Increase Germination Rates By Making The Best Seed Starting Mix
Getting good germination is the first-hurdle in the home-grower’s path to a bountiful summer harvest. Buying expensive seeds, only to find they rot in water-logged, mouldy compost is perhaps the most discouraging experience a new gardener can face – it was for me anyway – and over the past few years, I’ve tried several different growing media and created my own home-made blends to try and find the best seed starting mix and maximise seed germination.
Using the correct seed starting mix is the easiest way to improve germination rates. By getting a good balance of water retention, drainage and aeration, you can reduce the chance that seeds rot and maximise the odds that your seeds will sprout, grow strong roots and become strong, healthy plants.
By finding a potting mix or compost specifically designed for starting seeds, you can avoid problems with seeds or roots rotting or suffering due to extremely high nutritional content and also improve germination rates. But I find that we can do one better by blending in some extra ingredients.
In this article and in the video below, I’ll explain what ingredients go into my perfect seed starting mix, what ratios I use so that you can make it at home, and I’ll share the results of a small experiment to demonstrate just how good germination rates are with this mix.
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Choosing The Best Compost For Seeds
Seeds are small, fragile things and their roots and shoots are strongly affected by their environment. When sowing seeds at home, the growing media will have an enormous impact on the health and success of your plant. The ideal growing material needs to:
- have the right nutritional balance so plant growth is not stunted by too much or too little
- retain enough water so the seedling doesn’t dry out but also let go of water so that the roots don’t rot,
- be loose and permeable enough so that fresh air can be obtained from the surface and so that roots can grow through it freely,
- be solid enough for the roots to support and anchor the plant,
- limit bacterial problems from holding stagnant water and fungal issues such as surface mould
- allow microbial life to flourish which the plant can learn to utilise, ready for when it gets planted into the earth.
That’s a lot of competing demands! And the optimal balance varies from plant to plant. No wonder it can be so hard to find a good seed starting material…
Most compost suppliers will have their own mix which they will market as a ‘seed starting compost’. Using a general-purpose outdoor compost for adult plants that is too rich in nutrients and too stodgy can be harmful for seedlings. By swapping to a specifically formulated mix for seed starting, you will instantly get better germination rates and healthier seedlings.
There are quite a few seed composts available but for most plants, including annual vegetables, I find the organic seed & cutting compost from RocketGro* is best. In my experience, it has the highest germination, it’s peat-free and it has reasonable drainage so you can limit issues such as mould and rot if you don’t over-water.
But we can further improve germination, entirely eliminate mould issues and create a growing material that is perfect for a wide-range of plants by adding two more ingredients.
Improve Drainage & Aeration To Get Better Germination
Coconut Coir
When it comes to avoiding water-logging, you can’t beat coconut coir. It is one of the lightest, fluffiest, airiest growing materials available yet is exceptional at retaining water.
Coconut coir is a waste material that is transported in a environmentally-friendly dehydrated state. I spoke to Coco & Coir at a trade show, who claim that the carbon emissions shipping coconut coir from the Indian Ocean were lower than transporting a bag of compost on a lorry within the UK.
This also means that coconut coir can be a relatively cheap material to use* to germinate your seeds in and it can be used to bulk out a more expensive bag of compost.
If you wanted to only germinate seeds then you would be hard-pushed to find a better material. However the soil microbiology and nutrients are not present so if you were to use this to start seeds then, very shortly after germinating, you would need to transplant your seedlings using a different material. On the flip-side, mould and bacterial issues are uncommon as it is naturally sterile.
By adding coconut coir to a seed compost you can improve drainage and airflow around the roots, reduce the change of damping off or seeds rotting and make it easier for shoots to reach the surface. All while bulking out your seed starting mix and reducing the cost of growing food at home.
Perlite
You can further improve drainage by adding perlite, a volcanic glass which has been processed into small white pebbles. Perlite is great for opening up a soil and making it very difficult to compact, increasing air flow and drainage. As a result, roots grown in a perlite mixture can grow more freely, absorb more oxygen and have less risk of root rot or damping-off. You’ll also have less risk of seeds rotting too.
Perlite is an inorganic material so it doesn’t break down in the soil. It also needs to be mined (it’s literally a rock) and then requires heat treatment so it can expand into its usable form. It is perhaps the opposite of coir when it comes to its environmental credentials and its cost per gram.
I add perlite to my mix* to improve germination performance and help my seedlings grow but you could use an alternative such as horticultural sand or grit, or leave it out all-together or use more coir.
Creating The Best Seed Starting Mix
To make approximately 1 kg (2.2 lbs) my home-made and what I believe to be the best seed starting mix, I use the following ratios:
| Soil Component | Volume (heaped tbsp) | Weight (g) |
| Compost | 20 | 657 |
| Coco Coir | 20 | 303 |
| Perlite | 20 | 52 |
When making the mix I normally aim for an equal ratio by volume by identifying a level on a mixing bowl which, it turns out, was pretty close and is roughly equal numbers of heaped tablespoons for each component. However, for this article, I measured them out so that you can make the mix at home.
For the weights in this table, I measured the cumulative weight of the mixing bowl as each component was added and subtracted the previous measure to determine the weight of each component. This is the dry weight of perlite and the wet (but not water-logged) weights of compost and coir.
To make the mix you will need to add boiling water to your coconut coir brick and then let it cool down before mixing. I use a 1.7L kettle with 650g coco coir brick and find that about two thirds of the coir is wet and ready to use while the top third is still dry and can be pulled off and saved for later.
When the coir cools, add it to the mixing bowl and then heap the compost into the bowl. Then mix until blended. Then add the spoons of perlite and fully combine the mixture. Perlite is very dusty so be sure to work in a ventilated space and consider using a mask and eye protection. Alternatively you can wet the perlite before pouring it (although you will need to adjust the weights).
When you’ve finished making the mix, it should feel a bit like the sponge in a cake might if it were completely shredded up – somewhere between sponge and a crumble.
You’ll then have a seed starting mix which can be used for almost any seed, retains water but allows good drainage and airflow, enabling strong root growth, allowing the shoots to break to the surface, that limits the risk of root rot and has very high germination rates. You can add this to your pots, seed trays and cells – it should be really easy to spoon in or scoop up – and you can start sowing seeds.
However, to avoid surface-mould growth, be sure to cap the container with a layer of coconut coir. Doing this has the added advantage of creating a colour change when it the soil begins to dry out, which provides a clear and fool-proof signal that it is time to water your seedlings.
Testing Germination Rates
While this seed starting mix has never let me down and was a complete game-changer for my back-garden veg patch, anecdotal claims like this (especially accompanied by phrases like ‘game-changer’) should rightly raise some eye-brows and scepticism. Therefore I decided to test my mixture in a small experiment to quantify just how good this mixture is for sowing seeds.
There are a few caveats such as the sample size being so small (meaning a single failure creates an 8% rate drop) and this being a single set of growing conditions and using only one batch of (old) seeds each but, nonetheless, my results are below:
| Seed | Number Germinated | % Germinated |
| Tomato | 11 | 92% |
| Chilli Pepper | 12 | 100% |
| Sweetcorn | 12 | 100% |
| Lettuce | 12 | 100% |
| Cabbage | 12 | 100% |
| Peas | 12 | 100% |
A seed starting mix with only one failed seed is pretty good performance. Of course, the true failure rate can only be determined at a much larger sample size and will likely be seed dependent (I used Roma tomato seeds here, which I suspect have a lower rate anyway), but I’m pretty happy with these results in this sow-and-forget approach.
Other Ways Of Improving Germination Rates
Using the best seed starting mix is one way of improving germination rates. Controlling humidity at the appropriate level to enable the seed to take-up moisture, placing the seeds in an environment at the right temperature and considering their light requirements can also improve germination rates.
Using newer seeds that have been stored in dark, cool, low humidity environments and haven’t had much time to degrade on the shelf will also improve your chances – although old seed can still be viable and shouldn’t be discounted until the approaches above have been utilised.
To speed up germination you should ensure that seeds are germinating in a warm, moist environment and could consider pre-soaking (priming) before sowing to help the seed take on more water faster and to speed up germination.
And, of course, to maximise germination rates you’ll also need to make sure that you’ve nailed your sowing technique for whatever seed you are trying to germinate, including using the correct seed planting depth and orientation.
What To Do Once Your Seeds Have Germinated….
Of course, once your seeds have germinated, you’ll need to provide your seedlings with enough light to stop them from going leggy. It turns out that this is also dependent on the variety and in this study I performed on tomato seedlings, it turns out cherry tomatoes need less light than other varieties.
And, as always, Happy Gardening!
