ELI5: What is the “hardiness scale” in agriculture/gardening, and what do all those zone numbers and letters mean?

[u/Salty_Dragonfruit28]

I keep seeing plants labeled like “hardy to Zone 6” or “Zone 7b.” I think it has something to do with how cold winters get, but I don’t really get it, like if a plant is “hardy to Zone 6,” does that mean it dies in Zone 5?

[u/Cryogenian]

Yeah, at least it is more likely to. These Zones give the Annual lowest temperatures of the area, and lower zones will get progressively too cold for the plant.

Plants might still survive if the winter is mild or the time with low temperature is really small

[u/peskypneumonia_9 → u/Cryogenian]

it's basically a map of the coldest your area usually gets, so a zone 6 plant has evolved to handle that floor but not much lower. zone 5 would be pushing your luck, like borrowing a sweater that's two sizes too small. you can try microclimates though, tucking it against a south-facing wall can sneak you a half-zone sometimes

[u/Clean-Car1209]

here's the map, it'll helps understand what they are getting on about.. https://planthardiness.ars.usda.gov/pages/map-downloads

[u/lucky_ducker]

Maybe not quite "dies." Yeah, if you plant a zone 7 tree in zone 3, it's gonna die.

The zones reflect long term outlier extremes of cold temperatures. If you want your planting to survive for decades, it needs to be in a zone where it will never encounter temperatures cold enough to kill or damage it. Here's some examples.

I have a Golden Raintree that is right at the northern extreme of its range. One effect of that is it usually jumps the gun in starting new growth in early spring. Most years that's OK, but one year we got four successive hard freezes in April - early May. Four times the tree's new growth was killed, and I was concerned the tree itself would be damaged. It eventually leafed out OK, although it did not flower as profusely as it usually does that year.

Another example is the mimosa. This is an invasive small tree / shrub that no-one should be planting because it spreads so easily. They are common about 100 miles south of where I live, and I sometimes see volunteers sprout up in my area. Invariably, a really cold winter comes along that kills the tree down to - but not including - the roots. The following year an unsightly dozen or so new trunks sprout out of the roots, resulting in a gangly weed-looking mess of foliage.

So essentially the zone classifications are a way to ensure that your landscaping choices will grow optimally in your area.

[u/Heavy_Direction1547]

Means it can be expected to survive most winters in the region the number and letters refer to; lots of online maps. Climate change is shifting the zones and all contain microclimates so plants surviving in a zone 3 while designated a 4 etc. is not unusual.

[u/Onigato]

Hardiness zones take both the lows and the highs into account. If an area never gets above (arbitrary number) 85F, but gets down to -15F it's in one zone. If the highest is the same 85F, but the lowest is 5F that's a different hardiness zone. (all these numbers are completely fabricated and off the cuff, not the official ones. There is an official listing of what each zone is actually rated as the lows and highs.)

Plants that are hardy in zone 5a and 5b may get too cold in zone 4b, and get too hot in zone 6b, as an example. It may be fatal to the plant, or it may just be a simple "failure to thrive" and they don't grow well. They can usually be shepherded through "incompatible zones" with extra care, but it does take that extra care and usually additional equipment (like greenhouses or supplemental shade or watering) to accomplish what nature does in the normal zone.

[u/Petefjord]

its pretty much a temperature thing. plants do not like the cold, usually - but of course, some does better than others.

the zones starts at 1 and goes up as far as needed. For example, for my country we got 8 zones. (plus a mountain zone, as it were.)

1 is very fragile plants i shouldnt expect to survive the winter at all, 8 can effectively take anything.

so if a plant says "hardy to zone 6", it would mean that it thrives in zone 1-6 but i shouldnt expect much beyond that.

ofc, like other commenters has mentioned, its just a guideline. if i live in zone 3 area it could still happen that a particulary harsh winter kills a zone 4 plant in my garden.

and then on top of that, stuff like if its in a pot or in a flowerbed also affects

[u/moneyfink → u/Petefjord]

Zone 1 is the coldest, your comment has the scale inverted.