sounds like the seedlings are under good care.

      Good job Greg! Let me know if I can be of any assistance in the future.


  13. Heather

    Hi there Andrew.
    Help needed please. I have a heated propagator in which I have sown some sweet red peppers (5) but mostly lots of different tomato seeds. It’s a decent size unit so have plenty of room. I have plugged it in in my glass porch. glass from windowsill height with full length glass doors (all double glazed) so it is light and airy. It does get chilly in there at night though this time of year. I also have a small electric heater lugged in but has not been on as yet due to the heated prop. Now, my question is – today I have noticed that about 1/2 my toms have germinated (about 5 days) … so I have taken them out and put on a shelf level with the window and have switched n the little heater to 18 degrees. Will this be warm enough and have I done right here? Didn’t want them getting a shock coming straight of out heated propagator poor things!!! Am I right in moving them out into the light immediately they shrug their little shell……… and just keep the soil moist? Eventually will pot ion further then they will be going in an unheated Greenhouse probably mid to late April – what I don’t need I will give to friends. Any help would be much apprecuated. I am located in Manchester by the way.

    1. Andrew

      Hi, once the seeds have germinated they won’t need to be so warm. They are working toward the light at this stage. The ambient temperature in the porch should be enough and will harden them
      before planting out.

  14. Stephen Finch

    Hi Andrew,

    I have just bought from yourselves the large single layer vitopod propagator and this yesterday sowed a lot of different seeds ie impatiens,ageratum,begonias,petunias,chilli peppers,tomatoes and courgettes (to name but a few). I have watched your videos, read all of the comments above and taken note of your excellent answers which are very helpful to beginners like myself.
    From the information I decided to set the vitopod temperature controller to 20oC and placed the temperature probe into the compost of one of the pots. The vitopod is situated in my unheated glass greenhouse.
    Here is the problem, when I checked the reading today (wet and windy outside) it showed the compost temperature to be 29oC is this normal as I am worried that I am cooking the seeds? Should I reduce the temperature set on the controller which I thought switched off the heat once the set temperature was reached ?
    I hope you can help as I am a bit worried that I may have got off to a bad start and perhaps should have stuck to growing those simple little marigolds instead.
    Many thanks.

    1. Andrew

      Your temperature controller is working off the ambient temperature in the Vitopod and will adjust the heating to suit. It appears that your compost is retaining a bit of heat below the surface. I would suggest turning the thermostat down a bit and try a few settings until you get the correct soil temperature, normally around 20C.

    1. Andrew

      Hi Carmel. You need to reduce the heat and give them as much light as possible. You can also plant most of the stem under the compost when you pot them on to larger pots which will help to balance them up. I hope this helps.

  15. Baebara

    I have just purchased a Garland heated propagator. The natural temp is showing 20. I set required temp at 25 to start behind begonias off but the base is not getting warm. Any suggestions please. Never had a heated one before so a novice.

    1. Andrew

      Hi Barbara, it could be that the temperature inside the propagator is already at 25 if the temperature in the poytunnel/greenhouse is 20 so the thermostat (if you have one?) will have kicked in. Hoping I’ve understood that correctly. I’d check it out at night time and see if it’s working then – when the temperatures are lower.

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