Alabama extension publishes a germination table by crop. Lettuce germinates across a range of 40 to 80°F, with an optimum of 75°F and a hard ceiling of 85°F. Spinach runs 45 to 75°F, with an optimum of 70°F and the same 85°F ceiling.1 Utah State extension gives the practical band for leafy greens as 55 to 70°F, where emergence takes anywhere from 3 to 15 days.2 Penn State puts spinach specifically at 45 to 68°F.3
What happens above the ceiling
The failure here is not the slow, partial one you might expect. Above 80°F, the seeds of lettuce and spinach go dormant, and they will not germinate until cooler soil temperatures return.2 The seed is not dying and it is not sprouting. It is waiting, and it will keep waiting while you water an apparently empty bed.
That is the whole problem with sowing fall greens by the calendar. Late August can feel like the season has turned while the bed itself is still sitting above the number where the seed gives up.
When to sow
Penn State times fall spinach at six to eight weeks before the average first frost date.3 Minnesota extension puts collards and kale much earlier, direct seeded about three months before the average frost date, because those are crops you are growing to size rather than racing to a baby leaf.4
Both dates are anchored to first frost rather than to a month, which means they move with where you garden. The same calendar week that is right for a Nashville bed is early for Charlotte and very early for Houston.
If the bed is still too warm
Three honest options, in order of how much work they are. Wait, and let the reading come down on its own. Sow anyway and accept that germination will be uneven. Or cool the bed with shade and water. Penn State frames the whole fall crop this way: the same season-extending methods used in spring, run in reverse.3
Whichever you pick, take the reading rather than estimating it. Alabama extension notes that soil temperature is straightforward to measure with a soil thermometer.1 A single number from your own bed settles an argument that no planting calendar can.
The part that gets better
Kale and collards improve as the season goes on. Minnesota extension puts it plainly: for a fall crop the cooler the weather the better the flavor, until freezing temperatures stop growth altogether.4 The bed you struggled to start in the heat is the one still feeding you in November.