Top tips for successful germination

Top tips for successful germination

Late winter is when gardeners are thinking about the early sowing of certain seeds for the spring and summer. In this article we’ll take a look at tips for germinating a range of popular crops. Some herbs, some salad, and a couple of other favourites.

Firstly, it’s important to mention that all seeds should be fresh – the fresher the seeds, the higher the germination rate. Avoid old seeds.

Successful germination depends on a number of other key factors, including:

  • Warmth
  • Moisture
  • Light
  • Planting technique

What actually happens during germination?

It can be fun to watch the different stages of germination by planting in a clear container. Put some seeds down the side.

Roots prefer the dark, though, so once you have watched it for a while, transplant so that the roots don’t get any more light.

For those of you interested in the science, here’s a quick overview of what happens during seed germination:

Imbibition – The seed takes up water from the surrounding soil through a process called imbibition. This causes the seed to swell and rehydrates the embryo.

Metabolic activation – As water is absorbed, dormant enzymes within the seed are activated. This kickstarts metabolic processes like respiration and ATP production. This stage and the next is where additional warmth helps to help the enzymes work faster.

Mobilisation of reserves – Stored food reserves like starch and proteins are broken down by enzymes to provide energy and materials for growth.

Weakening of seed coat – As the embryo enlarges with water uptake, it pushes against the seed coat until it ruptures, allowing the radicle/root to emerge. Some hard-cased seeds can benefit from a little pre-soaking before you sow them.

Root emergence – The primary root or radicle is the first part of the seedling to emerge from the seed coat. It grows downward into the soil, anchoring the seedling. If your seed is on the surface, you might also see tiny white hairs, these are root hairs and are good! It’s not mould. They play an important role in helping the plant take up water and nutrients from the soil. Lettuce seeds sown on the very surface of the soil will often show root hairs.

If the seed happens to be sown upside down, nature works its magic and the root heads off downwards, in the right direction. Clever stuff. 

Shoot emergence – Next, the shoot emerges from the seed coat and grows upward. The plumule will develop into stems and leaves.

An exception to this is beans (and similar); where the root continues to grow for a bit longer, pushing the whole seed up above the surface. The leaves then emerge from the seed.

Early seedling growth – The young seedling continues cell division and elongation, producing new roots, stems, and leaves. It transitions from using stored reserves to generating its own energy via photosynthesis. If you’re growing with hydroponics (perhaps in an H-mini or the Harvst indoor garden) then this is the point at which it will need nutrients. If you’re growing in natural soil, make sure it’s rich in nutrients too.

So in summary, germination involves rehydration, growth resumption, reserve mobilisation, seed coat shedding, embryo emergence, and the establishment of the seedling. It marks the transition from a dormant seed to an actively growing plant. 

Now you know!

Do I need grow lights for germination?

Adding grow lights during the plant’s early life can significantly boost growth, and it will help prevent small seedlings from reaching too tall for the light. If they grow too tall too soon, they will become thin, weak and “leggy”. Lots of light on early leaves will pay dividends later.

S16 smart mini greenhouse - probably the biggest and best propagator you can get.

The S16 climate controlled mini greenhouse comes with LED grow lights, heating, and fan-driven ventilation, giving your young plants everything they need to survive their first few days and weeks of life.

Also available in a smaller model, the S8 mini greenhouse / propagator.

As for whether you should have additional LED grow lights on for germination – before emergence of the first shoots – different plants like different things. Some seeds need light to germinate, and some prefer the dark. Research the needs of your particular plant. If you’re not sure, have any grow lights off until the seedlings first appear. Too much light on damp, humid soil can lead to mould and fungus.

What temperature is best for germination?

Warmth is one of the environmental conditions that triggers germination of a seed. Different seeds prefer different temperatures, but right now it’s still far too cold outside for most plants to even start thinking about popping up. To get going early, you’ll need to use some kind of nursery or propagator. 

A propagator will help keep your seeds and seedlings warmer, will keep moisture in, and sometimes will provide additional light for when the first leaves appear. The simplest propagators are simply a cover, trapping external warmth via the greenhouse effect – the classic “sunny windowsill”. At the other end of the scale, premium propagators have built-in heating and grow lights.

If you’re propagating or sowing in a warm house, you might get away without additional heat, but at this time of year, most situations will need some kind of heating to reach the temperatures that seeds need to germinate (above 15C, and sometimes over 25C for things like tomatoes and peppers). You’ll definitely need additional heat if you’re sowing in a greenhouse or polytunnel.

Automatic temperature control is an important consideration – if the heater is simply “on” without any regulation, on a sunny day where there is additional heat, the seeds can get too warm very quickly, which can lead to the germination process stopping (no seedlings!), or the soil drying out too quickly (again, no seedlings!). Try and find a heated propagator with temperature control.

Here we have a temperature controlled heated seed tray in a Harvst H-mini indoor grow system, which works perfectly as a propagator.

Air movement and moisture control

All seedlings will benefit from air movement to prevent damping off disease (read more about damping off on the RHS site here). Any propagator dome used over a seed tray should be opened once the seedlings appear. If you’re using a Harvst H-series indoor garden, you can fit fans specifically for giving good air circulation.

Again, for all seeds – make sure the soil does not dry out too much. Cover the seed tray, usually with a clear cover, since many seeds need light to germinate. Once the seeds have sprouted, allow the soil surface to dry slightly between waterings to encourage healthy root growth downward. If the top of the soil is damp but the bottom is dry, roots won’t develop as well as they should. Bottom watering is best.

Since seeds are often germinated in small cells, thin soil, or even coir pucks; there’s not a lot there to hold moisture for very long. If it’s too warm, or you are germinating in an overly dry environment, you could find that your seedlings are dead within a day or two of no watering. Keep an eye on moisture levels daily, if you can.

This is another good reason for a temperature controlled propagator; it won’t keep heating the plants and drying the soil out too fast, if there’s already warmth in the sun or the surrounding air.

If the seeds have a hard outer coat (like spinach, beans, coriander, squash and even carrot), then soaking the seeds for a while before planting can trigger germination.

Here we have some coriander seeds that were still a little tough and have not all shed off the young leaves. The leaves need to open to get the light.

Planting tips

A general note on sowing depth … a seed has a certain amount of energy stored in it, and when that runs out, photosynthesis needs to have started. If the seed’s nutrients run out before the leaves get light, the seedling will die. Smaller seeds should be planted shallower (and in many cases, like some lettuce, can be sprinkled directly on the surface).

Coriander

  • Soak before sowing – Soak coriander seeds in room temperature water for 12-24 hours before sowing. This helps soften the outer seed coating.
  • Sow in fertile soil – coriander needs nutrient-rich soil to germinate well. Mix soil with compost or fertiliser before planting; we provide a liquid nutrient, or you can use a fertiliser/coir mix such as CoCo Grow+.
  • Plant shallowly – sow seeds no more than 5mm (1/4 inch) deep in the soil. Coriander seeds need light to germinate.
  • Keep moist – Water soil regularly to keep moist, but not saturated. 
  • When the seeds pop up, provide plenty of light. Coriander needs at least 6 hours of direct sunlight per day. Find a sunny spot or use grow lights.
  • Optimal soil temperature for germination is on the warm side compared to many other seeds; 20-25°C. 
  • Be patient. It can take 14-21 days for coriander seeds to germinate. Don’t give up too soon!

Basil

Basil is a favourite at any time of year; easy, plentiful and so, so much better when it’s fresh off the plant.

  • Soak seeds for a few hours before planting.
  • Plant shallow. Basil seeds should be sown just a few mm (1/8 inch) deep in the soil. They need light to germinate.
  • Basil germinates best at warmer temperatures; 20-25°C is good.
  • Once sprouted, basil needs at least 6 hours of daily sunlight or strong grow lights.
  • Allow 10-20 days for basil seeds to germinate. Wait 2-3 weeks before giving up.

Spinach

  • Before planting, try pre-soaking the seeds in room temperature water for 12-24 hours.
  • Sow the seeds no more than 5mm (1/4 inch) deep in fertile, well-draining soil. The key here is “well draining” – spinach needs oxygen to germinate so the soil must not be too wet.
  • Spinach thrives better in cool temperatures so aim for no more than 20°C on your heat mat or propagator. This will usually need a temperature controlled propagator, since non-regulated ones just keep the heat on regardless of ambient temperature.
  • Allow 7-14 days to germinate.

Lettuce (Cos)

There are so many types of lettuce, each with their own preferences for germination conditions. We love growing cos as it’s a great cut-and-come again salad leaf, and does well indoors in a hydroponics system.

  • Sow shallow. Often, it’s possible to germinate lettuce seeds just by placing them on the surface of a wet substance (e.g. soil or coir), and keeping them moist with a humidity dome or regular spraying.
  • Maintain cooler temperatures than some of the other seeds in this article – Lettuce germinates best at between 15-20°C. Avoid warm environments.
  • Lettuce seeds require light to germinate. Do not cover seeded beds.
  • Lettuce takes around 7-10 days to germinate, though with excellent conditions (an H-series nursery) we regularly have strong seedlings in well under 7 days.

Bell pepper

Slower growing plants like peppers need to be sown right at the beginning of the year. Get going early!

  • Pepper seeds should be planted a few mm (1/4 inch) deep.
  • Maintain warm temperature – Peppers germinate best between 20-30°C. Most people will need extra heating to get pepper to germinate well.
  • Allow good light exposure – Once sprouted, peppers require at least 6-8 hours of direct sunlight daily.
  • Pepper seeds can take 7-21 days to germinate.

Tomatoes

Who doesn’t love fresh tomatoes straight off the vine? Yum.

  • Tomato seeds should be planted a few mm (1/4 inch) deep.
  • Like peppers, tomatoes need warm temperatures to germinate. Crank the heat mat or propagator right up and aim for 25-30°C.
  • Keep soil evenly moist but not saturated. Alternate between watering and allowing the surface to dry out – but not too dry for too long otherwise the seedlings will die.
  • As soon as the seedlings appear, provide 14-16 hours of light daily. This is going to mean grow lights. 
  • Thin the seedlings to the healthiest one per cell/container once the first true leaves appear.

That’s an overview of just a few seeds you might like to start now. Enjoy your planting, and bringing your little babies to life!

Have a look at our propagation options

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    S16 Smart Mini Greenhouse

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    S8 Smart Mini Greenhouse

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    Premium propagator / cloche with LED grow lights

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    Smart temperature controlled heated seed tray

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    Propagator cover, high dome

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