Growing Vegetable Seedlings
Your ultimate vegetable garden seed starting guide:
What to start when, where to start it, and how to grow it!
Growing your own vegetable seedlings gives you the widest choice of varieties and ensures that seedlings are ready for planting out when you want them. Growing your your own seedlings is a commitment though, and as a general rule of thumb seedlings will require 5-10 weeks of daily care. You will need to have the right growth conditions at home to grow your own seedlings. Having enough light is likely one of the biggest struggles with starting seeds at home.
Light requirements
Seedlings stretching for the light become etiolated
South-facing windows may provide enough light if seedlings are directly beside the window. Seedlings will lean towards the light if light is not strong enough, causing them to be elongated. The stems of these seedlings are weak and easily attacked by damping off.
Supplemental light is ideal. High intensity fluorescent tubes and high intensity LED ‘tubes’ are now available to fit into fluorescent fixtures. Grow lights must be close to the top of the seedlings for sufficient intensity. As seedlings grow, lights will need to be raised. Use a timer to keep lights on for 16 hours per day.
Temperature and Moisture
Most vegetable seeds germinate in very warm conditions (21 to 30 degrees Celsius). Ideal germination temps are:
| 21 degrees Celsius | 24 degrees Celsius | 27 degrees Celsius | 30 degrees Celsius | 35 degrees Celsius |
|---|---|---|---|---|
| Celery | Lettuce | Carrots | Beets | Cucumber |
| Parsnip | Peas | Cauliflower | Broccoli | Pumpkin |
| Spinach | Onions | Cabbage | Squash | |
| Radish | Eggplant | Melon | ||
| Tomato | Peppers | |||
| Sweet Corn |
Once seeds germinate, they prefer slightly cooler growing temperatures (18 to 20 degrees Celsius).
Bottom heat can be used to increase the temperature for seed germination. Once seeds have germinated it is important to monitor temperatures to ensure temperatures do not remain too high. Maximum light levels are required though.
Growing in a Coldframe or Unheated Greenhouse
Grow lights can also be set up in cold frames and unheated greenhouses.
Temperature control in an unheated greenhouse can be a challenge. Heat mats can be used in combination with grow lights in this scenario to ensure temperatures never drop below 12 degrees Celsius overnight.
Clear domes on top of seed trays, in combination with reemay, can effectively keep temperatures warmer overnight. During the day the domes and reemay need to be removed to prevent seedlings from becoming too warm.
It’s important to monitor the weather if growing in an unheated greenhouse or cold frame. On a sunny day in late winter temperatures in an unheated greenhouse or cold frame can easily reach the high 20’s or low 30’s. It’s very important to open a door to help regulate temperature.
Starting Seeds: Direct Sowing
Many seeds do well direct sown: carrots, beans, pumpkins, corn, peas.
Direct-sowing other crops can also be very effective, but harvesting may be delayed compared to starting seedlings indoors or in a coldframe.
Birds can be an issue when direct sowing some seeds so using reemay to cover the soil until seeds have germinated can be beneficial.
Soil temperature is crucial when sowing seeds as many vegetable seeds will not break dormancy until the soil temperature has warmed. For warm season crops like cucumbers, beans and corn, resist the urge to direct-sow these seeds until soil temperatures are 15 degrees.
Soil Mixes
Use a seed-starting mix. There are now peat-free seed starting mixes available, made with coir. You can also make your own seed starting mix using a combination of: coconut coir/peat, vermiculite/perlite, and screen compost.
Note: If you use a soil mix that is too heavy seedlings will struggle to have a healthy root system. If the soil had too much fertilizer or organic matter in it, the root systems of seedlings can be burnt.
Damping off: A variety of soil organisms, that are most active in cool and wet soil conditions, can attack the stems of seedlings or germinating seeds. This can cause a total loss of seedlings in a container or seed tray. The best way to prevent damping off is to:
Take care not to over-winter
Good air circulation
Ensure soil is the correct moisture before sowing seeds
Thin seedlings that are congested
Germinate seedlings in warm conditions
Sterilized soil vs. soil with unpasteurized compost: Sterilized soil has reached high enough heats to kill pathogens and diseases. Historically, sterilized soil has been recommended to grow vegetable seedlings. In recent years, there has been more research being done on disease suppressing organisms in home-made compost, so there is no longer a hard and fast rule of using sterilized soil for seed-starting. More information.
When to Start Seeds
Our favourite resource is West Coast Seeds Planting Guide. Here is a rough guide to get you started:
| Jan/Feb | Feb/March | March | April | May (after frost) |
|---|---|---|---|---|
| Celery | Onion | Tomatoes | Melons | Corn |
| Celeriac | Leek | Eggplant | Zucchini | Beans |
| Peppers | Cabbage | Squash | ||
| Peas | Broccoli | Carrots | ||
| Cauliflower | Beets |
Hardening off
If growing in an unheated greenhouse or cold frame begin to harden off seedlings by opening the doors during the day.
Hardening seedlings off helps them to build up sun protection chemicals in their cells, grow a protective leaf cuticle, and grow stronger stems
For seedlings grown under grow lights, the growth rate needs to be reduced by growing them in cooler conditions.
Expose seedlings to sun each day, gradually increasing the length of time they’re outdoors everyday. Do this over at least a week.
Our favourite seed starting resources:
Linda Gilkeson’s email newsletter. Sign up here.
Seedy Saturdays - see dates here. A great source for local seeds and very knowledgeable vendors. Workshops are often available.
Linda Gilkeson’s Grow Your Own Seedlings presentation
