Wet feet, not cold, is what kills them
The sign says avocados hate wet feet and that is not a preference, it is the whole thing. Avocado roots are shallow and need air. In Florida sand that is easy; in the patch of your yard where water stands for an hour after a storm it is impossible, and the tree will look fine for two years and then decline over one wet summer with no obvious cause.
So plant high. Set the top of the root ball an inch or two above the surrounding grade and feather soil up to it, rather than digging a bowl that collects water. If the only spot you have holds water, build the soil up into a wide low mound a foot high and three or four feet across and plant into that. It looks odd for a season and then disappears under mulch.
Mulch wide and keep it off the trunk. Mulch piled against the bark holds moisture where the tree least wants it and invites collar rot.
Type A, Type B, and when it actually matters
Avocado flowers open as female one day and male the next, on a schedule that sorts varieties into Type A and Type B. A single tree does set fruit here, because the schedule slips in warm weather and the flowers overlap. A second tree of the other type sets noticeably more.
If you are buying one tree, buy the variety you want to eat. If you are buying two, make them an A and a B. Brogdon and Winter Mexican are Type B; Day is Type A.
Cold
Mexican-race avocados are the cold-hardy ones and the reason an avocado is worth planting in Lake County at all. A mature Winter Mexican or Brogdon shrugs off a night in the low twenties. A one-gallon tree in its first winter does not, whatever variety it is, because it has no wood and no root mass to fall back on. Protect it for its first two or three winters and after that you can mostly stop thinking about it.
Do not cut frost damage off in February. Wait until spring growth shows you where the live wood ends, because the brown twig you were about to remove is often the thing protecting the bud that will replace it.
