I meant that the heat tolerance was not what it means. The OP asked SPECIFICALLY about the Max Recommended Zone...the heat tolerance zone. They said they understood cold hardiness. Most people don't know the max heat zone, so they don't list it in their posts to the database.
I grow a lot of tropical plants and orchids, and this type of stuff is very important for that. Unlike more Southern climes in FL, I live in North Central where we have a real, albeit short, winter. Knowing the recommended heat tolerance is supremely important in growing a lot of orchids and higher elevation cooler growing Anthuriums and other aroids.
The heat tolerance of some genera is not absolute across species. That is why orchids are divided onto cold growing, cold to cool growing, cool to warm, and warm to hot.
I can easily grow a cold to cool orchid here in winter. But come summer, it will die a very painful death in the heat.
I can easily grow all of the lowland tropical plants year round in the greenhouse, where I can heat it in winter. But its a lot harder to make the cooler growing highland plants happy. They might easily like 48-50F in winter, but come summer, when its 90+F here day in and day out, they will die.
It's the same for many non-tropical plants. No one grows stuff like red hot pokers here, or Japanese magnolias, etc because while they would easily go through winter, they would not go through summer.
So I take the 'maximum zone' recommendation on the NGA site with a grain of salt. If someone actually puts a value in that spot, I would definitely do my own research and look that specific plant up, and see what its heat tolerance REALLY is. There is a reason why some plants are popular in the North, and others popular in the South. Its heat tolerance, more than cold. Because you can always dig up and store, heavily mulch, bring in a pot, or treat as an annual, a plant that is not cold hardy in your zone. But a plant that isn't HEAT tolerant somewhere like Texas, Louisiana and Florida is a waste of $$$
