USDA zone range explanation

hello,
I am french and i am new on davesgarden.
I know what is the USDA hardiness zone, but i don't understand why there is a range ?
To me, the USDA zone tell us what is the minimum temperature at which the plant will survive. why are there many hardiness zone for a same plant ?
For exemple, for the Bearded Bellflower, the hardiness are zone 3a to 8b ? I don't understand ?
thank you very much for your futur explanation and sorry for my bad english

Hi Neeqkowlah,

it is a very good (for me at least) question!
I asked myself this question for quite a long time.
I'll get popcorn, and I'll be right back to look at the answer!

I do soooo hope someone will answer this question fully and completely only using no more than three syllable words, please. Zones and the way the designations are used are thoroughly confusing, and in my zone (10b) most of it doesn't matter anyway.. we just seem to have to break the rules to grow anything. I understand a plant being hardy TO ZONE whatever - it can grow without winter protection in any zone number higher than that one. So, hardy to zone 8 means grow it outdoors in 8 and above, right? I hope that's right, I've worked so hard to get there. I don't get the range of zones, tho, and the way the range differs from one author to another. Thanks to neeqkowlah for asking and to anyone who can provide answers. Syb

The USDA zones are for cold hardiness. When it says 3a-8b it means the plant can be overwintered in that area. Overwintered meaning it may die back all the way to the ground or look fine all winter. The problem is that it doesn't take into consideration Microclimates...my area gets colder then 8a so it's microclimate is more like 7b or a. You just need to learn what happens temp wise in your area.

My understanding is that each plant has a minimum and maximum temperature it can survive, So a plant that was rated z. 3-8 would be a plant happiest within that range. Some plants don't thrive if it's too hot, or require a winter chill, and so on. Tomatoes don't set fruit if it's too hot. for instance, and crocus need a cold period to bloom.

However there are a thousand other factors affecting your plants' ability to survive, Humidity, soil pH, soil consistency, does it occasionally dip below freezing for a few hours or does it stay there for months, sun vs, shade, etc. USDA zones are a fast way to estimate whether an unfamiliar plant will do well in a particular zone. The US is huge, and has many many different zones and even places that are the same zone that have TOTALLY different climate. (I think the standard example is Seattle and Dallas.)

In the West Coast of the US, they have Sunset Zones, which take some of those other variables into account. And son't worry about your English; you REALLY don't want to hear my French.

I live in Zone 6b in Reno, NV. Our temps routinely get done into the single digits BUT, there is usually no snow on the ground and the humidity can get very low. My neighbor moved from Pennsylvania and also Zone 6b. Her temps also got down to single digits BUT her yard was covered by 5 feet of snow. My Zone 6 and her Zone 6 are not even close. The diffence is moisture. Its not cold that necessarily kills plants but dehydration. A wet plant can take more cold then a dry plant.

The USDA Zones are a guide. I can grow bell flowers in Reno and so can Sybil in Huntington Beach. Attempting to grow a Palm Tree in Reno would be a ridiculous idea but they grow great in Huntington Beach. The USDA Zones tell me not to waste my money.

I can grow great tulips but Sybil would find it difficult. The USDA Zones are not set in stone but guidelines to what could grow in an area. As Carrie pointed out, there are a lot of other factors - microclimates are caused by slopes (cold air moves downwards), buildings, roadways or areas with lots of concrete, or natural barriers like mountains or bodies of water.

So consider the USDA Zones as suggestions. With global warming and yearly climatic variations, Its always iffy for borderline plants. You may grow a tree for years and then an Artic blast hits your area and your 20 year old tree is gone in a matter of days. Or you can't deep water your 30 year old redwoods during a drought. Again, gone.

Another plant consideration is altitude. Some plants care about their elevation. You can grow a Valley Oak in the CA central valley but not in the mountains. You can grow a Black Oak in the mountains but not the valley. One of my favorite weird climate places is a little hanging valley at the top of Nevada falls in Yosemite Park. The elevation is 6000 feet. This little hanging valley is full of plants that never grow above about 2000 feet. A tiny Zone 9 in the middle of Zone 6.

Hope this isn't too confusing. Try not thinking too hard about it.

Daisy

Sunset zones are nation wide. They may be used more on the W Coast but they aren't limited to the west coast. Just google your location and Sunset Zone.

I didn't know that, Isaac, thank you!

Thanks for the explanations, guys, but what about the "least cold" zone with no upper limit?
I just don't get it.

Thank you for your quick responses.

I know there are many parameters, but what i didn't understand, this what can we "translate" of the second USDA zone. in my exemple of the Bearded Bellflower, the hardiness are zone 3a to 8b. i understand that the Bearded Bellflower will die if the temperatuire is below the zone 3a (-40°C) but what can i say about the zone 8b ? what happened to the Bearded Bellflower if the temperature go to the 8b zone (-6,7°C) ?it is just to understand , i know i have to take care with many others parameters, but i am curious and i would like understand what does that mean :)

thank you in advance

I think I see the confusion. The USDA Zones are based on an average of the MINIMUM temperature in each zone. So a plant hardy in Zone 3a to Zone 8b is a range of the minimum temperature requirements for that plant. The temperature range within each zone, for instance, Zone 3a says -40 to -37.2 C (-40 to -35 F), is the margin of error within that zone; the number came up somewhere in the middle. 8b has a range of -9.4 to -6.7 C (15 to 20 F). The Bearded Bellflower would be happiest in a zone with a minimum temperature within that range. But they don't like the heat as they are Alpine plants.

The high temperature is a different consideration and one the USDA does not take into consideration. But, if you live in Zone 3a, it's probably never going to hit 37 C (100 F) at your house although it will in Zone 8b. The USDA developed their zones to be used by farmers, not home gardeners.

The Sunset Climate Zones consider length of growing season, timing and amount of rainfall, winter lows, summer highs, wind and humidity. A much better choice for home gardeners. Unfortunately, I don't think Sunset has worked their way all the way to Europe. I was surprised when 1lisac said they were now nationwide.

Daisy

Wow, I'm not getting updates on all the forums...even the ones I posted on.