Brunnings Seed Raising Mix, and why seeds fail to sprout

Why seeds sit there doing nothing

You fill a tray, you put the seed in, you water it and you wait, and then you keep waiting, and after a fortnight of nothing you start wondering whether you've got a black thumb. Nine times out of ten it isn't you at all. Seed is fussy about a very small number of things and most of them come down to what it's sitting in.

The usual reasons, in the order they actually happen

• Sown too deep. A tiny seed buried 2cm down runs out of stored energy before it reaches the light and dies underground where you can't see it happen.
• Dried out at the wrong moment. Seed'll sit dry for years quite happily, but once it takes up water and starts moving, a single hot afternoon of drying out will finish it.
• Too cold. Germination follows soil temperature rather than air temperature, and a tray sitting on a cold concrete floor is colder than the room it's in.
• Old seed, or seed stored somewhere warm and humid. The packet has a date on it for a reason.
• Damping off, where the seedling comes up and then keels over at soil level a few days later. That one is a fungal problem and it gets its own section below.

What a seed raising mix actually is

Brunnings Seed Raising Mix is milled finer than a potting mix and it's deliberately low in nutrient. That second part is the bit that surprises people, because it sounds like the mix isn't doing as much. A seed's carrying its own food supply already and it doesn't need feeding until the first true leaves appear, so a mix loaded with fertiliser'll burn the fine new roots before they've got anywhere.

The fine texture matters too. A seedling root is hair thin and it's got to find its way between the particles, so a coarse mix full of bark chunks leaves air gaps that the root bridges badly, and the seedling stalls because there isn't much holding it.

Why the bag of potting mix in the shed doesn't do the job

An ordinary potting mix is built for an established plant in a pot, so it's coarser than seed wants and it carries a controlled release fertiliser on top of that. Put seed in it and you get both failure modes at once, because the mix holds more water than a germinating seed can cope with while the fertiliser sits against roots that are far too young for it. You can absolutely raise seed in potting mix and plenty of people do, but your strike rate drops and you'll never be quite sure why.

How to sow so it comes up

Fill the tray and tap it down lightly rather than pressing it hard, because compacted mix sheds water off the top instead of taking it in. Sow at roughly twice the width of the seed and no deeper, which for something like lettuce or basil means barely covering it at all. Water from below if you can, by standing the tray in a shallow dish until the surface goes dark, since a watering can will wash fine seed into a corner.

Keep the mix damp the whole way through rather than wet, and cover the tray to hold the humidity until you see green. Then take the cover straight off. Leaving the cover on after they're up is the single commonest way people give themselves a damping off problem.

A light dusting of vermiculite over the top of a fine seed is worth doing. It holds moisture right at the surface where a germinating seed needs it, and it stops the surface crusting over into a lid the seedling has to push through. It also lets light down to the seed that wants light to germinate.

Damping off, the one that actually kills seedlings

Damping off's a soil fungus and it shows up as a seedling that germinated perfectly well and then collapsed with a pinched, watery stem at soil level. Once a tray goes it usually goes quickly. The things that keep it away are all about air and water rather than any product: clean trays, mix out of a fresh bag rather than the open one in the shed, water in the morning so the surface dries by evening, thin the seedlings so they aren't touching, and get some air moving over them.

Moving them on

Once a seedling's got its first pair of true leaves, which are the ones that look like the plant rather than the two rounded ones it came up with, it's used up what the seed gave it and now it wants feeding. That's the moment to prick it out into potting mix. Handle it by a leaf rather than the stem, because a crushed leaf is survivable and a crushed stem is not.

One safety note that belongs on every bag

Potting and seed raising mixes carry a warning about Legionella for a good reason, and the risk is in the dust rather than the mix itself. Dampen the bag before you open it properly, keep your face out of it, wear a dust mask if you're doing a lot, and wash your hands afterwards. It takes 10 seconds and it's the only genuinely hazardous part of raising seed.

Shop related products

• Brunnings Seed Raising Mix 5L. Milled fine and low in nutrient, which is exactly what seed wants.
• Brunnings Vermiculite 5L. For dusting over fine seed and keeping the surface from crusting.
• Brunnings Premium Potting Mix 36L. Where the seedlings go once the true leaves are out.
• Brunnings Propagating Sand 5L. For anything that wants sharper drainage than the mix gives.

Did you know we also stock seed raising mix in 6 litre and 15 litre bags in the shop, plus a 30 litre block if you're doing a lot of trays. Those larger sizes aren't all on the website, so give us a ring on (08) 8536 4333 to check what's on the shelf, or have a look through the garden centre next time you're in.

Read next

• Seeds or seedlings, which should you start your garden with
• Choosing a potting mix, and what the ticks on the bag mean
• Brunnings Vermiculite, what it does and when to use it

I still sow more seed than I need, on the basis that something always goes wrong somewhere in the tray and I would rather be thinning than starting again.

Cheers,
Candeece

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