Well, that was hard, wasn't it? I did a little more research, and found a wonderful MS thesis from the University of Arkansas a few years ago, all about response of spinach to heat. People seriously growing spinach should have a look.

https://pdfs.semanticscholar.org/46a1/aacff7d84f0260b97ff4050a3fa6d92445ea.pdf

In this study, spinach seed germination drops dramatically at 35C, but is only about 30% worse at 30C, depending somewhat on the cultivar. Interestingly, the Ozarka cultivar has seed germination rates that are unaffected by temperature, up to 35C. The impact of temperature on leaf growth (measured by area of leaves produced) is less, more like a drop of 20% at 32C, for all cultivars.

So the net result is that spinach grows OK at higher temps, though it will eventually bolt. Also, it would seem that cultivars whose seeds germinate particularly well at high temperature do not necessarily grow particularly well at high temperatures. That is, the dependence on temperature of seed germination and foliage growth is different.

So germination at cool temperatures is smart, but no reason why seedlings can't be grown to plant-out maturity at higher temperatures.

Actually, I'm probably going to do Orach instead, as it is not only heat tolerant, but also frost tolerant (unlike NZ spinach, for example).

NZ spinach is not true spinach. Spinach is extremely hardy and will winter over in the open garden here without any protection whatsoever. In zone 6A, spinach would be sown in the open garden towards the end of September.

That's exactly right. But the first thing people tell you, when you say you want heat tolerant spinach, is "New Zealand Spinach!" My Malabar spinach isn't spinach either, and it serves the same purpose, except it's probably even more heat tolerant than NZ spinach. It seems that orach (which is also not real spinach) is just mildly frost tolerant, which is usually all I need here, and plenty heat tolerant.

I have not tasted orach, and would like to. I don't look at it as a substitute for spinach, I look at it as a reasonably hardy green. Spinach and warm temperatures just don't go together. I do already grow a lot of chard, which is great stuff, but was looking for some variety. Intensely colored leaves have some novelty. Orach has been gaining a lot of attention and popularity in culinary circles, so it can't be that bad. I do understand that it takes up salt, which most plants don't do, so if grown in saline soil, it'll be quite salty. Also, old leaves can be bitter. There may indeed be some soil or climate dependence on the taste. One more thing. Wild orach is understood to be kind of icky. But new cultivars are supposed to be much better.

Yes, I've heard the stuff self-seeds. If I like it, that's good. If I don't, not so good. I like my Malabar spinach, and it self seeds. I never plant the stuff. It just comes up by itself. So heavily, in fact, that the only real work involved with it is thinning.

I'll have to try the Malabar spinach next year and maybe some "improved" orach, too. The red leaves of one orach variety I grew added a nice look to salads, but was not good to eat by itself - at least not for me.