The formal definition of “hard freeze” is along the lines of:
" when the temperature reaches 28°-or-lower for at least a few hours. It usually means that many types of plants and most seasonal vegetation will be destroyed ."
The Almanac defines it as
“A moderate freeze (between 28 and 25 degrees F), sometimes called a hard freeze, can cause wide destruction to most plants.”
(and then there’s severe freeze, below 25)
It’s not a subjective thing. It’s not just frost (and if you’re dry enough, there may not even be any frost at all)
The point really is about the botflies - what does it take to (mostly) kill them off for the season.
This is all aimed at timing the Fall deworming to maximize the benefits.
Some years we don’t get a hard freeze until well into December . I don’t wait until then to use Equimax, because at that point, we’re in the same category as those who don’t get a hard freeze, so at most I wait about 7 months from the Spring deworming (usually April) to do the Fall deworming.
We had 2 hard freezes in a row about 10 days ago - 27 here. And while it has since been in the 60s to even upper 70s for most of the last 9/10 days, and most of those nights above to well above freezing, it’s still as good as done, and I’ll be Equimaxing everyone this coming week, especially since the next few nights will be in the freezing to slightly below, and with cooler days ahead.
We will still get an occasional botfly who found a cozy hidey hole, will see a few more bot eggs on for another few weeks. But I don’t want to wait 9+ months do kill what’s accumulated since the Spring.