Seed Germination - Role of Compost / Perlite / Vermiculite

For the first time I'm having a go at germinating some more challenging seeds. I have bought about 60 different packets on eBay. All come with helpful (if somewhat conflicting!) advice.

Many say to sow (and often to stratify) in "Good quality compost, Perlite or vermiculite"

Are they really just interchangeable?

I'd appreciate anyone's views on what the relative merits of the materials are.

I've only ever used Perlite to "lighten up" compost for cuttings, and I only ever cover seeds with Vermiculite (as it seems to provide a good covering, reduce the green algae growth, and I like to think its so light the weediest of seedlings can push through).

When I worked on a nursery raising Gypsophila we rooted the cuttings in trays of Perlite. Don't think I ever say a cutting fail! (can't remember exactly, but we probably did Chrysanthemums and other things in just-Perlite too).

Sorry .. .waffling again ... so that's the difference / benefits of them please?

 

Kristen - I don't really know the answer to your question. Its something I would like to know more about.

I grow lots of different seeds - some being difficult ones. Being close to the RHS library at Harlow Carr, I have read a number of books and articles on seeds, and whilst I can't remember exact details my feelings are that in general sharp sand, Perlite and Vermiculite are often interchangeable.

For seeds and cuttings I use multipurpose compost with sharp sand added. The percentage of sharp sand being 30% to 50%. I seem to remember in one book on seeds, the American University doing the research used exactly the same - so I haven't felt obliged to change.

Whilst I have a large bag of Perlite, I don't often use it. The reason is simply that sharp sand is so easy to get hold of and reasonably cheap whilst Perlite and Vermiculite are not.

I spray seeds regularly with Chesthunt (copper) compound. I once bought some really bad multipurpose compost that wouldn't drain, and I lost several batches of seeds inspite of adding sharp sand. The compost was very clearly bad as water just sat on top of it and wouldn't drain away at all. But other than that one occasion I have never had any problems. I always use cheap compost and sharp sand, and have never had any seedlings that damped off.

Germination, of course, is another matter. I find that 5% to 10% of my seed batches never germinate - but I suspect (indeed I know - from trying to germinate them in a polybag, and talking to other people who have had the same seeds from the same place) that many of those were dead in the packet. Sometimes I am too hasty. Before now, I have put seeds that wouldn't germinate outside and found that they germinated a year later. Beth Chatto said she never gave up on seeds for two years. My worst seed this year was Isoplexis sceptrum. The seed was so fine you could barely see it, but after 71 days it finally started to germinate.

 

As far as I am aware, Perlite and Vermiculite achieve the same end. It`s just that one ( perlite ) is manmade, ie, polystyrene and Vermiculite is a silica based product, ie, organic.:gnthb: BUT I`ve been wrong before.lol lol lol

 

John, cuttings don`t need nutrients to root. The exact opposite applies actually.:gnthb:

 

Mined though Dai? thus non-renewable (and there were some fairly ghastly stories about the miners suffering as there was frequently asbestos in the mined vermiculite IIRC - although that was a long time ago).

Thanks all, I'm surprised that Perlite and Vermiculite are described (not just by you guys) as being interchangeable. If I mix water with them and Perlite holds some water, but Vermiculite is sodden.

For me Perlite is cheaper than Sharp Sand I think?

A large bag of vermiculite (70L or so I expect) costs a fiver, and weighs nothing. Whereas I wouldn't be able to lift a bag of sand that big ...

Perhaps I should get a cu.m. bag of Sharp Sand delivered. Its always useful.

Right, well thanks for you input, I shall continue to mix 30% Perlite with multi-purpose compost for the majority of the cuttings I do. If I have sharp sand I'll use that instead - I agree, its what I have always seen recommended.

I tend to also throw in some soil-based compost too - typically some Seed Compost if I have a "spent" seed tray lying around. (Not sterilise, obviously, but I can't say I have ever had a problem with once-used seed compost - indeed, on that topic, I buy fresh seed compost at the start of the year and reuse the 1/4 size seed trays & compost repeatedly through the seed-sowing season - but only 'coz I'm a) lazy and b) tight!.)

 

John - interesting about sub standard stuff going into seed packets. It doesn't surprise me.

I always used to think that when seeds didn't germinate - it was my fault. Then I read that some seeds are dead in the packet. I have recently had proof of that. In another thread, I mentioned that sometimes I germinated seed in a polybag, which I always sprayed with a copper solution. Some difficult seeds would stay ungerminated in the bag for months - but never went mouldy because of the copper solution. But some seeds went mouldy in the bag just round the seed itself - but the mould never spread, again because of the copper. In a seed tray you would never have known where the mould came from. But in a sealed polybag, it is clear that some seeds carried the mould with them.

I do know from collecting my own seed, is that if you leave it too late the seed can carry mould spores. Sometimes, if you spray with copper solution, the seed can still germinate and grow through the mould. And sometimes the seed is too mouldy to germinate.

Dai - I think Perlite is a volcanic glass with traces of water in it. When it's heated the water explodes and it expands to 15 times its size in exactly the same way a popcorn does. Expanded polystyrene is a different chemical but it is almost identical in so far as it is also expanded and is very light. In all cases they are compostless materials that hold a certain amount of water but offer no nutrients at all. Vermiculite is a natural mica clay, which also expands when heated.

 

JWK - 85% sounds OK to me. It'd be difficult for any seed manufacturer to achieve close to 100% viability / germination rates, wouldn't it? There's only so much we can tell nature what to do before she starts ignoring us!

As for seed sowing, I just use ordinary multi-purpose for pretty much everything apart from very small seed like begonia. For that you need a very fine compost.

 

I use at least 25% vermiculite in any compost used for seeds, cuttings or pot plants to stimulate root growth. It seems to work and my seed germination is usually pretty good too, although I admit I don't grow difficult stuff.

I used to use perlite but it always looked a bit scruffy, as if I'd been cutting up rigid polystyrene nearby (you know how static electricity makes those tiny crumbs of polystyrene stick to everything). So it's not a very strong reason for prefering one product rather than the other but my plants seem to like vermiculite OK.

 

I know exactly what you mean!

 

Following this thread , no one has mentioned commercial seed compost :cnfs:
I use the B n Q seed compost. I cover with vermiculite for surface seeds , or if the seed requires to be covered I fine seive the compost over the seeds. I then spray the seed tray with Cheshunt compound , and water from the bottom. This gives pretty good results except for the "shock wave " petunia seeds I purchase from the USA (Ebay) , which have an erratic 10% to 90% success rate! Does anyone have any petunia germination pointers ? Also what are the merits of seed compost versus general purpose compost?

 

That's pretty much what I do for my veg seeds Harry. 1/4 sized seed trays that fit into my windowsill propagator and I use John Innes seed compost and cover the seeds lightly with vermiculite.

However the purpose of my original question was to try to better understand how perlite and Vermiculite (and I now realise Sharp Sand too) should be used in plant propagation.

Seems like its either a Black Art, or they are fairly freely interchangeable!

 

Harry - I use general purpose compost - with added sand, which for seeds I then sieve with a fairly coarse plastic sieve. It gives a nice fine result, and I suspect that the expensive seed compost that you buy is little more than that.

Kristen - been thinking a bit more about sand, Perlite and Vermiculite. They are, in essence, almost the same thing. They are all forms of silica (ie sand/glass). This means that they are very stable chemically and make no chemical or biological contribution to the growing process. They all leave air holes and I suspect the biggest contribution and difference between them is how they store and release water.... But that doesn't answer your question.

You must supply water and oxygen to roots. But really as long as you ensure oxygen is present and the plants aren't over watered (which removes air pockets) or underwatered - I suspect that it doesn't matter what you use. For instance you can grow plants very well with hydroponics - which is a very different system but delivers the same conditions of water and oxygen at the roots.

 

Ktisten - this had been a interesting question, which I posted on another board (UK Exotic Gardens) - where there were some useful observations :-

Perlite is good for drainage and aeration
Vermiculite is good for moisture retention and aeration
Sharp Sand is good for drainage

There are no hard rules, I personally just gauge what I need as I go along. In a gist, if a plant requires really good drainage and plenty of air on its roots then I use Perlite and Sharp Sand, if it needs a mix with more moisture retentive properties I add Vermiculite

I started with Vermiculite and then switched to Perlite, because it seems to hold its structure better and to drain better over the long term. In pots, I don't need an additive to retain moisture, I need it to improve drainage for Palms and xerophytes

I've become rather fond of cat litter Tesco sell a nice clay based variety that seems to provide excellent drainage whilst holding some moisture and it's relatively light weight too. This is in addition to perlite. I haven't used Vermiculite as I was always under the impression it holds a lot of water, which for most of the plants I grow wouldn't be beneficial.

[cat litter] The word seems to be that this is an excellent substrate for growing the Japanese orchid - Ponerorchis graminifolia. I find chopped sphagnum and perlite good enough for that. I have never like vermiculite - it retains far too much moisture and can become very soggy. Perlite is king as far as I'm concerned - superb for cuttings and an excellent additive to improve drainage and aeration.