Germination Temperatures

In all of my hours and hours of research, I still don't understand this. When it says 70*F is the germination temperature, how does that factor in daytime temperatures, nighttime temperatures and the heat caused by sunlight?

I have seeds in a window sil with a mini temperature meter that records max and min temperature. I've got a max of 109 (sunny day) and min of 48 (cold night)...

If plants are supposed to grow outdoors naturally, how can a single 70*F temperature be correct?

All it refers to is the distinction as to whether the seeds of a certain species require stratification (conditioning by exposure to varying temperature) or not.
The distinction comes from the seed germination studies of Dr. Norm Deno. He grouped the results of his extensive studies into whether a species required exposure to differing temperatures (stratification) to break down naturally-occurring chemical germination inhibitors, or whether it did not. (There are also some other categories, but temperature requirements are the major groups.)
For species that did not require stratification, his work and that of precedessors found that a temperature approximating room temperature (70 degrees F or 20 degrees C) was ideal for germination. It is from this work that you see seed germination instructions that specify "sow seeds on moistened medium at 70 degrees F" (or similar). Such species are also described as "easy, warm germinators" (meaning that stratification is not required).
This does not mean that a constant temperature of 70 deg F is required for germination - if so, seeds would never germinate in nature in colder climates; it only means that more complex cycles of temperature variation (e.g. a period of warm, then a period of cold or vice versa, or multiple cycles of either) are NOT necessary to induce germination.

Edit: Further...
Most annuals are "easy, warm germinators", with very few exceptions. Perennials, as a group, contain much variation in what it takes to break down germination inhibitors in seeds.

This message was edited Mar 4, 2011 8:24 PM

Thanks for the response.

I wonder if the sunlight exposure has burnt my seeds then. I might try indirect sunlight from now on and hopefully they can handle it. I've had some that cracked the shell but turned brown before going any further.

Sunlight through a window will certainly not burn seeds... it's usually quite the opposite; sunlight through a window is not usually intense enough to allow seedlings to grow properly beyond a short period. The seedlings become leggy and unhealthy.

I'm not quite sure what you are describing - sounds like your seeds have not yet germinated (no seedlings) yet? While you are germinating seeds, the pots should be covered with clear material (plastic lids, plastic wrap, glass, etc.) to keep the soil constantly moist; this is preferable even to daily watering.

Well it does get pretty hot in the windowsil, especially in a container that is sealed. I have them in a clear container, very moist soil. The problem is that they don't come out of the seed shells, and for the ones that crack open (or I do manually), they are green but turn brown instead of doing any growing.

I've looked at many sources for germination instructions, and I can't isolate the reason why they are doing nothing. I've got a container of lots of seeds, most plumped but none sprouting. The only thing I can think of is the afternoon sun trapped in the containers + indoor temperatures is cooking them. It has been over 4 months for some of them, when similar seeds of another cultivar sprouted long ago. I don't think I would have bothered with any of this if I knew how much work it is.

Well, like I said, 70 degrees F (20 degrees C), and a few degrees either side of it, is the ideal seed-starting temperature.
What is it that you are trying to grow? If these species are indeed "easy, warm germinators" (seeds that need no preconditioning, i.e. stratification, scarification, to germinate) then there is probably something wrong...

One is a columbine, aquilegia vulgaris Blue Barlow. Since it's the european type, it is not supposed to require a cold period, but some sources say it should get stratified, so I did.

The other is platycodon grandiflorus (balloon flower), and it is supposed to be 3 weeks at 70F in moist soil with light.

I grew other columbines with the exact same treatment as the Barlows and they turned out fine.

The 2 I'm having trouble with both came from one online store, so It's possible that they mishandled them. But I've had a couple that seemed to open up and just die.