Top 10 Tips For Seed Propagation

Modular seed trays

Top 10 tips for plant propagation

Propagating vegetable seeds using a heated propagator
Starting off seeds indoors has two main advantages; plants are protected from pests and disease in their most vulnerable stages and seeds can be started earlier in the year when outdoor temperatures are too low.

If you are starting plants off iVitopod heated propagatorn early Spring you will need to use a heated propagator as seeds need a minimum temperature to germinate and begin to grow. A heated propagator can be a heated cable laid in sand, a heat mat or an enclosed plant propagation unit with a clear perspex lid.

We supply a wide range of choices in our shop section and are always happy to give expert advise on any of our products should you need it.

The top 10 tips below are meant as a simple guide to learn the basics of plant propagation, we hope you find it helpful as part of your vegetable growing adventure!

Klasmann fine grade seed compost1. Use a seed compost as it’s finer and has lower nutrients than multipurpose compost. High nutrient compost can inhibit seed germination in some plants and damage young roots. An ordinary multipurpose compost can, however, be used in most cases if passed through a compost sieve to remove any large lumps and allow easy root penetration.

2. Sow seeds in modular trays with individual cells. Modular are seed trays containing a fixed number of individual cells each containing a single ‘plug’ plant. Plug plants are both easy to remove from the trays and easy to handle with minimal root disturbance when planting out.

Modular seed traysIf you are using a heated propagator modular trays are also more efficient as you will fit more plants on the heat than individual pots, you will also find moving plants around for hardening off or planting out much easier.

We find an 84 cell tray is the best fit for most crops with a 6 cell version (which has larger cells) for plants which need to spend longer in the tray like tomatoes, cucumbers, courgettes, peppers or squash.

3. Seeds sown in early spring will need heat to germinate. An average temperature of 18ºC is about right for most cool climate outdoor vegetables whereas 22ºC covers most warm climate crops like the aforementioned tomatoes, peppers etc….

Digital propagator thermostatUse a heat mat, heat cable or electric propagator with a thermostat added to achieve an even heat. Thermostats are much more energy efficient as the power is only turned on when the plant tray temperature falls below the the required level. You will find temperatures in a tunnel or greenhouse will often exceed those required on a sunny Spring day, extra heat simply isn’t needed.

4. Cover seeds with a fine layer of compost after sowing. Tamp the compost down gently and crape off the top of the tray wit a ruler to leave a loose structure for the new seedling to push through.

The exceptions to this rule are lettuce, celery and celeriac which will germinate better if left uncovered. In this case make depressions in the surface of the soil with your fingers and sow one seed per cell. It is helpful to water the compost before you sow as watering the uncovered seeds after sowing can wash them out of position.

Bottle top seed waterer5. Compost should be moist but not soaking, extra watering shouldn’t be needed until the seedling has emerged. Care should be taken when watering newly sown trays to avoid washing compost or seedlings out, use a watering can with a fine spray or the very handy bottle top waterer.

6. Turn the heat down! When the seedling has emerged from the seed turn the temperature down to 4-6 degrees or remove them altogether. Light levels are low in early Spring and too much heat but not enough light results in long spindly (leggy) seedlings.

For cool climate crops it is best to take them off the heat altogether, they only need the heat to germinate and provided they are protected from frost they will produce stronger and more compact seedlings.

Fleece plant frost protection7. Protect seedling plants with a good quality horticultural fleece at night when temperatures fall. Fleece will protect a plant down to -3 degrees and is much cheaper than a greenhouse heater. Fleece is a very light material and is simple draped over the growing seedlings without damaging them, it also emits light so can be left on the plants if daytime frost is a risk.

8. Always label your seedlings with variety and date of sowing, use a pencil so you can re-use the labels.

9. Under rather than over water young seedlings for the following reasons:
The plant grows more roots looking for moisture making it stronger in the long run.
Plants are less likely to suffer from ‘damping off’ disease, a fungal infection caused by excessive moisture. Drier compost tends to be warmer in cold weather giving better growth.

10. Most seedlings will be ready to plant out in 6-7 weeks in early Spring or 4 weeks when the season warms up in April.

If you would like to read more about seed propagation here is a good article about growing healthy vegetable seedlings. We hope you found this article helpful  – Andrew@quickcrop


    Cheers,Very detailed and accurate instructions on seed germination and plant husbandry.Once these
    vital and early hints are followed. the rest is easy.I have been germinating tomatoes.peppers,aubergine
    courgettes,cucumbers and many more tender veg. and half hardy annuals for the past 40 years with
    great success.A good propagator is a valuable asset.Thanks again.

    1. Andrew

      Hi Dermot. Thank you for your comment and for reading our blog. I am sure we could learn a thing or two from your 40 years experience! Thanks again, Andrew.

  2. Miranda


    Just wondering if you could give me some advice on some leggy cosmos seedlings? Can just see the first set of true leaves appearing (miniscule), can I prick them out now and bury them deep/ up to their leaves to strengthen them up a bit? They shot up just as the cloudy weather set in. Now got a grow lightbulb to help them out a bit and am going to build them a tin foil reflector box tomorrow.

    Also once I’ve pricked them out can they be potted in regular soil or do I need to use the seed soil again?

    First time growing anything from seed / anything much at all. Been doing lots of research but no practical experience.

    Any advice much appreciated, trying to fill our city studio courtyard with lots of flowers to draw!

    Great blog!

    1. Andrew

      Hi Miranda
      I am afraid I am not an expert on flowering plants but if seedlings have become very leggy I would always sow again, you should have more success as light levels naturally increase in April. I would also try replanting the seedlings that you have as you suggest but not when they are still very small. When the secondary leaves appear and the seedlings are 3/4 inches tall I would replant them in a potting compost up to the level of the second set of leaves and see what happens. The issue is you don’t have enough light and too much heat. If you are germinating indoors move them to a cooler spot once they have come up with as much light as possible. It might be worth investing in an outdoor cloche so you can put them outside in the daytime and move them back inside at night.

      I hope this helps


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