It’s seed-sowing time so I thought I’d share my tips for successful seed germination. I’ve heard a few people mention they often have hit-or-miss germination for certain plants (particularly strawflowers) but after a few years of trial and error, I’m getting excellent germination for almost all of my seeds, with non-germination usually being down to non-viable or low-viability seeds from the seller. Fortunately this usually happens only very rarely provided the seeds come from sellers who carry out viability testing before offering seeds for sale. (Note - some seeds require special treatment which I don’t cover here, and I also don’t discuss direct-sowing.) So, here are the main things to consider when starting your seeds indoors…

Seeds

Start with viable seeds. This is especially important for a few plants like helichrysum for which viability seems to drop off rapidly after the first year (at least that’s what I’ve found). Seed suppliers will hopefully indicate if certain seeds need to be sown in the same year as they were purchased. Some packets don’t come with use-by or packed-on dates so write on the date you bought the seeds to help your future self. Store seeds so you can keep what you don’t use for the next year. I store my seeds in an airtight box in my workroom which has low humidity and doesn’t get too warm. You can also store seeds in airtight containers in your refrigerator. If you’re not sure whether your seeds are viable, you can test them - Gardener’s World has a simple how-to guide.

Compost

Use seed compost or a potting compost that’s low in nutrients. Commercial seed composts are specially formulated with low levels of nutrients and should also be free of pathogens that can inhibit germination or kill young seedlings. You can usually buy seed compost in small volumes if you’re not sowing that many seeds. I only use multipurpose compost once my seedlings are ready for potting up into 3-inch pots.

Seed trays and pots - to clean or not to clean?

If you use compostable fibre pots or trays you don’t need to worry about this! But if you sow into re-useable seed trays and you’ve had disease in them in the past few weeks, it’s a good idea to clean them before sowing. I use durable cell trays and pots and personally, I don’t wash them before use. In an effort to save water and time, I stopped cleaning them a few years ago and I’ve not had a problem in the years since. Damping off of seedlings usually has more to do with environmental conditions (humidity, temperature, overcrowding) than dirty equipment.

Temperature

Check the growing advice to see what soil temperature the seeds need for germination. Because I start my seeds in my conservatory in late winter and early spring, I use heated propagators for all of my seeds unless we have really warm outside temperatures and lots of sun (usually not). After germination and some growth, I move some seedlings (the ones that can take some cold) out to my unheated greenhouse including cornflower, calendula and ammi. I keep heat-loving seedlings on heat for a while longer, or as long as air temperatures are cold, to encourage root growth and to help prevent damping off (which prefers cool soil).

Humidity

After sowing, keep the seed environment humid by covering the seed tray or pots with a transparent lid. Once germination occurs, remove the lid to allow air movement and help prevent damping off.

Watering

Watering from below helps prevent the compost from compacting and it also helps keep the surface from getting overly wet. Only water when needed, especially if your seeds are somewhere cool or not in a propagator. Overly wet and cold compost can cause root death, promote damping off and encourage fungus gnats (sciarid flies) which lay eggs on the surface of damp compost. I use clean tap water until my seedlings are well established and then I’ll switch to rainwater from my tanks (also to help avoid disease).

Vermiculite

I usually use vermiculite to top-dress any seeds that require covering to help as an additional barrier to moisture loss. This year, in an effort to deter fungus gnats, I’ve also been adding it after germination to top dress the seeds that don’t get covered at sowing (such as Helipterum). I know there are fungus gnats in the conservatory, but they’ve not yet caused a problem in my seed trays, so perhaps the vermiculite is doing the trick!

I’ve also used vermiculite (or sometimes perlite) to improve the drainage of compost for sowing and potting up of plants that don’t like cold, wet compost, like zinnias, ammobium and xeranthemum.

When to sow?

To decide when I’m going to sow annual seeds, I use the seed-starting date calculator on the Johnny’s Seeds website. This resource lists a number of popular vegetables and flowers with the number of weeks to sow seeds before planting out and the general time they can safely be planted out (e.g before or after last frost). All you need to do is enter your last frost date and the calculator tells you the calendar date ranges for sowing and planting out. If a flower isn’t included in the calculator, I’ll calculate the dates myself - based on what information I can find on the plant’s time to reach maturity - so I make sure I don’t start the plant too early (I don’t want to pot it up more than once) or too late because I want to make sure it will flower before the end of the growing season. Last year I had a crop of blue statice that didn’t reach flowering because of a combination of me starting them too late and less than ideal weather during the growing period.

For sowing biennials and perennials, follow the advice on the seed packet.

Potting up

I wrote in a previous post that I no longer use the ‘pricking out’ method when growing from seed. Instead, I sow several seeds in each cell of a tray and then thin to one strong plant per cell, leaving the seedlings to grow on in the trays. Then I pot them up into 3-inch pots once the roots fill the cells. Their robust root systems give the seedlings the best chance at surviving the potting-on process. This is when I change to multipurpose compost which provides enough nutrients until I transplant into the garden. After this step, it’s just careful watering and then eventually hardening off when the plants are ready to go out into the cut flower garden.

That’s all of my seed sowing advice. Do you have any other tips to share? Are there any particular seeds that you struggle to germinate?

Wishing you the best of luck with your seed sowing this year!