Gina1960 said: no, that isn't what it means.
The cold hardiness is EXACTLY just that- cold hardiness- and only that! It deals only with winter severity, and so it's only function is to help you know if a plant might survive through your winter.
If it survives your winter in zone 3, it WILL ALSO SURVIVE the winters in all higher zones, which explains the range- zone 3 to zone 9 (or 11.) It will survive the winter in all those zones.
If it could survive your summer- through heat and possibly drought, you'll need to find other sources for that info. This is an important distinction- while a plant with a hardness rating of zone 3-11 will survive wonderfully in all those zones through every winter, it may succumb to summer heat or dryness in all zones above 8!
USDA said:
The map is based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones and 5-degree F half zones.
FWIW, the chart makes no attempt to account for 'chill hours' needed for some plants to develope fruit or bulbs to flower. In other words, if a plant NEEDS a minimum cold for a span of time- this chart may not help you. The zone chart extreme cold temp. might only be for a day- but it could be that cold for a month- the zone chart can't tell you that difference. Other charts do- and so will your local extension service.
An attempt has been made to formulate a heat index, but it hasn't gained traction since developed in the 1990s by The American Horticultural Society.
https://www.gardeners.com/how-...
Drought tolerance is another factor that has become a rating for plant summer survival, but has no standardization.
The advice to ask a county extension agent is good.
