Each temperature range is where the result of the averaging of all the lowest temperatures for each year during the period of measurement falls. So probably just about every area gets colder than the lowest temperature in the range for the zone that area falls in. And there is no such thing as a Zone _ temperature, as the full range of actual temperatures for each zone is liable to be broader than the average range - 10 degrees F is no more unique to Zone 8 than it is to any other zone where it gets that cold.

Last time I looked the USDA web pages on the hardiness map and zones recommended choosing plants rated one zone colder than your zone.

Yes, nurseries and others who should know better look at the USDA zone average ranges and assign zones to plants based on those, rather than how much cold each plant can actually come through in one piece, and how cold it actually gets in each zone. If a plant freezes at 10F, it's rated Zone 7 and so on. And if a garden or nursery site sometimes gets below 10F, then it is in Zone 7 - and the plants offered, that have survived that site are therefore hardy to Zone 7.

When in reality it may not even stay above 10F in parts of Zone 8, such as where I am - I have seen it below 5 a couple of times - admittedly over a period of decades. But that has been without a maximum/minimum thermometer showing the coldest it got each night, just what it happened to be when I looked out the window (and assuming the thermometer being looked at was accurate. Elsewhere near here the official weather stations have recorded 10F inside Seattle and 0F at the airport south of Seattle).

And when you are planting trees, shrubs and other plants you want to last indefinitely - and planning to be on the site for decades yourself, or at least not leaving a future dead tree for somebody else to have to dispose of - those occasional steep drops do matter, more than the fact that most of the time it is not that cold.

This post was edited by bboy on Wed, Sep 4, 13 at 23:57

Good summary, bboy. That's pretty much what I'm saying - the averages aren't all that matters. In fact, I'd be more interested in a map that puts us in zones based on, say, the all-time record low, or at least "coldest temp in last fifty years" etc.

For Akron that would be -25F. For here, if you use BWI Airport as a guide, I think it would be -7F, but it gets several degrees colder here in Howard County.

I worry less about straight temps than say exposures, winter moisture levels and timing of leafing out. Those are more easily identified through the plant's genetics and microclimates in your town/city/yard etc.

In my zone 5a, I typically find plants that can at least withstand -20 and I'm what you'd call a collector.

With that said I'm not going to not plant because there is a chance over a 20-30 year period that might possibily cause dieback or kill a certain plant when we're talking about a 10 degree variance in whether a plant is zone appropriate or not. I feel like you'd be splitting hairs at that point.

However I like the suggestion of planting a zone hardier for more indefinite plantings and larger plantings. All my shade trees/screening plants are zone 4 or can handle the lower end of zone 5.

At the end of the day don't let a 10 degree published variance in zones effect your plant diversity.

That's why experimentation with marginal plants is called "Zonal Denial". If it floats your boat, have at it. But don't start telling other people, who don't want stuff they plant to die or die back in a cold winter that a plant is "perfectly hardy" etc. because you have been able to overwinter it for a time. It's not that hard to find people doing this who have only gotten a suspect plant through a single winter! Plant purchasers who are not enthusiasts often really do not like post-planting failures. And even some plant nuts are exasperated or mystified by any losses that may occur: "The dumb thing died!".

I have some (6) Live oak hybrids doing fine in zone 6;. If I live long enough to see them thrive in a warm trend, at least 25 years I'll be happy.

Bboy - me being in canada, anything I grow seems like "zone denial" for most people!

Some of my marginals in zone 2-5 (my yard is roughly a 4)

Fargesia rufa ( bent and tarped and buried in snow and leaves)
Phyllystachys alata (same as above)
Blue boy/girl Holly
Musa Basjoo (first winter, wish me luck!)
Magnolia tripitala (survived the winter in a dixie cup outside with no protection besides snow)
Figs (first year with chicago hardy)
Seedling pear (it was from a bosc pear. Seeing as all european pears minus one or 2 "wont grow here", the seedling is hardy as hell so far)
Himrod/niagra grapes (himrod makes it here to the snow line. If i bend the cane down to the ground and mulch im sure it will surive. The niagra is new this year.

I just keep thinking that one hard winter like one of the mid-80s or mid-90s would seriously damage a LOT of the landscape plants around here. Crepe myrtles & Cedrus deodara in particular. The Trachy palms that are scattered around the District too.

Enjoy them while you can!

Screw zone denial. Educate yourself on the purpose of the planting and go from there.

"Tis better to have loved and lost, Than never to have loved at all."

Edit: I should clarify (which I've already said in another post) that you can/should experiment within reason. For example the proposed optimism for the zone maps are likely in that 10 degree range.

So for me in zone 5a, I may push and try a 6a plant. Keep in mind that by experimenting the community may find that a plant is actually more hardy than what was originally reported. Take Bosnian pine for example. That was and is reported to be a zone 6 plant. Now its reported a performer in zone 5 and in some cases warmer areas of zone 4.

For me to try a zone 7 plant well thats just silly.

This post was edited by whaas on Thu, Sep 5, 13 at 20:51

A lot depends on how well characterized the plant is in the references being consulted. If you take one kind of plant and start reading about it in every source encountered that mentions its hardiness you can end up with a range of designations.

Hardiness of seedlings of wild species can vary significantly from one plant to another. Representatives from different parts of the natural range can have differing levels of cold tolerance - and related siblings may also not be uniform in this respect.

Well to be fair, even something "hardy" can die in its first winter if it was grown in the wrong climate, or it didnt have time to haden of. Even native plants can be damaged in this way.

Experimenting w/caution is OK. Some observations on zone-denial here fully exposed to wind:

Longleaf, loblolly, pond & Bhutan (white) pines, after some bud-damage here in z6 when very young, are now established & show no cold effects for yrs. S Magnolia & umbrella magnolia also do well. Granted, haven't had a truly severe, ground-freezing winter yet, like Dec-Feb 76-77, Jan 85 or Jan 94...

This post was edited by beng on Fri, Sep 6, 13 at 10:56

Check THIS out:

{{gwi:429174}}

The "original" 1960 map looks a lot like the 2012 map. Which makes sense, since, esp. in the eastern half of the country, the 30s, 40s, and 50s saw warmer winters on average than would be seen in the 60s through the 80s.

Location of production facility where stock was sent from generally has no effect on hardiness - except when stock is sent north during spring - in an advanced state of growth - gets frosted after being set out in northern sales yards. Key point is genetics of plants not changed by being grown in a milder area. If it is the same clone, a plant is still inherently hardy to Zone 6 whether the examples bought came from a grower located in Zone 6 or Zone 9. Where southern origin could have a lasting effect on northern outcomes is when a specific batch of seed-raised plants of a wild species has not been previously tested by northern winters - and it is a species that varies that much in hardiness. Cedrus deodara, for example.

Or a cultivar is represented by more than one clone, all being sold under the same name, but not having equal hardiness. The stock from the southern grower would not have revealed itself to be less hardy in the north than other clones, until that stock was sent north. But this is not something to expect to be getting stung by on a regular basis.

This post was edited by bboy on Fri, Sep 6, 13 at 17:14

Bboy - I beleive it wasnt the hardiness of the plant itself, but the hardiness of the growth in the zone. Almost all the plants I get here are grown in toronto or BC. Even hardy plants need to be babied in their first year. It is possible I am thinking of in the spring. I was thinking a long the lines of the current growth not being used to the temps going north.

Again, we could be talking about the same thing... lol

A large amount of the outdoor stock sold here originates in California, because it doesn't, in fact matter that it started out there - if a plant is a hardy kind. Chance of being caught by cold because growth is more advanced in the south goes away as soon as weather in new location warms or growth hardens - plant will not be more tender months later because it used to be living in a softer climate. If plants shipped in from farther south are still vulnerable to cold a year later it is for reasons other than them having once been growing in a different climate.

I thought this article was interesting. It may show what the weather would be like in the near future.

Here is a link that might be useful: The Climate-Grain Production Relationship Quantified

Here we were said at one time to be going to have shifted to cold dark springs and hot summers by 2050, seems like it may be starting already.

In my zone 6 the USUAL lows are about 20 F and some 7-8 F here and there. There are (some years) below zero a few days. I may be thick-headed, but if they are going for the AVERAGE lows, not the LOWEST lows, for me about 8F at the most intense part of winter, I should be at least zone 7. It only makes sense to me to go by the ultimate lows you can usually expect at the worst of winter, for me up to -10 F. Otherwise you plant for YOUR AVERAGE lows, and then your plant croaks in a under average temps. I can't see why you shouldn't plant for the lows that are not average but have been experienced every 6-9 years. Of course you may experiment with marginally hardy plants, that you know may not make it.

That would be why they tell you to plant for the next zone colder. Somebody is in Zone 7? Then they would plant stuff rated hardy to Zone 6, and so on.

I've always wondered why they don't use something like "all-time record low" or "lowest absolute temp in the last XXX years" instead of "average lowest".

I think it has to do with how plant hardiness is determined. It's more than just a specific temperature. It also has to do with the duration of the cold, etc.

If it gets to 10 below zero here in MD, it's short-lived and usually rebounds into the teens or twenties the next day, and closer to freezing by the day after.

In Ohio, there are many times when it's -10 for a low that the HIGH stays in the single digits.

Hence, the DURATION of the cold. Warmer zones might have an all-time record low some 15 degrees below their zone rating, but generally short-lived. That doesn't mean zone-hardy plants can't sustain damage though.

As far as people saying "well I'm z6 but it rarely gets below zero" I've seen that too.

The last 10 years or so has been an anamoly. I actually charted the last 40 years of winter lows (lowest temp from each calendar year) for the three major airports around Washington, DC as well as Akron, OH.

The zones are accurate as they are now shown, using the data from '76 to 2005. As I said above, I think it's optimistic, knowing how cold it CAN get, and considering that the 70s and 80s were QUITE cold, and the 90s and 00s QUITE mild, compared to the norm (except '94 and '96).

Yet, it works. For Akron, OH, the "average lowest" temp is about -8F over that time period, but has ranged anywhere from -25 to +11 or so.

For DCA (Reagan National) it averages out to about 9.9F if I remember right. I have the spreadsheet on another PC and will try to copy a pic of it later. However, the way it works out is in a ten year period, you have 7 years where the lowest temp is in the teens, and three in the single digits or colder, and it averages out as above.

Dulles averaged out to around zero. It actually DOES work when you chart the numbers. The MEDIAN lowest temps are a few degrees higher than the MEAN (AKA average), but not by much. That just means you have quite a few "slightly above normal" years for every one or two WELL below normal years & "normal" years.

It seems going back some years that overall trend actually has held true, with the distinction that the 40s and 50s were warm, the 60s, 70s, and 80s quite cold, and the 90s, 00s, and 10s (so far) rather mild.

HM4EV

Your first few sentences are right on.

A plant that is rated at lets say zone 5a which is -15 to -20 may easily survive a -25 degree day or two. It depends on so many other factors like duration, exposure (inclduing sun and wind that day), moisture, plant species, etc. Should we even get into root vs. bud hardiness and how snow cover effects this!?!

You may only experience a few days within a given decade in which you even hit the -15 to -20 range. My short tenure with gardening in the last 10 years has yeided a low of negative 13.

If I had to plant one zone lower that means I'm rolling with zone 4a which is -25 to -30. Why would one plant their garden based upon the possibility of a one time record extreme in a 30 year period dictate their enjoyment over that time with just the possability the plant might die? I mean thats potentially a lifetime for many gardeners.

I do support this strategy with long term shade and privacy plantings though!

I am much more concerned about having a dry, windy winter that is AVERAGING lower than normal temps throughut the winter....ie having many day time highs in the single digits than a couple extremes in the -20 range. If my area is notorious for these types of winters then I have to 1) Understand the plantings purpose 2) Avoid species that fold to these conditions and 3) Use my micro-climates to their fullest potential.

At the end of the day this is my view point as a zone 5 gardener. The options really drop off when you go into zone 4 so I tend to keep a more optimistic view point. For someone in say zone 6 as far as I know its not like you enter a whole new world in zone 7.

A zone 7 for me is somewhat different than a zone 7 in NC in that my average winter temps are colder, but the overall temperature swings a bit smaller, hence the bare winter minimums being about the same, more or less. Summers are about the same. NC has a bit more rain than I do.

Then my zone 7 is DRASTICALLY different than a zone 7 in New Mexico, much more so than NC.

Whaas, for me the difference in zone 6 and 7 are really only noticed when it comes to some zone 7 palms like windmill palm and a couple other palms. The Southern live oak is said to be zone 7 hardy. I am trying out another live oak hybrid that is okay so far from surviving last winter, so far so good. Some Cannas and Elephant ears are zone 7 hardy. Most common perennials are zone 6 or even 5 hardy, though.

The one thing I did not see in this thread, is the best indicator of the long term climate. That indicator is the trees and plant life you see in the wild area of the place where you live. These areas have developed over hundreds of years, and only those plants and trees that you find are the ones that have adapted to that area.

So put down you zone maps and take a hike through the woods on a nice day and carefully observe what is growing.

For trees I use the Arbor Day zones

Reviving this old post...

My better half and I have looked into SE PA (anywhere from Lancaster to the western Philly 'burbs) as a possible future home. Much of that area shows up as z7a on new maps, but 6b on old ones.

However, while the native vegetation is nearly identical, landscapes differ a bit in that you rarely see crapemyrtle there, and it's common here, and you NEVER see a Cedrus deodara up there outside of arboretums, but they're common here. We have family there and spend time up there. You also can pull off Douglasfir and Giant Sequoia there a bit more easily than here, mostly because summer temps are just a couple degrees cooler, esp at night.

While it might be that people simply haven't tried those plants, I think this is evidence that calling SEPA zone 7a is a bit optimistic anywhere outside the Philadelphia city proper & a few areas to the south of there.

Does anyone remember 1994? The temps in parts of Lancaster, York, Bucks, and Chester counties approached -20F. Most "true" zone 6 plants would weather that OK if established, but most zone 7 plants would perish or be severely damaged at those temperatures...and they can happen again...even if 1994 was a once-in-a-century event for many of us. This winter, in many ways was worse for marginal plants than '94, even though minimum temps weren't nearly as bad, the DURATION of the cold was far worse.

I'm not saying I wouldn't plant something rated to my "new" zone, just that I would still consider it zone-pushing at the end of the day, and adjust my expectations accordingly.

Final note - I think zone-pushing is much easier for an herbaceous, rhizomatous, or bulbous perennial (like the Cannas, etc poaky mentioned) since it's a lot easier to protect roots or rhizomes/bulbs in the ground with mulch, than protect a 40' tall tree.

I don't have time to read this entire thread, but I wanted to make a few comments.

We're seeing USDA zone creep (updated maps) based on recent temps (not counting this winter!) or maybe based upon heat islands.

And growers, in hopes of making a sale, will extend the hardiness rating of a plant by a half to a whole zone, perhaps based upon a few mild years testing.

We're also seeing people planting things that have survived numerous mild winters. But not really zone-defining winters.

Also, hype in on-line discussion boards and availability of border-line hardy plants from vendors encourages people to try border-line hardy plants that just may not survive long-term.

And new plant introductions with claims of hardiness that just have not been proven.... (I'm thinking of claims by one grower of zone 6 hardy gardenias, for example).

This post was edited by dave_in_nova on Fri, Mar 14, 14 at 11:53

Dave I vaguely remembered this thread and thought it might get bumped.

Your points are all valid especially regarding vendors trying to inflate hardiness ratings, in some cases. Some examples are more innocuous than others. Take for example, Gardenia 'Kleim's Hardy'. I got mine at the old Styers, before it was bought by Urban Outfitters. They wisely put a disclaimer on the tag saying their regular 1 year warranty did not apply. However in my location the plant has never shown a lick of damage, down to 7F. And I have no doubt that somewhere, probably in the upper south like the high piedmont of NC or TN, it withstood 0F with minimal injury and thus its typical trade zn 7 rating was reasonable. However, after the 8th coldest January in Baltimore, coldest winter overall since 94, etc., it looks almost completely dead now. Even though it was fine after the first round of 3F, each additional freeze and rain/ice/snowstorm event (remember, most of China doesn't have the winter precip we do) sucked more life out of it. So even though this plant was reasonable rated zn 7 hardy, it wasn't hardy in a bad (for my area) zone 7 winter. FWIW, a smaller 'Chuck Hayes' is slightly less damaged seeming.

OTOH, there are some merchants who grossly inflate hardiness in a purely dishonest way. No need to revisit that, although I have to say it's pretty hilarious to see the introductory catalog statement of a certain very popular mail order business this year has an "I told you so" when they've been telling their customers the opposite for 20 years.

As for the map itself, I do think it's more accurate than any previous version, but still not perfect. How can it be? The USDA zone system is very limited. I posted somewhere else on the internet once that with the ability of computers to crunch data these days, the nursery industry really _ought_ to be able to come up with something far more advanced. Sort of taking the Sunset zones as a starting point, but considering many additional factors like overall duration of winter, likelihood of sudden freezes at critical times of year, etc. But that will never happen; as it is, many people are "confused" by the current USDA zones, as Bboy is always (correctly) reminding us.

David - who's this nursery that has the "I told you so"?

Clues are fine if you don't want to spell it out...or email me through my GW page.

Where duration of cold becomes significant is when it results in deep freezing of the the soil and root zone. This is why plants that come through one or two really cold nights in the south are not then proven to be hardy in northern climates that have similar lows - in the south it may be summer-like again the next day or a few days after the hard frost. Once the roots freeze evergreen plants in particular cannot keep the top hydrated unless they are northern adapted and have special coping features like the tight rolling up of rhododendron leaves.

Otherwise it only takes a few hours below a plant's minimum temperature for damage to occur. The least hardy part of a plant is the newer roots on the outer edge of the root system; the next zone of vulnerability is the mature roots on the inside of the ball and the most hardy portion is the top. This is why plants in small containers have been found to be about 20 degrees F less hardy than the same kinds growing in the ground. And why when potted stock exposed to possibly destructive cold "came through fine" because the tops look okay that may be a mistaken assessment, the first place to look is inside the pot.

I think even the "upper end" zone rankings of many plants can be modified. Most of the time, the warmest zone in a plant's zonal range is due to either winter chilling requirements, summer heat, or both.

A plant that might wither in summer and fail to meet it's chilling requirements in winter in z8b in Savannah, GA might do just fine in a z9a in Everett, WA.

The reason being - A - cooler summers in WA and B - the AVERAGE winter temps (daytime highs in particular) in the PNW are much cooler, meaning more chilling hours will accumulate than in GA, where many daytime highs reach the 60s & chilling mostly only occurs during the night-time hours.

Sunset-type zones are a good start - but even the Sunset zones are a lot less useful east of the Rockies, IMHO.

It gets even more complicated when you consider that insufficient summer heat can have a negative effect on winter coldhardiness - that might be why +5F degree temps in Seattle kill the same cultivars of Cedrus deodara that survive -5F in Maryland (just a basic example).

The Zone maps are based on average low temps. Takes the lowest temperature of each year, adds them up, and divides by the total number of years to get the average low temperature. That number will always be higher than a year when we have extreme low temperatures. For permanent trees we don't want to risk losing, it's best to go down one zone for hardiness.

The number is calculated for the last 30 years I think, and the average came out to be warmer calculated for the last 30 years.

Like other mentioned, much easier to protect smaller plants. Also short dips are less harmful than prolonged dips. And of course there are local microclimates.

WannabeGardnr, it is probably advisable for permanent focal plants to plant a zone colder, but, HONEST nurseries and publications take that into account when rating plants - a plant that has an absolute hardiness limit of -20F is NOT a z5 plant, although many less-than-reputable nurseries might try to say it is. More realistically, it's a z6 plant at best, since a colder than average winter would kill it in zone 5.

If you want to really play it safe, try to find the all-time record low for your location and go off that.

What Zone 9a in Everett WA? It appears to me you can tell where Zone 9 starts because the Cordyline australis are able to keep their tops indefinitely and make Joshua tree-like specimens. I personally haven't seen this consistently demonstrated north of Coos Bay.

Here is a link that might be useful: Intellicast - Historic Average - Everett, Washington

This post was edited by bboy on Fri, Mar 14, 14 at 16:26

I stand corrected - the new map says 8b. The colors for 8b and 9a are close...still, the example I gave works...

{{gwi:429175}}

Meh,

We are in a cooling trend first of all, but that's why the Warmers call it climate change...duh ...climate is always changing - so just a money making carbon tax scam. Saying that :-).....I use Arbor Day for trees, USDA model (new) for plants. I think it's fairly accurate but also believe Sunshine index etc are also important. I seen your 7a temps on the east coast and for averages that's fine but we already had 80 degree and multiple 70 degree days here in SW Missouri supposedly a 6b area USDA or 7 in Arbor Day. Far warmer than you Maryland folks have been and that seems consistent year to year.

What I think is more important is cold duration, and high temps, not averages.

No. The maps arre based on scientific data, which is petty non bias. The data is over the last 20 years, and generally speaking the trend is probably going to continue, or change at the very least, just like it did since the previous 20. Short term data is easier to use for short term averages.

bboy is right -- potential soil-freezing is perhaps the real limit. Soil freezes roots which can't draw up water & foliage suffers or dies, especially evergreens (so newly planted, unestablished stuff is very vulnerable). This winter has been fairly brutal, but fortunately enough snow cover to limit soil freezing, so my zone-challenged evergreens show little effect -- longleaf, loblolly, pond and Bhutan white pines, and Southern mag.

BTW, 3 inches of new snow this morning.

As I have stated before in this and above in this thread. The data used during the generation of the new zone map was during the "warm" phase (warm for the eastern US) of a multidecadal oscillation of the Atlantic. This alone is enough to "inflate" the averages. We have now entered the "cold phase" which this factor alone would trend the averages downward all else being equal. With that said, global climate change is real and IS having an impact in many predictable as well as unexpected ways. One of which is weakening of the jet stream which make it easier for cold air to spill down SOMEWHERE, resulting in regional cold while the system as a whole warms. The question then become WHERE? This winter was eastern North America. Last winter was Europe.Winter before that was China. In each of these, while these areas were very cold, nearly everywhere else was not. So based on this, I'm sitting tight on any long term plantings. The average may go up, that variation will still kill as many are seeing this winter in the eastern US.

Arktrees

This post was edited by arktrees on Mon, Mar 17, 14 at 11:28

8 inches here this AM, beng.

The soil probably froze to 4" or so - a lot for us, but not deep enough to hurt an established evergreen for the most part.

Lol ....scientists are dropping off the climate change bandwagon faster than you can say snowflake. The earth has been far colder and far warmer thousands of times throughout history no different than the change in climatology of today. I could give a cr*p if we have climate change ....IT'S suppose to change, it's not suppose to stay the same. Every planet in the solar system heated up the last decade because of the sun - its natural.

All the carbon dumped into the atmosphere since the industrial revolution began equals 10 times less than one volcanic eruption of Krakatoa.

Sorry Joe, but you have little understanding of the science, nor the amount of CO2 from Krakatoa, nor are actual climate "scientists dropping of the climate change bandwagon". Believe what you choose, but that does nothing to change that there is a great deal of evidence supporting human caused climate change. It's true not everything is due to human induced climate change, but it is absolutely just as true that a single 7.2+ Billion species of large industrious mammal inevitability has massive consequences to the planet.

Arktrees

This post was edited by arktrees on Mon, Mar 17, 14 at 22:04

Long forecast shows more cold waves on the way:

Here is a link that might be useful: More cold waves

We didn't exceed minimum lows for zone 5, yet I have plants dieing left and right after 4 years in the ground. Some of the plants are listed to zone 4 too.

Granted these plants aren't fully established but cold duration and overall averages seem to have much more bearing.

Roughly seven years ago we saw similar lows (approaching -15) but the overall averages were...average. No damage that year what so ever.

Local nursery trade with legacy associates there since the 70s said this is by far the worst they've seen. Some what subjective but none the less it was the coldest on record since the late 70s.

The winter was surely a zone buster. Hmm, I must have March Madness on my mind. Go Badgers and Panthers!

I am not educated in the scientific data. I just want to ask, isn't El Ninio and La Nina, responsible for the direction the path of winter winds. One has winds originating from the SW being warmer, and the other originating from the N.W., bringing the colder air to the N.E. of the U.S? I thought that ice ages had come and gone before the industrial age and the car. My zone 6 low of -10 was exactly a zone 6 minimum. We were forecasted for -11 F, but my yard was -10 F exactly. My local news channel said this winter has been the coldest in decades. I am confident in being zone 6. I think next winter won't be as much of a b**CH.

Well, I had a zone 6 winter here in SW Pennsylvania. We had -10 F for at least 3 nights, and -8 F for 7 nights approximately. The live oak "late drops" survived, but have LATE leafout this spring. One is just starting to leaf out. The other 2 will leafout in June. Be patient, they are going to leafout. The harsh winter has stalled them quite a bit though.

I found a high resolution copy of heat zone map. http://repdomino.monrovia.com/MonroviaWeb.nsf/8c104835579b67e18825685f006acdf8/f99d2ba9eb2a9ab58825692000737444!OpenDocument.

Since this got bumped, I'll just add that the Live oak "late drop" had died back to the ground, and sent up new tops, 4 leaders in all.

You need to remove the period (full stop!) at the end for that URL to work.
It really isn't that high rez. anyhow.
But also, even more than the USDA hardiness map this AHS heat map is of very limited utility. Number of days over 86F is essentially meaningless for a lot of plants. Right off the bat, it would have made more sense to plot the average summer nighttime dewpoint temperature. At Sonoma Horticultural Nursery in - surprise surprise - Sonoma County CA, they can grow the Himalayan cinnabarina series Rhododendrons. Or the hardy big leafs like R. rex. At Rarefind in NJ, you cannot. Even the ones that would be winter hardy there. The two locations probably have a similar # of days above 86F, but their summer climates are hugely different as are their winter climates. The cool nights and lower dewpoints in Sonoma County make all the difference.

Yes, David. In my experience, unless you are talking hot enough and/or sunny enough to actually scorch leaves, hot nights are a far bigger issue as is humidity than daytime max temps. Especially for some cooler/drier climate conifers like Sequoiadendron, many Abies species, etc. Soil temp, type, and moisture is also a factor on both ends. Some plants tolerate heat better with higher moisture, some, due to fungal and root rot issues, actually prefer to be drier if it's hot.