The temperature window, and what sits outside it
Updated
The published range
USDA Guide 6 gives 70 to 75F as the temperature to ferment at, and about three to four weeks to finish at that temperature. That is the figure to design the room around rather than a target to hit on average.
- 70 to 75F: the recommended window. Three to four weeks.
- 55 to 65F: acceptable, and slower. Five to six weeks.
- Below 55F: the ferment may not start at all.
- Above 80F: the pickles become soft during fermentation.
Read that list as two different risks rather than one scale. Cool is a scheduling problem: the ferment still works, it just occupies the vessel for half again as long, which matters when the vessel is the constraint and the picking window is short. Warm is a quality problem, and it is not recoverable. A batch fermented at 85F does not become firm again when you move it.
Why warm fails the way it does
Fermentation is a competition, and salt and temperature are how you fix the result. The organisms that produce the acid you want are favored at the lower end of that range. Warmth favors the ones that soften tissue, which is why softness rather than sourness is the symptom that reports a hot ferment. The published problem tables name fermentation temperature explicitly as a cause of soft pickles, alongside too little salt and scum left on the surface.
The practical consequence
A ferment is a three to six week occupancy of a space you can hold within four degrees, not a space that averages the right number. A garage that runs 68F at night and 84F in the afternoon is not a 75F room; it is a room that softens pickles every afternoon. This is the reason the crock location page comes before this one: the decision about where a ferment lives is a temperature decision, and it is made before any cucumber is bought.
Measure it before the season with a logger rather than a thermometer you read when you walk past. What you need to know is the daily maximum across a week, because that is the number that does the damage, and a spot reading at the wrong hour will tell you the room is fine.