I just came across the following link which may be helpful to some people for sowing / germination temperatures, which from the chart shown, would appear to be rather different for various types of vegetables.

Temperatures are shown in degrees F, which may not be what people are used to. It would be interesting to see if this chart bears out individual experiences when sowing. It certainly proves the theory that sowing later when soil temperature has warmed up means that later crops often catch up fairly quickly.

http://tomclothier.hort.net/page11.html

That's really useful Primrose. I've printed it off. I was surprised at the high temperature for lettuce as I'd read they wouldn't germinate at high temperatures, although I've just looked it up and one experiment said that subjecting lettuce seeds to a temperature of 30C is that about 90F? for 15 minutes and germinating them in the dark was very effective.

It will be interesting to test the info on the table out. I wonder if leeks will be the same as onions.

I sow lettuce regularly throughout the summer for succession and in heatwave periods have often found that the seeds simply haven't germinated at all. So now if it's very hot when I'm due to sow, I tend to sow the seeds in trays in my shaded potted shed area where the temperature is a little lower, and the seeds don't get baked to death in the scorching sun. I suppose one could always keep a small polythene bag full of compost in the fridge for 24 hours before sowing to keep the temperature down, but there's never room in our fridge for that kind of activity. .

A really useful chart Primrose, thank you.
the packets are a pretty good guide too, I just planted some mooli radish outside under fleece, they are hardier than the normal type and the hardy winter lettuce are doing well too

Hi Primrose,
The one thing that is wrong with the chart is that it pertains to the USA and their temperature zones and not ours.
A few years ago I sent out a very simple seed germination chart which gives all the same information for this country.
The USA have either 9 or 10 temperature zones to deal with and I suspect that the chart deals with a place that is infinitely colder than anywhere in the UK.
JB.

Hi Primrose

I have to agree with Johnboy that North American data is misleading. They have a much greater variety of conditions to cope with than we have - some more favourable many much worse.

One particular set of figures which causes me to doubt the usefulness of the data is the suggestion that the "optimum" temperature for lettuce is 77F which will give 99% germination and emergence in two days. Not if you sow at that temperature

Hi Alan,
One of my many goofs was with Lettuce. I sowed them in February in the tunnel and there was a beautiful sunny day and I had left the property for the day. They actually germinated in October! I just threw the trays aside and didn't get back to them and resowed. That was 20 trays each with 576 in each tray so just a few. I agree that 77F is too high a temperature. Things like lettuce should not need added warmth in this country nor should any of the range of Brassicas. They are better left to germinate in their own time and by doing so you get a far better root structure and a superior root structure is the basis of a healthy plant.
JB.