Grow Perfect Seedlings Every Time

, written by Benedict Vanheems gb flag

Onion seedlings under grow lights

There’s no better sight than seedlings finally pushing through – a sure sign that homegrown harvests can’t be too far off! But what’s the best way to start seeds? Where, even, do you begin? Read on or watch our video for our tips…

Essential Seed Starting Equipment

Let’s start with the basics: your seed-starting mix. You can use an multi-purpose potting mix, or a mix that’s been specifically formulated for starting seeds. A seed-starting mix has a finer particle size that’s particularly beneficial for smaller seeds. It’s also a lot lower in nutrients that an multi-purpose potting mix. Seeds don’t need any added nutrients at all to germinate – the seed contains all everything it needs to get going – so this works just fine.

Whichever potting mix you use, do make sure it’s peat-free. Extracting peat releases carbon and destroys natural habitat.

When it comes to your sowing containers you’ve got a few options: you can use pots, seed flats, or plug trays. Pots and seed flats are more space-efficient than plug trays, taking up less space on your windowsill or in your greenhouse. It also means there’s less wasted seed, because every single seedling that germinates can be transferred into its own pot or plug tray once it has germinated.

The benefit of using plug trays is that there’s less need to pot seedlings on. You can simply sow into the plug trays and then either grow the seedlings on as a cluster, or thin them out to leave one seedling per plug. It means that the seedling will remain in place until the roots have filled the plug (at which time the whole thing can be pushed out and transplanted into a larger plug or pot), or until it’s time to plant them out, meaning less root disturbance.

Sowing into plug trays results in less root disturbance later

Sowing Seeds

It’s essential to sow seeds at the correct time. Sow too early, and plants may have outgrown their pots before the weather has warmed up enough to plant them outside; but start seeds too late and they won’t have enough time to reach maturity before the end of the growing season.

It’s a balance, but the back of the seed packet should give you a good guide as to timings. Our Garden Planner can also help. It looks up climate data from your nearest weather station and then calculates the best range of planting dates for each crop in your plan. Your Plant List shows recommended sowing, planting and harvesting times for the plants in your garden. The blue bars show when each plant can be started indoors, while the green bars indicate the best times for both sowing and transplanting outdoors.

Fill your pot or seed flat to the brim with your potting mix, then tap it down and firm it level. It’s hard to over-firm it, and seedlings prefer plenty of well-filled potting mix to sustain them. Sprinkle the seeds very thinly, then cover thinly with a little more potting mix. Most seeds need to be covered to a depth of around twice their diameter.

Some seedlings are easier to sow into a plug tray. I like to use this method for cluster-grown crops like beets and salad onions. Fill the plug trays right to the top then tap it down to settle. Top up with a little more potting mix, brush off the excess, then use your fingers to make little depressions into the mix. Sow your seeds, then cover them with a thin layer of potting mix.

Stand containers in a tray of water to allow them to soak up the moisture they need

Make sure to label your sowings - this is important! You might think you’ll remember but it’s way too easy to get confused, particularly if you’ve got seedlings with very similar leaves. Include the date of sowing and the variety you’ve sown.

Use a watering can fitted with a fine rose and go over the pots and trays a few times. Leave to drain through from the surface and then repeat. You want to really wet the potting mix so the seeds are woken up from their slumber! Don’t worry, if it’s a good mix it’s hard to overwater at this point – any excess will just drain out of the bottom.

Speed Up Germination

I’m impatient and want to see those seedlings push through as quickly as possible! The best way to achieve that is to give your seeds as close to ideal conditions as possible, which in most cases means a little warmth, so bring these early sowings indoors to germinate.

The seed packet should give an indication as to ideal temperatures. In most instances, tucked away in corner of a warm room should work just fine, or you can place it somewhere with a little extra heat to speed things up – on top of appliances like refrigerators or freezers, or on a mantelpiece, for example.

Cover seed trays and pots to retain moisture and place them somewhere warm

Keep the potting mix from drying out by covering the plug tray or pot with some sort of clear cover. You could use a purpose-sold propagator for this, or simply secure a clear plastic bag over the top of the container. Just be sure to check regularly and water if necessary.

Seed packets usually give you an idea of how long germination should take, but nevertheless, nothing beats regular inspections – and that’s half the fun anyhow! Once around half of the seedlings are up, remove them from the humidity dome or plastic bag and move the seedlings to somewhere with good, strong light.

Start Your Seedlings Off Right With Plenty of Light!

Growing seedlings on a windowsill, especially early in the season, rarely gives them access to the same quality of light as outdoors. The result is leggy seedlings that often struggle to recover, so if you don’t have a suitable outdoor protected structure such as a greenhouse or cold frame, it might be worth investing in some full-spectrum grow lights.

The lamp unit on most grow lights can be adjusted up or down. Ideally the lights should be about 4-6in (10-15cm) above the canopy of the seedlings – far enough away that the seedlings don’t get too hot, but close enough to give good, strong light. Move the lamp unit up as the plants grow.

A growlight is a useful tool when light quality is poor early in the season

Keep the lights on for up to 16 hours a day. The longer you leave them on, the quicker seedlings will grow, so this is a good way to catch up on growth early on in the season. I tend to switch lights on when I get up in the morning, then switch them off as I head up to bed, which means they’re on for around 15 to 16 hours. Or you could, of course, put your growlights on a timer.

You can move plants outside or into a greenhouse once the weather warms up or, in the case of cool-season crops, once light levels have improved and it’s less gloomy. For cool-season crops, poor light levels are often the killer at this time of year, rather than low temperatures.

Transplanting Seedlings

In most cases you’ll need to transplant – or 'prick out' – seedlings into their own plugs or pots once they have germinated. Don’t procrastinate too long before doing this. It’s important not to allow them to become overcrowded, as this can cause them to become leggy and can encourage disease. It’s also a lot easier to separate seedlings out when they’re small, before the roots of neighboring plants begin to knit together. It’s usual to do this once they have two pairs of leaves (their seedling leaves and their first ‘true’, or adult leaves), but I often like to move seedlings on a lot earlier than this.

Fill new pots or plug trays with multi-purpose potting mix. Make holes to nestle each seedling’s roots into, then carefully remove the seedlings from their nursery pot. If you aren’t transferring all of seedlings, then lift out only what you need at any one time.

Always handle seedlings by their leaves to avoid crushing the delicate stem

Carefully separate the seedlings then transfer them to their waiting holes. Only ever handle seedlings by their leaves, because if you damage or crush the stem, the seedling’s had it. Try to avoid damaging the roots too. This is one reason why working with really young, small seedlings is often better: their root system are nowhere near as extensive as more established seedlings, so there’s less root to damage.

Firm in around seedlings then water from above, or alternatively set pots or trays into a container of water. This enables you to water from below by allowing the water to be drawn up through capillary action. After about 15 minutes, pour off any water that remains in the container.

Check moisture levels regularly. Push a thumb into the potting mix or simply lift the plug tray or pot up to gauge how heavy it is. You’ll get a feel for this with more experience, but the heavier it is, the more water it contains and the less likely it is to need watering.

Acclimatize seedlings to outdoor life gradually

Hardening Off Seedlings

Seedlings of more tender crops must be gradually introduced to outside conditions before they are planted – a process known as ‘hardening off’.

Take seedlings and plants outside for about an hour a day to start with then, over the course of a week or more, gradually increase the period of time they are outside until it’s time to plant. Cool-season crops like lettuce need less of this hardening off than warm-season favourites like tomatoes.

A great way to toughen up plants – whether indoors or under cover in a greenhouse or cold frame – is to occasionally run your fingers lightly back and forth over the foliage. This mimics the action of the wind to create sturdier plants. Indoors you could also use a fan for this.

The Garden Planner provides recommended growing dates for your garden as well as spacing requirements

A Few Final Seedling Tips…

Experienced gardeners always hedge their bets. Successful gardening depends on so many factors. Is the season unusually warm or cold this year? Are your first seedlings going to be eaten by pests like slugs or birds? That’s why it’s a good idea to sow seeds in small batches a few weeks apart, and it’s worth making an early start because then if you lose those seedlings it doesn’t matter so much – you can always sow some more.

The Garden Planner shows you exactly how many plants you’ll need for the space you have. But hey, sow a few more than you’ll need as spares, just in case, or so you can select the very biggest, healthiest seedlings to plant out.

Many quick-growing crops like lettuces or radishes are harvested regularly. Sow a small plug tray every couple of weeks throughout the growing season, and that way you can look forward to a succession of harvests, rather than having them all come at once. That’s smart planning!

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Show Comments


"I am using your Garden Planning App and have high hopes for a successful vegetable garden. it will be small--one raised bed 80" x 40". I am using the square foot method, so it looks like I can still fit a lot of vegetables into that. I also plan to do a fall planting of some of them. I am following the planting schedule in your plan for my area. So, have ordered my seed starter kits with domes and self watering for my window sill. I now need to order the seeds and get to planting late February-early March. I'm enjoying your many helpful articles like this one. I am a seasoned gardener, but have concentrated mostly on dahlias, roses, perennials, etc. We have a real deer problem, so I have had to wait to install a deer fence to grow these veggies. I'm excited and am glad you have the message board when and if I have questions as I go along. "
Gloria Crawford on Thursday 9 February 2023
"Hi Gloria. So pleased that you are starting a new vegetable garden. I'm sure it will bring plenty of bounteous harvests. :-)"
Ben Vanheems on Friday 10 February 2023

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