How to Make the Best Potting Mix for Starting Seeds

It’s that time of year again: seed sowing time! Sowing the first seeds of the growing season is incredibly exciting – just think of all that fresh, tasty homegrown produce to come!

To sow into containers you’ll of course need some seed starting mix, and a good mix can prove expensive. Unless, that is, you make your own.

Seed Starting Potting Mix Recipe

The perfect seed starting mix mustn’t be too high in nutrients, which could harm delicate seedlings. The mix should also hold onto moisture without becoming soggy. Overly wet conditions can rot seeds and encourage fungal diseases such as damping off.

Our seed starting mix uses a soil-less recipe, so it’s beautifully light and fluffy and will promote good, strong growth and happy seedlings.

Begin with two parts compost as your base. All parts are measured by volume, so it doesn’t matter what you use to measure your ingredients, so long as you’re consistent. The compost slowly releases nutrients into the mix, which will help to feed seedlings as they grow. You can use your own garden compost, or buy some in. Break up clumps with your hands or, better still, screen or sieve the compost to get a fine, even texture.

Now add two parts coir (coconut fibre). Coir is extracted from coconut husks, making it a sustainable, plentiful alternative to peat or peat moss. Extracting peat damages fragile ecosystems and contributes to climate change, so we like to avoid using it. If your coir has come in a block, rehydrate it first by soaking it in a bucket with water until you can easily break it apart. If you prefer, you could substitute well-rotted leaf mould in place of the coir. Both coir and leaf mould contribute bulk to the seed mix, and are great for moisture retention.

Finally, add one part perlite, which will both lighten the mix and improve its air content. If you prefer not to use perlite then you could substitute sand, though it will give a heavier mixture.

Use a spade or your hands to mix all of the ingredients together. Take your time and be thorough – you want a consistent mix, with all of the ingredients evenly distributed. Once you’re done, store the starter mix in either a lidded container or in old compost sacks (or any other strong plastic sack) with the top rolled down tightly and secured. Store your mix in a dry, cool place.

Using your Seed Starting Mix

Moisten the seed starting mix before you use it, so it’s damp but not sodden. The mix can be used in plug trays, plastic pots, seed flats, or any recycled containers suitable for seed sowing.

Gently press down your seed starting mix as you fill your container, and take particular care to properly fill at the corners. Top up with more mix if required. Don’t worry, this mix isn’t easily compacted so don’t be shy about firming it down so there’s enough mix for roots to explore.

Sow your seeds according to the packet instructions, then water. Watering requires some care – you don’t want to blast the mix out of the container, so use a mister or a watering can fitted with a very fine rose. Alternatively, make a watering bottle by piercing holes into the cap of a plastic bottle using a pin. Fill with water, screw the cap back on and you’re good to go.

Once the seedlings have germinated it’s best to water from below. Sit your containers in shallow trays of water for a few minutes until you can see the surface of the mix is moist. Remove containers from the water once you’re done so excess water can drain away.

Many seedlings need potting on into larger containers at least once before it’s time to plant them out. Most will be happy potted on into the same seed starting mix but hungrier seedlings like cauliflowers or tomatoes will appreciate something a little richer. Adding some worm compost to the mix like this gives it just the nutritional boost you’re after.

Potting Mix for Containers

Try this simple potting mix for plants to be grown on in larger containers. Thoroughly combine two parts garden compost with one part coir or, better still, leaf mould. Now add some perlite for drainage – about two to three generous handfuls to every 10 gallons (45 litres) of the coir-compost mix. A similar amount of worm compost can also be added for hungry plants, or incorporate a slow-release organic fertiliser according to the manufacturer’s instructions.

Plants grown in the same container for a long time need a potting mix that holds its structure and is buffered against nutrient imbalances. Loam or good quality garden soil offers this. Simply combine one part loam – or screened or sieved garden soil – with one part garden compost then add some slow-release organic fertiliser. And that’s it – a versatile potting soil suitable for many containerised fruit trees, bushes and perennial vegetables.

Making your own seed and potting mixes like this can save you a lot of money. But perhaps most appealing of all is that you can tweak these recipes for what you’re growing. Of course, there are lots of other seed sowing and potting mixes out there, so if you’ve got one that works for you, why not share it in the comments section below?