Germinating Vegetable Seeds in a Heated Propagator

Vegetable seed propagation supplies

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 but 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 Klassman 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 Horticulatural 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. Alsos 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.

Wooden raised garden pond shop

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4 Responses to Germinating Vegetable Seeds in a Heated Propagator

  1. Adrian Masterson says:

    hi Niall, I like your propagator but the one shown is too big for my purposes. I wonder if you do the same kind of propagator but smaller. I only need 30 to 40 seed pods. Any luck and if so, how much does it cost, please.

    Best wishes


    • Andrew says:

      Hi Adrian. Yes, we do stock a range smaller propagators, I include a link to our propagation department where you can see our full range. There is a smaller version of the large vitopod propagator pictured or we also have much smaller windowsill or compact kitchen propagators. Please let me know if I can be of any more help, you can email direct at or call me in the office on 01 524 0884. Thanks for getting in touch. Andrew

  2. Adrian Masterson says:

    Hi Andrew, I am about to buy your small heated propagator, with double height cover. Can you tell me if I need to order a container for seedlings as well and if so, what is the appropriate size to fit in the small propagator? Is there anything else I should include while I’m at it?



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