A bag of commercial seed raising mix runs about $8 to $12 at the garden centre. I make roughly the same volume for the price of a cuppa, and mine works better because I mixed it for my own conditions. Here’s how I do it.
What you need
Three things, and you can find all of them cheap or free.
- Coir (coconut fibre) for the bulk. A compressed brick costs a few dollars and swells up to about 8 to 10 litres once you soak it. This holds moisture without going claggy.
- Fine compost or worm castings for a little food. A couple of handfuls per batch. If you’ve got a worm farm or a compost bin, this part is free. A worm castings tea brew uses the same stuff if you want a liquid feed later.
- Coarse sand or perlite for drainage. A small bag lasts ages. Sand is cheaper, perlite is lighter.
That’s the lot. Total cost for a decent batch sits well under what one shop bag would set you back, and you’ll have leftovers for the next round.
You don’t need fertiliser pellets, fancy additives, or anything with a brand on it. Seeds carry their own food for the first couple of weeks. All the mix has to do is hold them upright, stay damp, and drain.
The method
I mix mine in an old bucket. Takes ten minutes.
- Soak the coir brick in warm water until it breaks apart and fluffs up. Give it half an hour. One brick is plenty for a few seed trays.
- Measure roughly two parts coir, one part compost, one part sand. I don’t weigh it. A scoop is a scoop. Close enough is good enough here.
- Mix it through with your hands until the colour is even and there are no dry clumps. Break up any lumps of compost as you go.
- Check the damp. Squeeze a handful. It should hold together and feel moist, but no water should run out. Too wet and your seeds rot. Add dry coir if it’s sodden.
- Sieve it if you’re fussy. For tiny seeds like lettuce or basil, I push the mix through an old kitchen sieve so the texture is fine. For big seeds like beans, don’t bother.
Fill your trays, firm it down gently, and you’re ready to sow.
The one or two things people get wrong
Two mistakes sink most home batches.
First, too much compost. People think more food means stronger seedlings. It doesn’t. A rich mix grows mould and damping off fungus, and your seedlings keel over at the stem. Keep the compost to a quarter of the mix at most. Lean is the goal.
Second, mix that’s too coarse or too dry. If the texture is chunky, small seeds fall into the gaps and never surface. If it’s bone dry when you sow, the seed sits there and does nothing. Damp and fine, every time.
Adjusting for your patch
Where you garden changes the recipe a touch. The climate zone growing guides sort this by your zone if you want the detail, and the what to plant now calendar tells you which seeds are worth raising this month before you mix a batch.
Up in the tropics and the subtropics, compost breaks down fast and mixes stay wetter, so I’d lean harder on the sand or perlite for drainage. Maybe go one and a half parts coir instead of two. Damping off loves humidity, so keep it lean.
Down in the cool temperate and cold zones, Tasmania, the tablelands, the southern states, evaporation is slower and the risk is a soggy tray that never warms up. Same fix, more grit, and start your trays somewhere warm. A sunny windowsill does the job.
Arid and semi arid growers have the opposite problem. Your mix dries out by lunchtime. A bit more coir helps it hold water through a hot afternoon, and a cover over the tray keeps the moisture in.
One brick of coir and a scoop of your own compost will raise a whole season of seedlings for a few dollars. Once you’ve mixed your own a couple of times you’ll stop buying bags altogether, and you’ll start saving that compost for it. If you want to watch what those seedlings would have cost you at the till, the price tracker keeps a running tally crop by crop. The only hard part is remembering you made too much and finding trays for the rest.
