Donald ...
Yes, there is heat zone information available. In a sense, it further defines "full" sun. Full sun in heat zone 8 is more intense than full sun in heat zone 6.
I first learned about it from a book recommended by another ATP member, The A-Z of Garden Plants published by The American Horticultural Society. According to the chart at the back of the book heat zones are defined ad "Average number of days per year above 86F (30C)." I am guessing the formula is calculated the same way cold hardiness zones are calculated with no adjustment between day and night temps.
I have to look up every plant before I even think of purchasing anything for the garden because even tho' my temps cool off at night, gardening in a more arid climate with a high heat zone number truly limits the plants that will grow well for me.
Smiles,
Lyn
