You need to be able to measure your soil temps not the air temps. They are very different.

Numerous charts are available for per crop "soil temp for ideal germination vegetables". Measuring soil temps is easy to do with any sort of thermometer that can be stuck in the ground 4" down. They make many thermometers expressly for measuring soil temps, some quite inexpensive. But even the cheap stick-type wall thermometers, the ones filled with red liquid that climbs a glass tube, work well.

Dave

The beets and carrots I planted have only germinated so far in the shade. Lettuce and brassicas came right up.

Thanks Dave and Peter!

Yeah I kind of figured I should probably get a thermometer for that purpose. Will invest in one.

But once you get a thermometer, you need to know what soil temperatures are best. Such tables, of max, min, and optimum soil temps for germination are available on lots of ag extension sites. What I see is that for carrots, 95F is max. Optimum is 45-85F. Radishes have the same max, and 45-90F optimum temps. It really depends on the crop. Okra, corn, cukes, turnips and watermelons have maximum germination temp of 105F, while for lima beans, beets, celery, lettuce and spinach it's only 85F. Interesting that while limas do fine in very warm weather, they really don't like germinating in very warm soil.

I'd find it hard to believe that soil temps in Sonoma CA would be outside this optimum range. You may have heat during the daytime, but it cools off a lot at night. Also, even near the surface, moist soil temp will be close to the dew point, which is well within the optimum ranges.

102 is too hot for me too ;-)

Consider the age of the seeds also. I like to soak my seeds for 1-3 days depending on the seed. You could also try germinating some in a moist napkin or ever starting them inside. Daily moisture on the outside seeds is very helpful (even inside ones).

Here is one temp guide...
https://ag.arizona.edu/pubs/garden/mg/vegetable/temperature.html

I prefer digital thermometers for soil temp...
http://www.walmart.com/ip/Taylor-TruTemp-Digital-Instant-Read-Thermometer/16541966

I am sure Amazon also has them.

HTH

I'm in S Santa Rosa and it is STINKING hot here! 105* today! I'm putting off planting anything til it cools off later in the week. Sometimes with carrots with our warm Sept/Oct I will soak the soil, do the seed, then cover lightly with cardboard or weed cloth, watering as needed. Nancy

I will always maintain that the main cause of seed death in hot/dry areas is drying out (to any degree) after once being wet. I measured soil temps here today. It's hot, arid, and all in full sun. The drip areas that have moisture were about 85 F an inch down and 84 F six inches down. The dry-as-dust plots that aren't being used were cooler. Coolest were drip areas with mulch. Not bad for direct blistering sun.

Hey grubby, wow I'm surprised at how not-too-hot the soil is in Tucson now. I lived there for about half a year, going into the beginning of summer and it got crazy hot. I guess maybe the monsoons cool the soil down?

The soil doesn't get as hot as you'd think a few inches down. The water at the bottom of my pool is downright cold because of the soil temperature that deep. It's just the surface that gets hot. I've got kale seedlings already that I direct-seeded and they're fine. I'm in PHX. So, sow away and just keep it moist and it should be fine.

Again, moist soil temperature ends up around the dew point, unless sunlight is shining directly on it, and you're looking at the very surface. Not a soil temp problem.