There are many germination temp charts available online but I'm not familiar with the particular one you refer to. If you attach a direct link to it in your post when citing specifics it would help to answer your question and saves responders from having to dig it up. :)

However in general the charts are not "lab" results. Most will post the ground soil temp range for those seeds which are normally direct sowed and the indoor soil temp range required for those seeds which are normally grown for transplanting, ie. in the house or in a greenhouse. And for the most part, depending on the info source, they are quite accurate even in real life.

So for example, tomato and pepper seed temps on the charts are for indoor growing using a supplemental soil heat source while spinach, peas and similar crop temps on the chart refer to the garden soil temps range required as they are normally direct seeded.

While indoor soil temps can vary greatly depending on the heat source used, outdoor soil temps are much slower to respond to changes - both warming and cooling.

Hope this helps.

Dave

Forgot to mention - be sure to check out the Growing from Seed forum here as that is where you'll find most of the discussions about seed germination info and lots of FAQs on all the basics.

Dave

Here is a link that might be useful: Growing from Seed

Yeah, those numbers are basically best case scenarios. You can germinate seeds in temperatures much lower than that successfully, it just takes longer.

Pepper seeds typically prefer a good 80-85 degree soil temp to germinate, but I've sprouted even the difficult nuclear-hot peppers in room temperature soil (high 60's to low 70's) during the winter months.

I'd say as long as you keep just about anything at room temperature you will eventually germinate it, assuming it doesn't rot first from too much water.

They are probably showing the optimum average soil temperatures for germination. Usually a state university will post the daily average soil temp for your area. Seeds do germinate over a large variety of temperatures. Onions will give you 90% germination at 32 degrees, but it will take 136 days. (This is why Alliums do well planted in the fall, as long as soil temperatures are above 32 degrees they will make some growth.) Here is an interesting chart. http://tomclothier.hort.net/page11.html

Sorry dig but the charts I was referring to are in the Johnny catalog. They may have them at their online store. Not sure.
I was wondering more about daily variances. Particularly in regard to tunnels. They're going to bring the heat way up in the day but will not prevent the low night temps. I was wondering if that would make seeds germinate before they could tolerate the low night temps

As I said above, While indoor soil temps can vary greatly depending on the heat source used, outdoor soil temps are much slower to respond to changes - both warming and cooling.

That is why tunnels and clothe type covers work so well. The soil heats during the day and retains that heat, remains warm at night.

Just as it takes several days for the soil under cover to warm up well to be ready for planting, it also takes several days of really cold weather for it to cool down much.

Dave

Don't get sucked in what theexperts say. Every expert contradict each other. Their information are generated under the ideal condition. From my experience while using heat mats and growing lights every plant has different time to germinate. For example I found that tomatoes germinate faster than Eggplant, pepper is in the middle.
Abe