There are a couple of things you need to get right when propagating vegetable seedlings but once you are set up the rest pretty much takes care of itself.
1. 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.
If 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. 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 (64.4°F) is about right for most cool climate outdoor vegetables whereas 22ºC (71.6°F) covers most warm climate crops like the aforementioned tomatoes, peppers etc….
Use 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.
See also: Soak the seeds before planting them
5. 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.
7. 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.