Seed Starting Masterclass: My Blueprint For Success Every Time

, written by Benedict Vanheems gb flag

Cucumber seedlings

It’s something most of us can relate to: sowing seeds with keen anticipation, only to be disappointed by patchy germination. It’s the most frustrating experience, right? But why does it happen? And how can we take our seed-sowing game from ‘just OK’ to absolutely outstanding?

Read on or watch our video to discover ten surefire tips that will put you on the right track for seed-sowing success. Get set, sow!

1. The Right Temperature for Sowing

One of the biggest mistakes that many of us make is sowing at the wrong temperature. But by sowing at the right time of year and keeping the potting mix close to the temperature sweet spot for your seeds, you can really improve both germination speed, and success.

The table below shows the range of ideal germination temperatures for some of our most popular crops. You can clearly see the dramatic difference in needs between cool-season vegetables like cabbage, parsnip, peas and spinach, and warm-season or frost-susceptible crops such as most beans, melons, and peppers. By keeping your seeds within the optimum range you can really boost your germination wins!

Table showing ideal sowing temperatures
Keep within the recommended sowing temperatures for each vegetable to maximise germination success

With the optimal temperature range for most cool-season staples beginning from around 45-50ºF (7-10ºC), you can see why delaying sowing in a cold spring makes sense. Better to wait until the soil is consistently above those temperatures than trying to jump the gun. Of course, this is where cloches and horticultural fleece can really help, because they essentially shift the whole growing season forward by as much as two weeks.

Warm-season favourites, on the other hand, need minimum temperatures more like 60-70ºF (16-21ºC). It’s one reason why in a cool or temperate climate we need to sow them indoors or with the aid of some additional warmth such as a heat mat or a propagator.

If you don't have a heat mat or propagator, there are likely to be warm areas in your home that fit the bill. Have a look around – perhaps you could place seeded pots or trays near a radiator or above a fireplace, or maybe near a gas boiler or something like that. Include a minimum-maximum thermometer in your chosen location so you can keep an eye on temperatures and check how close to those optimum temperatures you are.

Soaked vs dry pea seeds
Soaking seeds improves germination rates by softening the seed coat

2. Soaking Seeds

As well as warmth, seeds need air and moisture to germinate. Soaking before planting can soften the tough outer coating that seeds have, speeding up germination by making it easier for that all-important moisture to penetrate. Soaking will also help to wash away the natural germination inhibitors that many seeds have.

Soaking seeds is super straightforward. Take a clean jar or bowl and fill it with warm-to-the-touch water – comfortably warm but not so hot it burns, as that won’t be great for the seeds either! Drop them in then leave for eight to 12 hours – overnight is usually convenient. Make sure the seeds are properly submerged – you can just poke them under, or alternatively wrap them in paper towel first and then pop them into the water. The paper towel soaks up the water, which then weighs the seeds down. Sow your seeds immediately after soaking, don’t delay.

Seeds that benefit from soaking include larger seeds like squash, cucumber, beans and peas, and beetroot. It’s also worth soaking garlic cloves and onion sets to plump them up in preparation for planting.

Chamomile tea
A chamomile tea soak mimics the way chilli seeds are naturally dispersed

Soaking Chilli Pepper Seeds in Chamomile Tea

And then there are chilli pepper seeds. Trawl the chilli chatrooms and you’ll see that chamomile tea comes up time and again! Apparently chamomile tea replicates the guts of birds - yes, really! Wild birds eat the fruits then disperse the seeds via their droppings. Both a bird’s guts and chamomile tea are mildly acidic, which helps to soften up the seed coating ready for germination.

Simply make a mug of chamomile tea and drink it. Then use the same teabag to make a second mug. Allow the tea to cool off until it’s comfortable to the touch, then go ahead and soak your seeds.

Soaking isn’t recommended for all seeds. While it may not do any harm, soaking tiny seeds like lettuce or basil will just cause them to clump together, making sowing fiendishly tricky!

Nicking squash seeds
Breaking down the tough seed coat helps moisture to enter and triggers germination

3. Scarifying Seeds

We don’t condone violence on this channel, but a little targeted roughing-up could be just the what’s needed to persuade your seeds to get a move on!

This is another way to breach that tough seed coat. Try wearing down the seed coat of larger seeds like nasturtium seeds using a nail file, or by rolling the seeds between sheets of sandpaper. This process of breaking down the seed coat has got a pretty cool name: ‘scarification’. In the wild this would naturally happen as seeds bump up against small rocks or other abrasive things in the soil.

With flatter seeds like squash seeds you can just nick the rounded edge of the seed several times using nail clippers to break off some of that woody outer coating. Now, of course, this needs to be done very carefully so you’re only nicking the seed coat and not damaging the interior of the seed. All this does is create an entry point for moisture. You can then soak these seeds prior to sowing them, dramatically hastening germination and reducing the risk of seeds just languishing and potentially rotting away.

For peas and beans, nick the seed case with a sharp knife on opposite side to the little 'eye'. And once you’ve scarified your seeds and soaked them, you’re good to sow – promptly, so they don’t dry out!

Hydrogen peroxide
Hydrogen peroxide can be used as a safe seed treatment

4. Hydrogen Peroxide Seed Soak

An alternative, perhaps unexpected way to boost germination is to soak seeds in hydrogen peroxide. That might sound extreme – none of us like to use nasty chemicals – but hydrogen peroxide is quite benign as you can see from its chemical formula, H2O2, which means it breaks down safely into little more than water and oxygen.

It seems crazy – this stuff is normally used for dying hair or treating wounds – but it’s awesomely effective for treating seeds too!

Mix together two cups (half a litre) of water with a quarter cup (around 60ml) of hydrogen peroxide solution. Make sure what you buy is in the range of about 1-3% concentration, which is normally how it’s sold – you don’t want it any stronger than that. Add your seeds, then leave to soak for just half an hour. Once the time’s up, continue soaking in plain water for up to 12 hours as usual.

To make your hydrogen peroxide go further, pop your seeds and hydrogen peroxide solution into ice cube trays, allocating one compartment to a single type of seeds. Just make a note of what’s in which compartment to avoid confusion!

Potting mix
If your potting mix is dry, rehydrate it before use

5. Keep Potting Mixes Hydrated

I have found that some potting mixes, especially seed-starting mixes, can get dust-dry if the bags they’re in aren’t properly rolled down after each use. Seeds need moisture to germinate, so dried-out mixes need careful preparation before sowing. However, it’s not a good idea to plunge it into a bucket and waterlog the mix, because over-wet soil can cause small seeds to rot.

The simple solution is to pre-moisten dry mixes by spreading out what you need onto a tray and then watering or spraying the mix and massaging all that moisture in until it is consistently moist.

Potting mixes on heat mats or under grow lights can dry out quickly, so check pots and trays every single day – no excuses! – and be disciplined about keeping them moist. If seeds have started to germinate but then can’t get enough water to continue growing, they may well falter and fail – and we don’t want that!

When seeds are tiny, watering with a fine spray works best so they don’t drown or wash away from the surface.

Keep seeds inside propagators to keep the potting mix warm and moist

6. Cover Seeds to Keep Humidity High

Covering seeds with a propagator lid or clear plastic is a great way to retain some of that vital moisture. Make sure to remove the cover once the seedlings have germinated to reduce the risk of disease taking advantage of the humid conditions.

Watering seeds
Give seeds a warm shower to encourage them to sprout

7. Use Warm Water

Water that’s warm – not hot – to the touch won’t shock your seeds like cold water could. Warm-season crops particularly benefit from using warm water because this avoids checking precious progress by suddenly cooling the potting mix with chilly water straight out of the faucet. At the very least, draw your water then leave it to come up to room temperature before using it.

8. Seed Planting Depth

Many seeds benefit from light, which means sowing them at the correct depth so enough light can penetrate the soil or potting mix to reach the seeds beneath. In most cases this just means burying seeds to the equivalent of one seed-diameter’s depth. Some seeds, especially tiny ones, must be sown on the surface of the soil. If in doubt, check the seed packet instructions.

Lettuce seeds
Some seeds need light to germinate

9. Test Seed Viability

Old seeds can give patchy results, especially if they haven’t been stored properly. Check seed packets for packing and sow-by dates, which are often displayed on the back – if you have haven’t accidentally ripped it off when opening the packet! If you’re saving your own seed, bear in mind how long seeds tend to remain good for:

  • 2 years: Leek, onion, parsnip, shallots
  • 3 years: Corn, peppers
  • 4 years: Beans, peas
  • 5 years: Beets, chard, cucumber, eggplant, tomato
  • 6 years: Brassicas, carrot, celeriac, celery, lettuce, pumpkin and squash

If you’re unsure whether seed is good or not, carry out a simple germination test. Sow a test sample of seeds into plug trays or pots, contrasting how many seeds are sown with how many seedlings appear – this way you can work out a rough percentage germination rate.

Or, easier still, lay your seeds onto moistened paper towel, fold it over, then roll it up and place into a sealed plastic bag to retain moisture. Don’t forget to label the bag. Germinate in ideal conditions for that particular seed type, then unravel every few days after the earliest expected germination time to check progress. Re-moisten the paper towel if it starts to dry out.

Once you’re confident that most seeds should have sprouted, work out your germination rate. I like to keep things simple by test-sowing in multiples of 10 – it makes working it all out a lot easier!

Chitted seeds
Chitted seeds can be planted straight away

10. Chit Seeds

Many gardeners like to use the above method to chit (or pre-sprout) seeds that tend to be tricky to get started. This can really improve overall germination rates, giving you tiny, ready-to-go seedlings for planting at their final spacings.

It can be hard to transplant larger seedlings of things like parsnips, but pre-sprouting means you know how many seedlings you’re growing and can plant them straight away without any transplant shock. It also saves having to sow thickly and then thin out.

This is a popular technique for crops including parsnip, chard, coriander, cabbage, and pretty much all warm-season crops like tomatoes, peppers and squash.

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