The Complete Guide to Sowing Seeds

, written by Benedict Vanheems gb flag

Starting from seed in a plug tray

Spring is almost here, which is our cue to bust open the seed packets and start sowing. Want to find out how to succeed with seeds? Great! I've put together a few simple sowing guidelines, so let’s get started...

Time It Right When Starting From Seed

I start most of my vegetables off in plug trays under cover because it means that when it’s time to plant I can guarantee strong, thriving seedlings and an almost 100% success rate. I do sow directly outside as well, but only vegetables with long taproots like carrots and parsnips, which don’t transplant well.

When sowing, the first thing to consider is timing. There’s little point in sowing too early because you may end up with seedlings that are ready to plant while it’s still too cold outside for them. Conversely, sow too late and there won’t be enough time in the growing season for the crop to develop fully.

The optimal timing is different for each type of vegetable. If you’re unsure of the best dates for your location, our Garden Planner can tell you when it’s safe to begin sowing under cover or outdoors using your location and local weather station data to pinpoint timings suited to your garden.

In most areas, early spring means a host of cool-season crops are good to sow: everything from salad onions and leeks to lettuces and peas. Some fruit-bearing warm-season favourites like tomatoes can be started indoors too.

Sowing many seeds to a pot saves on potting mix

Essential Seed-Sowing Equipment

Let’s talk equipment. You have three options: sow into pots or flats then transplant the seedlings into bigger pots or plug trays; sow direct into plug trays; or, if they’re really large seeds like zucchini, into individual pots.

Why would you sow into a pot first? It means that germinating your seedlings takes up less space – a small pot could hold dozens of seedlings, which is ideal if you’re starting seeds off on an indoor windowsill. Then once those seedlings appear you can then transfer exactly one of them into each plug. I start off tomatoes, most salads and cabbage family crops in this way because ultimately I want just one of those seedlings in each plug tray or follow-on pot and this allows me to get that.

Sowing into plug trays on the other hand is great for veggies that you want to multisow. Multisowing is when you sow a pinch of seeds into each plug with the intention of planting the seedlings out as a cluster. Crops like beetroot and onions grow really well this way.

Some crops grow well when sown as a cluster

Alternatively, if you only want one plant per plug, sow two or three seeds into each plug then, if more than one germinate, simply snip off the excess to leave one in each.

Plug trays have excellent drainage through big holes at the bottom, which is great for encouraging healthy roots. Most seeds are fine with smaller sizes of plugs, and if the seedlings are going to be planted outside fairly promptly this makes for a far more efficient use of pricey potting mix than a larger plug size would. However, larger plugs are necessary for big seeds that produce chunky seedlings like beans and squashes.

It's worth investing in solidly made plug trays that will last – cheaper ones easily crunch up and split (although you can stack one inside the other to create a double-strength tray). Better, I reckon, to buy a sturdier one which should remain in good condition for years to come. If you’re going to use plastic, at least make sure it’ll last!

Most general-purpose potting mixes are fine for seed sowing

What Kind of Potting Mix For Seed Starting?

The best seed starting mix to use is a topic of hot debate among gardeners! Personally, I start most of my seeds in a general-purpose peat-free potting mix because the results are generally very good. However, sometimes the mix can be quite lumpy with woody bits that get in the way of delicate seedlings struggling to push through to the surface, so if necessary I’d suggest passing your mix through a screen or sieve to leave a fine-textured potting mix. I sometimes mix in some vermiculite to help aerate the soil and retain water.

For really tiny seeds you may want to consider using a seed-starting mix because the particle size of the mix will be similar to that of the seeds themselves.

One way to improve potting mix for seeds of crops that need really good drainage (for instance peppers) is to mix your standard potting mix with an equal amount of coir. The coir is really light and fluffy and will help to open up the potting mix a bit, making it better draining.

Larger seeds that quickly grow into substantial seedlings are best sown into individual pots

How to Sow into Pots and Plug Trays

Fill pots with your potting mix and firm it down. Space the seeds as evenly as possible across the surface. It’s harder to do this with smaller seeds, so just take a pinch and scatter them as evenly as you can across the potting mix. Cover your seeds with a little more mix. For most seeds you dou don’t need much – just enough to cover the seeds so they go out of view.

To sow into plug trays, first fill your tray with potting mix. Don’t be shy about pushing the mix down into the plugs - really load the tray up. Level off the surface, then make depressions for the seeds in each plug with your fingertips.

Don’t forget to label your pots or plugs! Include the date too, so you know roughly when to expect them to pop through.

Grow lights are useful if you don't have a sunny windowsill

Give your seeds a drink, quickly watering across them then allowing the water to drain through before passing over two or three more times. I always use a watering can fitted with a rose and get the water flowing before I begin my pass over the sown trays. Pass over fairly quickly to avoid blasting out the potting mix and unsettling the seeds too much.

Cover your pots or plugs to retain moisture and keep conditions warmer in the pot. You can use a little polythene, held in place with an elastic band, or rest a sheet of glass on top of the pots or plugs. Germinating your seeds indoors, in the warm, will encourage them to pop up a lot quicker. Warm-season crops like tomato seedlings will need to stay indoors for a while before heading out when there’s no chance of frost and temperatures are reliably mild, while cool-season crops like lettuce are safe to go out into a greenhouse or cold frame the moment they’re up – light frosts won’t harm them.

Crops that will be staying indoors for a while need to be grown somewhere with plenty of light. If you don’t have a bright sunny space in your house an alternative is to use growlights, which enable the young seedlings to grow strong and sturdy instead of reaching for the light and becoming leggy.

Handle seedlings carefully by their leaves

How to Transplant Seedlings

The best time to transfer seedlings into individual pots or plugs – what gardeners call ‘pricking out’ – is while they’re still quite small, because it’s a lot easier to feed smaller roots into the holes you make in the plugs or pots.

Start by pre-filling your pots or plugs with all-purpose potting mix. This saves time so the seedlings aren’t left out of their pot for too long – you don’t want their roots to dry out.

Carefully lift out a clump of seedlings using something pointy like a pencil or sharp stick. Try to get under the roots and then, supporting them from below, move them out and carefully place them on the surface you’re working on. Use your stick to tease apart the seedlings (often they will just fall apart by themselves).

Start your seeds off right and they'll soon turn into strong, healthy seedlings!

Pick up a seedling in one hand by a leaf – never the stem, because the plant won’t recover if the stem gets damaged – then make a hole using your stick. Carefully lower the seedling into position, using the stick to guide the roots down into the hole. Firm in around the seedling to close the hole and bury the roots.

Seedlings can go in quite deep, so long as the leaves aren’t covered. If your seedlings have got a bit lanky, set them in fairly deep so they’re supported all the way up the stem.

Give your transplanted seedlings a thorough watering. Don’t worry if they look a bit bedraggled afterwards – they’ll soon recover. At this young stage they’ll shake off any disturbance and settle in really quickly.

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