Perlite is a good medium for sowing seeds, but it favours a particular style of seed and treatment of seedlings.
I started buying perlite in 100 litre bags for my aquaponics, as it’s a great medium for the pots. As I have plenty I started branching out into using it to extend potting mix, and also as a medium for striking cuttings. I was cautious about it as a cutting medium as I had a misconception that it would dry out too rapidly (and coarse sand was working perfectly well for me) but actually it’s very good for cuttings, especially mixed with sand.
I thought about using it for seed sowing after I had picked some seed pods of one of our local flowers, Hovea acutifolia. It’s a lovely small shrub with blue pea flowers in Spring, and I thought I might as well try to spread it along some of our tracks where it already grows sporadically. The seeds are similar to sweet pea seeds, and like sweet peas they germinate better if treated with the seemingly drastic measure of pouring over boiling water and leaving the bowl to cool.
But that’s the thing with them; they have a lot of dormancy to break and so germination tends to be spread out. If I diligently sowed one seed in each tube pot I’d waste a lot of pots, and have to be separating out seedlings as they appeared anyway, for growing-on.
With perlite I just filled a seedling tray (about 30 x 40 cm) with pure perlite, mixed in the hovea seed quite thickly and covered with more perlite, then left it, with regular watering, on my seed raising bench.
The first seedlings appeared in about a week, and when there were a few there I just lifted them out with a wooden skewer and planted them into standard potting mix in tube pots. Then I smoothed over the perlite surface and put the seed raising tray back on the bench. The 50 I lifted out are all growing on well, and it’s time to lift the next lot out.
The advantage of perlite with this method is that it’s very easy to lift the seedlings without damaging them. It also stays remarkably moist (with daily watering) but is both sterile and airy so the chances of seed pathogens getting a hold are reduced.
So, fine seeds (up to say 1 mm) would probably prefer to get their roots straight down into seed raising mix, but for large seeds, perlite is a good option.