I've grown plants of many types for more years than I want to admit (the number would show my age) and I just realized there's something about hardiness zones I fail to understand (it might just be my elevator isn't going all the way up right now).

Anyway, I live in zone 7b which I know means it has averaged 0 to 10 degrees F for the winter cold over past years; I get that, and I understand that a zone 8 or 9 plant isn't gonna make it through the winter on its own here. I understand "close" and mulching the daylights out of plants and have been successful many times.

What I realized I DON'T understand is this:
A plant is "rated" for say zone 4-9. But what does the zone 9 at the end mean? I don't recall ever seeing a top end (summer) for zone temps. I could see where a plant native to a zone 4 locale might bake in zone 10 but I haven't ever seen "top" temps/zones. So what am I missing here? And if there IS a top zone number could someone please point this old guy to it? Thanks!

Hi Bob,

Let me see if I can make this more understandable (or more confusing) . As you have figured out, the 'zones' are for the lowest temps not the high temps. So a plant with a zone range of 4-9 would be happy with minimum temps in those ranges. Usually that means that the plant needs a certain number of 'chill' hours (hours under 45 degrees). The zones were originally invented for farmers by U.S. Department of Agriculture. Fruit trees need chill hours (ie: peaches) or need temps above a low temp (ie: oranges).

Someone made the translation to everyday gardeners: think peonies. Peonies will be so happy in zone 3 to 7 (zone 8 is iffy). They need the cold to be happy. Or tulips... Tulips grow great in Reno but when I lived in California (zone 8), they lasted maybe 2 years at the most (and then they just don't come back).

But there is a high temp a plant will be happy with (remember that farmers are only interested in low temps). That's where the American Horticultural Society comes in. They have come up with a heat zone map that will be helpful on the other end:

http://www.ahs.org/gardening-r...

But not all zones are created equally. I live in Zone 6 or 7 but, our winters are dry and cold. The humidity often gets to zero with no snow. My friend, also zone 6 or 7 but in Minnesota, has 10 feet of snow every winter. Snow insulates. Without snow, our ground freezes to 24". In Minnesota, the freeze depth is 6".

Daisy

I'm Watching this thread to find out the right answer.

My guess is that if a plant needs some chilling to thrive or bloom, the "upper zone limit" might caution us that that plant needs the coldest cold each year to go below a certain temperature.

My beef with Hardiness Zone zone info is that it shows an AVERAGE of the last 20 years' coldest moments.
So it is a statistic that tells you that 50% of all your winters will kill a given plant (ignoring micro-climates and heroic measures).

I would be more interested in a statistic that told me that 90%, or 95% of the last 20 (or 50) winters stayed below X degrees. Then I could pick plants that would only be killed by cold every 10-20 years, where I live.

How many gardeners are such gamblers that they would DELIBERATELY pick plants with a 50% chance of dieing EVERY year?!?

"Dead AGAIN? Seems like this happens every other year!! The tag SAYS Zone 8 and I AM Zone 8!!!"

DaisyI said:
But there is a high temp a plant will be happy with (remember that farmers are only interested in low temps). That's where the American Horticultural Society comes in. They have come up with a heat zone map that will be helpful on the other end:

The problem with Heat Zone Map is it's not very widely used. I have never purchase a plant that has state what heat zone it is adapted to.

I choose plants that are in the middle of my zone requirements. I don't have to worry too much about heat (our highs are a couple 100 degree days a few times a year). I live in Zone 6, but plant only plants hardy to zone 7. Even then, I choose planting locations with care. The 'iffy' plants are on the south side of the house against the house wall. The heat intolerant plants are on the east side and north side where there is more shade. Microclimates... get to know your yard and find the hot spots and the cold spots, the sunny spots and the shady spots, and plant accordingly. And be prepared when the weather does not cooperate with the plan. That's gardening in the great outdoors!

Jay, I think eventually the plant people will get the idea and tell us what the upper temp limits are. In the meantime, you will have to figure that out through trial and error.

It isn't always the heat that makes a difference, sometimes it's humidity. Or even the angle of the sun and light hours that change the viability of a plant in a zone. Desert plants draw their moisture underground to wait for moisture and coolness, tropical plants sunburn and blister in their water retaining cells in the same zone.

Thanks Daisy, that helps some. Thanks to all the rest as well.

I figured there is more to hardiness than just cold as you mentioned with the examples of snow and insulation. I made the mistake of leaving Elephant Ears in the ground over the winter a few years ago. When I had them growing somewhere else in the yard they came back for years, but rotted in the new location. Now they get lifted.

I found one of my Bleeding Hearts sometimes dies in the summer and comes back when it cools back down; other times it stays green until fall. I tried to pay attention to such patterns but could not make a correlation with the zonal info - other plants with "lower" maximum zone numbers weren't affected, which is part of where my question came from.

As far as micro climates within my yard, I look at light requirements when deciding where to plant, then locate based on that need. Soil & water can be then be adjusted for the spot if necessary.

I too try to get plants that are hardier than my zone. I've had disappointments with zone 7b plants not being quite hardy enough for my 7b locale. I try to pay attention and put extra protection on any questionable areas when we have weird cold spells, so most do well over winter.

I try to make the best possible decisions when getting plants. I've selected alternative varieties many times because my first choice's zone was iffy. I would rather have a successful, happy plant than one that caught my eye but was not the best for our area. Gardening is supposed to be fun; seeing a plant die because of a lack of some research and making a poor choice isn't enjoyable.

And you mey well have places in your yard that are warmer or colder than 7b. I'm a 7b or a, myself, up in Maryland, but a garden against a south facing house wall is easily more tropical.

I like the try it and see (within reason, after some understanding) method.

Great question, Bob. My interpretation is that the USDA zones are exactly what they say they are "hardiness" zones. So they really only tell you - on average! - what will survive dependably through the winters where you live. This info is much more useful in colder zones than for me, for example, living in Florida.

So when a plant at a nursery is labeled "zones 4 to 9" well sure it will probably survive winters in those zones, but that doesn't mean it will grow as well in zone 9 as it will in zone 4. The operative word is "survival" and you don't want your ornamental plants OR your fruits and vegetables to just survive. You want them to thrive.

I live in zone 9a or 9b (depending which map you look at) now but lived (and still garden at my daughter's) in Salt Lake City, Utah, which was then zones 4 or 5 but is now designated zone 6 depending what part of town you live in. Altitude makes a huge difference to the success of plants, as well as humidity - or lack thereof - and summer high temperatures. A windy exposure can also make the sink or swim difference to a plant especially in a dry climate.

So I guess what I'm saying is I'm with Sally, as she says above "try it and see". If I eliminate a plant by the trial and error method, I don't keep on buying it just because the label says it's hardy in my zone. I might try it in a couple of different areas of my garden, though. For example daylilies need a little bit of winter chill at least, and so I've now found that I can grow them on the north side of my house where they are in the shade all winter, so the soil cools off, plants stay cooler in the daytime and get colder at night than they did when I planted them in a sunnier spot.

On the other hand, I can't make lavender (or sage) survive our summers dependably even though it is supposedly "hardy". It just doesn't do well with the high humidity we suffer for at least 4 months. If I can place it where it gets very little rain, such as under an overhang, and strictly control how much water it gets, it might limp through a summer (once or twice in 15 years) but never grows well the next year anyway. So it's treated as an annual in my garden. I buy it in the fall and just let it peter out come June or so.

DaisyI said:Hi Bob,

But not all zones are created equally. I live in Zone 6 or 7 but, our winters are dry and cold. The humidity often gets to zero with no snow. My friend, also zone 6 or 7 but in Minnesota, has 10 feet of snow every winter. Snow insulates. Without snow, our ground freezes to 24". In Minnesota, the freeze depth is 6".

Daisy

Where in MN is there a zone 6 or 7?? And FWIW, if the freeze depth were only 6" here, we'd have a LOT of unhappy ice fishers with trucks at the bottom of lakes. I don't know.. maybe dirt is different than ice. I get your point though, the snow insulates so the coldest cold is kinda 'mulched'. That said, you can't count on that here. We've had some pretty snow-weak winters lately and the months of below 0 temps on bare ground are harsh. Then on the flip side, the plants get May when it can be -10 one day, 58 the next day, and -10 again the next day. Then a week later it is all 85 degrees days for a week. Then it might snow on us. Then it gets super dry and doesn't rain for 3 weeks. Then it is humid and sticky and 98 degrees with no rain but it feels like you're walking through a swamp. I respect any plant that can grow here, no matter what the zone rating says!

That's exactly why the hardiness zones really aren't worth much - because they are "averages" and no winter is going to be average. There are too many variables.

And were designed for farmers, not home gardeners.

>> When I had them growing somewhere else in the yard they came back for years, but rotted in the new location. Now they get lifted.

Lavender is supposed to be sensitive to wet feet, at least over the winter. Two plants side-by-side, getting the exact same weather, will live through the winter or not depending on how well-drained their soil is.

(I assume that is more relevant when the winter cycles around freezing much of the time. I can't imagine roots caring about drainage if they are frozen rock-hard early in Winter and stay that way until Spring!)

I really delights me that plants (and weather, and soil) are so complex that even experienced people have to "try it and see", or experiment. Horticulture MAY be a science if you know enough about some plant varieties in certain regions, but it sure seems to be an art!

For hardiness itself, if a plant can survive a zone 4 winter then it better be able to survive a zone 7. My initial question was the second number and as I understand Daisy's explanation it has to do with needs of the plant for blooming and/or other things which have nothing to do with just surviving winter.

I agree with all of you that planting is about location, location, location. One spot may be shaded nearly all day while another is sun all day. Exposure to/protection from wind will affect them positively and negatively depending on temperatures & humidity - the same wind that requires protecting a rose in winter is wonderful for helping dry it and preventing fungi problems in summer. No spot is necessarily bulletproof year round, we have to be diligent and aware every season.

Gardening is rewarding; it is also challenging. If all that was involved was sticking a plant in the ground and walking away it might be easier, but how much fun would it be? "Having" to tend the garden gives us time to stop and spend a moment with each one we planted, to appreciate its uniqueness, maybe to reflect on the idea that sometimes it is growing in spite of us. It can be humbling to realize we might have planted for "me", but an entire community of bugs, birds, and critters are living out their lives from and around it. The challenge of placing it where IT needs to grow rather than where it's best for US helps remind us the world does not revolve around us. I appreciate being able to walk out on my porch to look at orchids rather than having to tromp through a rain forest for days to see it in its "real" world, same with any plant in the garden. The privilege of having the plants of our choice close at hand is worth the little bit of effort to determine which ones to plant and exactly where.

You can't even depend upon "if a plant can survive a zone 4 winter then it better be able to survive a zone 7" because even though the winter may be longer and colder, it could be more consistent. It might stay warmer much longer in the zone 7 location, but if the plant keeps growing Into December, and then is subject to freeze/thaw cycles where the similar plant in zone 4 might stay frozen (or have been mulched better or buried deeper or have snow cover) the plant in zone 7 might not fare as well as the zone 4 plant that died back earlier but stayed cold.

Another example would be a hardy perennial that still thrives in my zone 9-ish might only live 3 years for me, where it will last 10 years in zone 4 or 5 or 6 because here in Florida it puts on close to 3 seasons' worth of growth in one year.

You've hit the nail on the head in your last statement, really. Planting the right plant in the right place is what it's all about and the challenge to find those two things is what keeps us fascinated all our lives.

The higher number is the upper limit. Not all plants can handle tropical conditions and need a winter rest. There are several plants I can't grow here in south AL that were common to me in OH. Lilacs (Syringa,) tulips and Hyacinth bulbs come to mind readily. There's not enough chill hours for the buds to form, which happens during winter rest/dormancy. These plants common to a central OH landscape are nowhere to be found near the FL border. If one wants a tulip or Hyacinth to re-bloom here, it must be artificially chilled.

There are other plants that just don't tolerate high temps, but I can't offer a specific, personal anecdote. Plants called alpine are a good place to start an investigation though, as they are generally averse to high humidity, low elevations, and high temps.

From wiki vis USDA

A hardiness zone is a geographically-defined zone in which a specific category of plant life is capable of growing, as defined by temperature hardiness, or ability to withstand the minimum temperatures of the zone. The zones were first developed by the United States Department of Agriculture (USDA), and have subsequently been adopted elsewhere. They are categorized according to the mean of the lowest temperature recorded each winter, termed the "average annual minimum temperature". Thus if five successive winters reach respective minima of −14 °C, −12 °C, −8 °C, −16 °C, and −13 °C, the mean coldest temperature is −12.6 °C, placing the site in zone 7.

Benefits and criticisms
The hardiness zones are effective in that, for many situations, extremes of winter cold are a major determining factor in whether a plant species can be cultivated outdoors at a particular location. However, it does have a number of drawbacks, most significantly in not incorporating summer heat levels into the zone determination. Thus sites which may have the same mean winter minima, but markedly different summer temperatures, will still be accorded the same hardiness zone. An extreme example is the Shetland Islands and southern Alabama, which are both on the boundary of zones 8 and 9 and share the same winter minima, but very little else in their climates; in summer, the continental climate of Alabama is about 20 degrees Celsius hotter than the oceanic climate of Shetland, and there are very few plants that can be grown at both locations. Due to its maritime climate, the UK is in AHS Heat Zone 2 (having 1 to 8 days hotter than 30 degrees Celsius), whereas Alabama is in Zones 7 to 9 (61 to 150 days hotter than 30 degrees Celsius).

Another problem is that the hardiness zones do not take into account the reliability of the snow cover. Snow acts like an insulator against extreme cold temperatures, protecting the root system of hibernating plants. If the snow cover is reliable (then present during the coldest days), the actual temperature to which the roots are exposed is not as low as the hardiness zone number would indicate. As an example, Quebec City in Canada is located in zone 4 but can rely on an important snow cover every year, making it possible to cultivate plants normally rated for zones 5 or 6, whereas in Montreal, located in zone 5, it is sometimes difficult to cultivate plants adapted to the zone because of the unreliable snow cover.

Low temperature is not the only determinant of plant survival - other factors such as high summer temperature, humidity, soil temperature, etc. may be equally important. Also, many plants will survive in a locality but won't flower if the day length is inappropriate or if they require vernalisation (a particular duration of low temperature). The low temperature statistic is only appropriate for woody perennial species, and even then its use is limited. With annuals the time of planting can often be adjusted to allow growth beyond their normal geographical range.

What I am seeking is REQUIRED minimum temps for certain plant species. For example, Junipers are often used as bonsai specimens and sold for indoor use. If a Juniper species is listed as Zone 4-9, I understand how much cold it will tolerate. However, how do I determine how much cold is REQUIRED for it to survive? Experience tells me that normal home temps are too warm for it and many other plant specimens that require cold winter temps.

If the zones say 4-9, the minimum chill requirement would be that of a normal Z9 winter. Those Junipers shouldn't be sold as indoor plants, IMO.