What to Plant Seeds In (And What to Avoid)
The growing medium is the one thing most beginner guides skip over — and it's often why good seeds fail before they ever have a chance.
When germination doesn't happen, it's tempting to blame the seed. But the seed is usually fine. The issue is more often what it was planted into. Choose the wrong medium and seeds rot quietly, dry out too fast, or suffocate before anything shows above the surface.
This guide covers every common option — what works, what doesn't, and why it matters more than most people expect.
What a Good Seed-Starting Medium Actually Does
Not all growing media are equal — and the differences matter most right at the start, when seeds and new roots are at their most vulnerable.
A good seed-starting medium does three things: it drains freely so water doesn't pool around the seed, it holds just enough moisture to keep things consistently damp, and it keeps air available around the seed and developing roots — because roots need oxygen just as much as they need water.
Most standard potting products don't meet all three criteria. That's not a flaw in them — they're designed for established plants at a different stage of growth. But it does mean that grabbing the nearest bag without checking what it's actually formulated for is one of the most common reasons seeds don't germinate.
Which Growing Media Work for Seeds? A Quick Reference
Here's an honest overview of the most common options:
|
Growing Medium |
Good for Seeds? |
Why |
|---|---|---|
|
Seed raising mix |
✅ Best option |
Fine texture, low nutrients, free-draining — made for this job |
|
Coir pellets / blocks |
✅ Excellent |
Great moisture balance, biodegradable, easy to transplant from |
|
Potting mix |
⚠️ Not ideal |
Too nutrient-rich — can burn delicate first roots |
|
Garden soil |
❌ Avoid |
Compacts badly in containers, drains poorly, inconsistent results |
|
DIY mixes |
⚠️ Inconsistent |
Can work but results vary — adds unnecessary risk for beginners |
The short version: seed raising mix is purpose-built for germination and is the best starting point for most beginners. Everything else involves a trade-off.
Seed Raising Mix vs Potting Mix: What's the Difference?
These two products look similar on the shelf, and they're often confused — but they're formulated for very different purposes.
Seed raising mix is designed specifically for germination. It has a fine, consistent texture that makes good contact with seeds, a low nutrient level (because seeds carry their own food supply and don't need fertiliser at this stage), and a structure that drains freely while holding just enough moisture.
Potting mix is designed for established plants. It holds more moisture, contains higher fertiliser levels, and often has a coarser texture. All of that is exactly right for a tomato plant growing in a pot. For a seed just starting out, it's too much.
Why potting mix can damage seeds
Seeds have their own built-in food supply — enough energy to power germination and the very first days of growth. They don't need nutrients yet. What they need is a gentle, low-nutrient environment where delicate first roots can develop without stress.
The higher nutrient levels in potting mix can burn those first roots before they've had a chance to establish. It also retains more moisture than seeds need, which increases the risk of rot. Using potting mix for seeds doesn't guarantee failure — but it introduces a risk that's completely avoidable.
|
💡 If potting mix is all you have Mix it with perlite at roughly one part perlite to two parts potting mix. This improves drainage and dilutes the nutrient concentration. It's not as good as seed raising mix, but it's a workable substitute in a pinch. |
Why Garden Soil Doesn't Work in Containers
Garden soil might seem like the obvious choice — it's free, it's available, and plants clearly grow in it. But it's one of the worst options for starting seeds in pots or trays.
The problem is structure. Garden soil compacts when watered repeatedly in a small container. Once compacted, it drains poorly, holds too much moisture, and leaves almost no air space around seeds and roots. It can also introduce pathogens that aren't an issue in the open ground but cause problems in the enclosed environment of a seed tray.
Leave garden soil in the garden. For seeds in containers, it simply doesn't perform.
Coir Pellets and Blocks: A Reliable Alternative
Coir — compressed coconut fibre — has become increasingly popular as an alternative to traditional seed raising mix, and it earns that reputation. It holds moisture well without becoming waterlogged, has a naturally fine and consistent texture, and is completely biodegradable.
Coir pellets are small compressed discs that expand when soaked in water, each becoming a self-contained seed-starting pod. Seedlings can be transplanted directly into the ground without disturbing the roots — you plant the whole thing. Coir blocks work the same way at a larger scale.
Both are clean to handle, easy to use, and widely available in Australian garden centres. For beginners especially, coir pellets take a lot of the guesswork out of the process.
|
💡 One thing to know about coir Coir is very low in nutrients — which is ideal for germination. But once seedlings develop their first true leaves, they'll need feeding. This is expected and normal. Just keep it in mind as seedlings grow on. |
What to Look for When Buying Seed Raising Mix
Most branded seed raising mixes from reputable garden centres perform well. In Australia, products from Yates, Searles, and Richgro are widely available and reliable starting points.
What to check on the label: fine or uniform texture, low nutrient content (avoid anything with added slow-release fertiliser), and good drainage. If it's marketed primarily as a potting mix or garden soil — even if it says 'suitable for seeds' somewhere on the packaging — look for something more specific.
Price isn't a reliable guide to quality here. A mid-range seed raising mix from Bunnings or a local nursery will outperform an expensive potting mix for this specific job every time.
Can You Reuse Seed Raising Mix?
Sometimes — but with care. Seed raising mix that's been used once will have a different structure than fresh mix. Repeated watering compresses the particles over time, which affects drainage and airflow. It may also carry residual pathogens from previous plants.
If you do reuse it, refresh it first by mixing in a small amount of fresh seed raising mix or perlite to open the structure up again. Only reuse mix that came from healthy plants — don't reuse anything from a tray where seedlings damped off or showed signs of disease.
For best results — especially when starting out — fresh seed raising mix is worth the small extra cost. It removes one variable and gives seeds the cleanest possible start.
How to Fill a Container for Seeds
Seeds don't need depth — they need contact with a moist, stable medium. For seed trays or punnets, fill to within about 1 cm of the rim. This leaves a small lip that helps with watering, giving water somewhere to go before it soaks in.
For deeper containers, you don't need to fill the whole thing with seed raising mix. A 5–8 cm layer on top of a base of regular potting mix works well and keeps costs down. What matters is what's around the seed — not what's at the bottom of the pot.
One thing to avoid: don't pack or compress the mix down firmly before sowing. Seeds need air in the medium. Fill the container, tap it gently on the bench to settle things, and leave it at that.
Common Questions
Is seed raising mix necessary, or can I use something else?
Seed raising mix is the best option for germination, but it's not strictly necessary. Coir pellets or blocks work just as well. Potting mix can work if you mix it with perlite to improve drainage and reduce nutrient levels. What you want to avoid is standard garden soil in containers — it compacts too badly to give seeds a reliable start.
What's the difference between seed raising mix and potting mix?
Seed raising mix has a finer texture, lower nutrient content, and better drainage — all specifically designed to support germination. Potting mix is richer and better suited to established plants growing in pots. Using potting mix for seeds introduces a risk of root burn and excess moisture, both of which can prevent germination or cause early seedling loss.
Can I make my own seed starting mix?
Yes. A simple DIY mix: one part coir, one part perlite, and one part fine compost or worm castings. This produces a free-draining, low-nutrient medium that works well for most seeds. The main risk is inconsistency — if in doubt, a commercial seed raising mix takes the guesswork out of it.
Do seeds need fertiliser added to the growing medium?
No — not at germination stage. Seeds carry their own food supply. Adding fertiliser at this point risks burning delicate first roots rather than helping growth. Wait until seedlings have at least one or two sets of true leaves, then introduce a diluted liquid fertiliser at roughly quarter strength.
How do I know if my growing medium is draining properly?
Water it thoroughly and watch what happens. Water should begin draining from the base within a few seconds. If it pools on the surface and takes a long time to absorb, the mix is too dense — add perlite to open the structure. If it runs straight through and the mix dries out within hours, it may need a little more coir or moisture-retaining material to balance it out.
Next Step: Choosing a Container
Once the growing medium is sorted, the next decision is what to put it in. Containers matter more than most beginners expect — specifically drainage and size.
Choosing the Right Container for Planting Seeds
For a complete guide to growing from seed — from sowing to transplanting — start here:
Growing from Seed: A Complete Beginner's Guide
