There have been a few posts pointing out that you should not plant out squash into cold soil, but does anybody actually have a number to put to it.
I have decided to regally measure my soil temperature, and then sow the seed indoors 2 weeks before it looks like the soil will be warm enough.
I had a google, and I know I need to measure at a depth of 4 inches, but I can't find an actual temperature, as most of the authors are obviously in warmer places than the UK.
Why don't you just put lots of fermenting matter in the places you are sowing? A good bit of grass cuttings or manure under the sowing spot can give off quite a bit of heat.
I have never measured the temperature, I just sow them outdoors in Mid-May when the sun is shining ;D