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When buying plants I sometimes see info about the plants hardiness described as "Hardy in zones 3-8". From what I understand this means that the lowest temperature this plant can withstand is -40? But what about zone 8? Is that the highest temperature it prefers during winter? Or why doesn't the label just say "Hardy from zone 3 and higher"?

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The USDA zone system classify US climate. The real definition is about lowest temperature, but in US the zones are also nearly uniform on climate, so it is used also to describe typical climate range for plants.

So with a range and the usual USDA hardiness map, you have an idea of where the plant should grow. Note: it is not 100% accurate, there are always local conditions (and climate), but it is a good idea.

Note: some plants need some freezing. You cannot plant tulip bulbs if the bulbs will not have some cold periods. Grape vines will not produce grapes, if they will not reach some cold temperature in winter. So also a range of minimal temperature is necessary: deciduous plants requires some cold, to let them to start again the yearly cycle.

But mainly because of the first reason, USDA hardiness zones are not really useable in the rest of the world. E.g. in Europe (because oceans and mountains), a place with the coldest temperature as in US, will have a very different length of the cold period (so also different deep of freezing), so plant limits are different.

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Because the USA is a large country, there is a wide variation in weather and particularly average upper and lower temperatures between regions. Therefore, the USDA zone does not only refer to cold hardiness; some plants will not perform properly in too high a zone with its higher temperatures. For example, they may not survive harsh heat in summer, preferring cooler more temperate conditions, or may require a cold period during winter in order to trigger flowering, so if it says Zones 3-8, it won't do well planted in, say, Zone 10 because of the higher temperatures.

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