In message
, Rod
writes
I'm probably going to give more questions than answers.
Are you talking about sowing now? If so, unless you intend to grow
your lettuces under glass it's too soon anyway. Onions for big bulbs
or show is OK now.
Going through the list in sequence.
Compost - for consistency I'd change from your mixture to a good
proprietary multipurpose compost like Levington multipurpose. Homemade
mixtures are for special needs when you have enough experience to know
what those needs are. The grit sand will not be sterile and commercial
JI composts are notoriously variable from just OK to bloomin awful.
Sow thinly. I've never had any use for perlite or vermiculite, if the
seeds need covering then a bit of the compost sieved over them is all
you need.
Water management will be easier with the Multipurpose compost - I
suspect you've erred on the wet side - experience will be your guide.
Temperature and light levels - the seeds you mention don't need high
temperatures, in fact lettuces will fail at high temperatures. You
only need the warmth to get the physical and biochemical processes
started. Uncover the seedlings as soon as you see that roots are
starting down into the compost and after a day or so reduce the heat
or take the trays off the heat altogether. Your propagator needs to be
in good light but *never* direct sunlight - that turns it into an oven
in minutes. Watch those temperatures - don't rely on the thermostat
being accurately calibrated - it isn't. Use a soil thermometer in your
trays and an air thermometer under the propagator cover. Soil temp
around 16-18C, if the air temperature reaches 30C you're aleady
heading for trouble.
As soon as the seedlings are cooled off and they are big enough to
handle - usually well before you see any true leaves get them out of
the tray and into plugs or whatever that you are going to grow them in
and use the multipurpose compost as before. Continue gradually cooling
them off subject to the usuall caveats about frost, cold winds etc. If
damping off starts you can limit the damage by pricking out
immediately, avoiding the parts of the tray where the seedlings have
gone off and carefully examining every seedling (wear your reading
glasses if you need them) - if you see any kink at soil level then
it's a gonner. If you follow this advice you shouldn't need any
fungicide.
Google for Thompson and Morgan's old seed germination database for
special needs and general advice on growing from seed.
Whenever you change anything watch for the results of the changes and
learn.
That's a distillation of over 40 years as a professional gardener and
nurseryman and reflects what I've been doing with decent success for
several decades now. But others will have their own equally successful
methods and materials and if they work then fine. The above will give
you a reasonable start upon which to build your own techniques.
Good luck and enjoy - every seed that germinates is a little miracle
and if it turns into something like the 40ft Cedar I'm looking at now
that I sowed 35 years ago it's a great big miracle and a joy.

Many thanks for the very informative post. My recent first attempt at
growing from seeds (two varieties of echium) was a bit of a disaster due
to damping off. From the above it is clear that I did not help matters
by letting temperatures get too high and not ventilating adequately.

As recommended by the seed supplier I used JI number 1 and added about
30% mix of grit and sand which I did not sterilise which was probably a
mistake. I have just got some replacement seeds and this time I will
sterilise the mix and take more care with the growing conditions.

--
Robert