Seed compost sterilisation

Ok. As I mentioned yesterday, I have been making up some seed compost using leaf mold, which I have steam sterilised. This leaves the odd dead woodlouse and millipede to be seen, and presumably kills a lot you can't see. but - is it needed or beneficial ?

I asked a long while ago how people cleaned plant pots etc. I was surprised that many people took a pretty relaxed view. But what about the compost you put in them ? I am very precious with my chilli seeds, and always use a sterilised mix - don't want anything eating them, or fungal problems in that warm damp propagator. But I don't bother for anything else. What bothered me a little while ago was a post on another forum, where someone tried a number of germination tests, mostly aimed at proving peat is best to support a vested interest. But the interesting thing was the comparison included homemade compost, both sterilised and "raw", and the sterilised was the worst emergence. Now, it could be a poor and biassed trial, or I suppose it could be that a compost full of living biota is somehow beneficial. Anyone have any thoughts ? Do I need to do my own trial

Interesting CH - but I can't see why sterilised compost should affect the emergence of seedlings. After all, you are planning to test some elderly broccoli seeds on just a piece of kitchen paper. That's hardly full of living organisms.

I've been gardening and growing from seed for many many years, but I would say I have quite limited experience. If something seems to work, I keep on doing it, and if something doesn't work I'll try another method, but I rarely experiment just for the sake of it. I'm also not great at making sure things are clean, as I'm nearly always in a hurry. But I've almost never had diseases at seedling stage - damping off once or maybe twice in my entire gardening career. I've certainly never sterilised compost, nor the seed trays.

So if you fancy doing a trial, I for one would be very interested in the results.

freebird wrote:Interesting CH - but I can't see why sterilised compost should affect the emergence of seedlings. After all, you are planning to test some elderly broccoli seeds on just a piece of kitchen paper. That's hardly full of living organisms.

Yes, I see what you mean.  I would have thought that the texture, and especially the way it holds moisture, would be the most important thing for germination.  I too have had very few problems with "damping off".  For later growing stages I can just credit the idea that a "living" compost might be better - after all the use of mycorrhizal fungi is quite trendy in the gardening press.

I've been gardening and growing from seed for many many years, but I would say I have quite limited experience. If something seems to work, I keep on doing it, and if something doesn't work I'll try another method, but I rarely experiment just for the sake of it

It is tempting just to continue doing what seemed to work and not be too analytical about it.  But I am a scientist at heart, and worry that just believing what seemed to be true without putting it to the test is the way that myth and superstition arise !

I had heard (I don't know if it's true...) that nutrients in compost can effect germination, so the stuff you buy from the garden centre specifically for sowing seeds is deliberately low in nutrients. (I suppose that was one of the qualities of peat, but I don't use it.) At the moment I'm using a mixture of garden compost and a bit of top soil with a layer of coir at the top where the seeds sit. This is probably a bit eccentric, but I like it because it's fine. Nothing is sterilised.

I always get a bag of seed compost from a nursery near where I used to work. It's quite a trek now but I still make a trip because I've always had fantastic germination since I first used it. They make it themselves & I would guess that it is sterilised.

I'm afraid I'm pretty lackadaisical about cleaning things though I always try to do my small pots & trays with bleach. Everything else just gets a swill out in the river. I'm like FB - if it works I carry on & if it doesn't I keep trying.

Dandelion wrote:I had heard (I don't know if it's true...) that nutrients in compost can effect germination, so the stuff you buy from the garden centre specifically for sowing seeds is deliberately low in nutrients. (I suppose that was one of the qualities of peat, but I don't use it.) At the moment I'm using a mixture of garden compost and a bit of top soil with a layer of coir at the top where the seeds sit. This is probably a bit eccentric, but I like it because it's fine. Nothing is sterilised.

I think that the reasoning is that having a seed compost high in nitrogen is no use to the seeds whilst germinating - they are using their own reserves - but it does encorage rotting just as it would on a compost heap.  Because of this true seed composts aren't very good for growing on for any length of time - they are designed around sowing in trays and pricking out fairly early.  So not really suitable for module growing.  I think this may be part of why it seems to be getting harder to find a specialist seed compost rather than general purpose composts - it is part of a growing sysrtem that is bit out of fashion.  

Also, seed composts usually use peat (John Innes formula) because it is, as you say. low in nitrogen.  It also unfortunately has a texture/water retention property that is unsurpassed by anything else. Which is why I want to make my own (as well as that I'm perhaps a bit obsessive about homemade stuff).   Composted coconut fibre (coir) and leafmold are passable substitutes. Maybe a good use for that reclaimed peat dredged from resevoirs ?

Following on from this line of logic ... the seeds I have used for a germination trial have chitted quite nicely on damp kitchen towel. I presume this is pretty clean, and doesn't contain nitrates to help the seed rot. It is also very easy to keep it somewhere warm until they sprout.

... Which makes me think. For these older seed, why not chit them first on damp paper in a warm place, then transfer them to modules with tweezers to grow on. Then there are no misses because of unviable seed, perhaps lower losses from rotting, and I maybe don't need to use a seed compost, so that they don't run out of nutirnts so quickly. Only obvious drawback is that it is a tedious job.

Have you come across the method of germinating them in either wallpaper paste (without fungicide) or flour and water paste, then piping the resulting paste into the soil using a polythene bag with a corner cut off?

I read about that in Geoff Hamilton's The organic garden book (my go-to book in case of doubt ...) I've not yet tried it though.

I do like module sowing for brassica, they give them a bit of a head start before the slugs and/or pidgeons find them

Being very new, I don't want to cause confusion by posting in odd places or in threads where info doesn't belong so thought I'd ask.
I have some information on seed and cutting composts from the latest Which report and thought it might be useful to share. I've looked around for a suitable place to put it and wondered about here. Rather than start a new thread. Unless that would be preferable.
Promise I'll stop badgering with questions soon .

If it merits a new thread don't hesitate to start one. The search function on here is not the greatest, and it helps if the thread titles are a good key to the thread content.

I'd be interested in what Which have to say, as although I make my own seed compost, and 100-200 litres of tomato compost, I always run outand have to buy some. I've usually gone for Sinclair's New Horizons, but like a lot of the peat free composts it seems to get a bit worse each year.

I have just discovered an excellent reason for sterilising seed compost.

I ran out of the stuff I had steam sterilised, and rather than do some more, I used some unsterilised garden compost in a sowing mix. I now have two lush trays full of seedlings, but I have no idea which ones should be there, and which are "volunteers" !

Can anyone show me what Rudbeckia and Knautia seedlings should look like please

My Rudbeckia seedlings have been a complete wash-out this year: I tried a dwarf variety which just haven't grown (ha ha - the lot I tried last year didn't grow either!) Anyway, I can't send a photo, but they have oval leaves which are greeny grey, look slightly rough (I.e. not shiny or fleshy), and have tiny hairs visible around the edges.