Hardiness Zone Phoenix question

[wrigjef]

Hey Ya’ll,

  I live in Phoenix and for as long as I have lived here (20 years) we have been universally considered a 9B zone.  
  It’s my understanding in weather we measure in 30 year increments. (Correct me if wrong).  Also it’s my understanding that Hardiness zones are measured by the average annual extreme minimum Temperature.  Which means you take the lowest temp recorded each year over a 30 year period add them all up and divide by 30 to get the average minimum extreme temp.  
  Once you have the average min extreme temp you refer to a hardiness zone map to establish the zone you are referring too.  

Hardiness zone 10b Average minim 35-40

Hardiness zone 10a average minimum 30-35

Hardiness zone 9B average minimum 25-30

If this information above is correct then Phoenix would be considered a 10a zone because the average minimum temp in Phoenix over the last 30 years is 34.6 degrees.

I have researched the minimum temps in Phoenix each year extensively plus I keep 3 thermometers in different spots around my property to measure at home so I can concur with the results I read.

I was very surprised most years Phoenix did not see a freeze 20 of 30 years.  Most years when we did see a freeze it was very close to    32 degrees.   One time in 1990 Phoenix got to 26 degrees but since then 29 has been the lowest (1988-2018)

So the question is this;

Assuming 34.6 degrees over the last 30 years is an accurate average minimum for Phoenix, are we in a 10a Zone?  OR because we hit 26 degrees at any point over a 30 year period we are considered a 9B zone?

Responses appreciated,

    Jeff.  
 

Edited November 22, 2019 by wrigjef

[kinzyjr]

@wrigjef According to the definition of a hardiness zone, yes you are a 10a.  If you round up to the nearest degree, you could even brag and say low 10b.  As you noted however, zones don't tell the whole story and planting based on zones alone can lead to some pretty bad results if you get that proverbial 26F.  Most of us in Central Florida's urban areas could make a similar zone argument for our respective locations.  As an example, see the graphic below for Lakeland, FL over 15, 30, and 40 year periods:

While I love living here, my advice to local growers would not be to plant an entire garden full of coconut palms or Adonidias based on these charts.  I'm also keeping it in the back of my mind that 2019 isn't quite in the books yet, so that 36F could change.

[Jeff985]

You’re also looking at different years than the hardiness map.  It uses 1974-2012. Houston has the same issue. If you look at the last 30 years the average is between 28.5, but it’s still classified as 9a. The next time they update it you should officially be 10a. 

[RJ]

The 70's and 80' from a historical perspective (prior as well) were quite cold decades. As kinzjr alluded to, averages are just that: averages. Your plants could care less about averages they are mostly concerned with absolute lows. That's what the USDA maps tell us though. They give us a range of average lows for a given area. Like it or not most of us here in the states live in a continental climate, and with that comes some pretty wild temperature swings. 

[AZPalms]

Hi Jeff! I also live in the Phoenix valley. I agree that inside the loop 101 area could be considered a 10a most years. I’m also curious as I’m sure you know, the valley here is so diverse in elevation, population density (urban heat island), mountain protection etc. I travel across the valley daily for work. In the winter months when I leave my home in Waddell, it’s not uncommon for me to see temps begin at say 31f, climb to 34 by the 101/i10, 37f by the tunnel, back down to 33 by the 202 etc and so on. Similarly, the same for summer month temperatures. 

I’ll be curious to see when they update the zones how the scale the area. 

 

Max 

[wrigjef]

13 hours ago, kinzyjr said:

@wrigjef According to the definition of a hardiness zone, yes you are a 10a.  If you round up to the nearest degree, you could even brag and say low 10b.  As you noted however, zones don't tell the whole story and planting based on zones alone can lead to some pretty bad results if you get that proverbial 26F.  Most of us in Central Florida's urban areas could make a similar zone argument for our respective locations.  As an example, see the graphic below for Lakeland, FL over 15, 30, and 40 year periods:

While I love living here, my advice to local growers would not be to plant an entire garden full of coconut palms or Adonidias based on these charts.  I'm also keeping it in the back of my mind that 2019 isn't quite in the books yet, so that 36F could change.

Thanks for the info.  Our winter extremes are very similar.  The Latitude of Phoenix is about as far north as Myrtle Beach, SC but our winter weather on average is almost identical to St. Augustine.  I now believe Phoenix is in fact a 10a zone but that is not a true indication of how our weather works here.  Unlike where you live in Florida, we may only get down to 34 degrees in a given year as an extreme but we will have dozens of nights in the 30’s in an average year.  

Central Florida in contrast will only see temps in the 30’s or below for very short periods of time and then bounce back to the mid 50’s quickly.  It’s not common anywhere in central Florida to have night after night in the upper 30’s like Phoenix does see in an average winter.  
 So being considered a 10a zone is great on paper but the reality is areas in Florida in 9B zones have a much better chance of growing tropical Palms because your normal average winter lows are much warmer the Phoenix.    Jeff

[wrigjef]

7 hours ago, Jeff985 said:

You’re also looking at different years than the hardiness map.  It uses 1974-2012. Houston has the same issue. If you look at the last 30 years the average is between 28.5, but it’s still classified as 9a. The next time they update it you should officially be 10a. 

This makes perfect sense now.  If you look at the years prior to 1988 - Phoenix would a 9B or even a 9A if you go back into the 1950’s and 60’s.  We had minimum temps just about every winter in the upper 20’s. 1950 hit an all time low of 17 degrees.  
  Today Phoenix is a mega heat island and doubt we will ever see temps below the mid to upper 20’s ever again.  

[wrigjef]

7 hours ago, RJ said:

The 70's and 80' from a historical perspective (prior as well) were quite cold decades. As kinzjr alluded to, averages are just that: averages. Your plants could care less about averages they are mostly concerned with absolute lows. That's what the USDA maps tell us though. They give us a range of average lows for a given area. Like it or not most of us here in the states live in a continental climate, and with that comes some pretty wild temperature swings. 

Agree about the 70’s and 80’s being cold but how much does that have to do lower populations and less heat island effect?  Orlando comes to mind.  Orlando today is a different world compared to back in the 70’s and 80’s.  They are successfully growing tropicals there now they could only dream of back then.   

[Jeff985]

15 minutes ago, wrigjef said:

This makes perfect sense now.  If you look at the years prior to 1988 - Phoenix would a 9B or even a 9A if you go back into the 1950’s and 60’s.  We had minimum temps just about every winter in the upper 20’s. 1950 hit an all time low of 17 degrees.  
  Today Phoenix is a mega heat island and doubt we will ever see temps below the mid to upper 20’s ever again.  

Exactly. The hardiness map is just one thing to look at. In Houston the lowest temperature on record was 1989. Since then it’s been a warm 9b. Where I live, a couple blocks from Galveston Bay it could be 10a when the next hardiness map is released. Maybe. But all that means is the average is 30+. One year may have a low of 34 and the next year 26. The average would be 30 which is 10a, but a coconut won’t survive the 26, so I still can’t grow them even though they’re classified as a 10a palm. 

[wrigjef]

5 hours ago, AZPalms said:

Hi Jeff! I also live in the Phoenix valley. I agree that inside the loop 101 area could be considered a 10a most years. I’m also curious as I’m sure you know, the valley here is so diverse in elevation, population density (urban heat island), mountain protection etc. I travel across the valley daily for work. In the winter months when I leave my home in Waddell, it’s not uncommon for me to see temps begin at say 31f, climb to 34 by the 101/i10, 37f by the tunnel, back down to 33 by the 202 etc and so on. Similarly, the same for summer month temperatures. 

I’ll be curious to see when they update the zones how the scale the area. 

 

Max 

Hey Max,

   You are so right about the temp differences across the valley.  When I was just dating my girl friend I would leave her house early morning in Old Town (Scottsdale for those who don’t know)  and the temps in Jan would be like 48 degrees.  By the time I got to my house in North Scottsdale It would be in the low 40’s and I was only 6 miles north of her but right on the edge of the heat island.  Old Town was the heat island 

[Jeff985]

I just saw your area on the weather channel. That’s a crazy amount of hail. It looked like snow. 

[wrigjef]

53 minutes ago, Jeff985 said:

I just saw your area on the weather channel. That’s a crazy amount of hail. It looked like snow. 

Yeah that was on the west side of Phoenix we did not get any hail near me.  I saw some kids sledding which was a funny sight with Palm trees in the background.  

[Steve in Florida]

11 hours ago, Jeff985 said:

You’re also looking at different years than the hardiness map.  It uses 1974-2012. Houston has the same issue. If you look at the last 30 years the average is between 28.5, but it’s still classified as 9a. The next time they update it you should officially be 10a. 

The 2012 USDA zones map actually uses data from 1976-2005.  Anyone wishing to update his location can try to obtain the last 14 years of observations to update his local zone map.

[Jeff985]

13 minutes ago, Steve in Florida said:

The 2012 USDA zones map actually uses data from 1976-2005.  Anyone wishing to update his location can try to obtain the last 14 years of observations to update his local zone map.

You’re right. I typed that at 4:00 am. Numbers get mixed up at that hour. 

[edbrown_III]

and here I sit stuck in 9A

 

[Jeff985]

10 minutes ago, edbrown_III said:

and here I sit stuck in 9A

 

I’d be shocked if Jacksonville is still classified as 9a after the next update. But the map really doesn’t matter. What will and won’t grow in your area isn’t going to change if they suddenly update your zone on a map. 

Edited November 23, 2019 by Jeff985

[edbrown_III]

yeah ---  still have alot of 25s and lower within the last 10 years ----  I have a Arenga pinniata that is starting to      get some width and may eventually make a trunk ---  

salute

[RJ]

On 11/22/2019 at 1:04 PM, wrigjef said:

Agree about the 70’s and 80’s being cold but how much does that have to do lower populations and less heat island effect?  Orlando comes to mind.  Orlando today is a different world compared to back in the 70’s and 80’s.  They are successfully growing tropicals there now they could only dream of back then.   

That is actually why I stated prior as well. The 70's and 80's were cold decades even when compared to any other decades in the 20th century. 

You are however, correct the HIA effect certainly has changed what they can grow. You can watch it happening to any fast growing city even further north like Atlanta, GA and Charlotte, NC.  It has essentially created a micro climate that in this case shields much of the area from more extreme colds. Much int he way a body of water can do that. 

[Walt]

When I moved here (Highlands County, Florida) in 1997, my area had a 9a USDA hardiness map rating. Some years later a new USDA map placed it at 9b.

In any event, the barometer I go by that definitely indicates a given area is 10a and up is the coconut palm -- regardless of what some USDA hardiness map may say otherwise.  

When I first moved to Highlands County my wife and I would drive everyday looking at property/home listings so as to buy the property we have today. We traversed the county from north to south and east to west. In all that travel I observed about three mature coconut palms (all on or close to big lakes). Today they are far more plentiful, but mostly around the warmer at night lakefront areas.

One exception to this are two coconut palms growing in Sebring in an area that is not benefited by a lake.  I've been tracking these two palms now for almost 16 years. I've only seen them cold damaged one winter (although I don't necessarily check on them each winter), and that was in 2010. The fact that the below two coconut palms have stood the test of time (16 years that I know of) indicates to me that the area they are growing in is a solid zone 10a (at a minimum) -- with the exception of December of 2010 when lows may have dipped (for short duration) into the upper 20s.

The first pic below I took on 1-30-2004. I took the second photo on 11-22-19.

[Merlyn]

Even a relatively small distance can make a big change in minimums.  My house sits (more or less) 1-2 miles from the urban heat island and we are typically 2-3F below their temperatures.  So a relatively "normal" for the area 27F cold snap could be 24-25F at my house and torch a lot of plants.

The same can be said for frost, I know one member here says that he lives in a "frost hole" where a mile or two away there's rarely ever frost.  I'm fortunate that while I get colder than normal I almost never see frost.  Although I did see sleet and snow a few years ago...   

[sonoranfans]

Having lived in gilbert chandler area for 10 years growing palms I agree with AZpalms.  Inside the 101 loop is 10a outside 9B.  Gilbert saw 21F in 2007 yet in phoenix inside the loop it was more like 31F, while  north phoenix saw 23F and scottsdale was warmer at ~ 26.  The heat island is a huge effect it all depends where in the metro area you are.  Down south in casa grande they saw 16F on the same cold front in 2007.  If you are inside the 101 loop 10a palms have been growing there for a while.  

[ruskinPalms]

Walt has it right. Look for indicator plants, preferably things like long lived Zone 10+ palms and trees. Small zone 10 plants do not count. I tend to know I’m in a solid zone 10 when I see larger banyaning ficus trees and seeing them self propagate. I always love seeing the other trees getting swallowed by a ficus tree . Large royal poinciana trees also help define zone 10+ as well. Can’t always go by seeing large, mature palms in a landscape unless you know they were planted as small plants and have survived there for at least 10 years in my opinion. Royal palms and foxtail palms are pretty solid for warmer 9B areas when they are planted as larger trunking specimens so seeing those around doesn’t help define an area as zone 10. Large, long term coconuts do but that really doesn’t help for out west. I guess there would likely be other indicator palms for out west to define solid zone 10 but I think the ficus trees as indicators still stands for Florida and out west.  

 

 

[ruskinPalms]

And on the topic of Arizona, has anyone ever seen or tried a coconut in the Yuma area? I kinda think they would have a good shot there 

[sonoranfans]

too cold in winter in yuma.  Temps away from the heat island can really drop off.  I dont think yuma has much of a heat island.

[Silas_Sancona]

13 minutes ago, ruskinPalms said:

Walt has it right. Look for indicator plants, preferably things like long lived Zone 10+ palms and trees. Small zone 10 plants do not count. I tend to know I’m in a solid zone 10 when I see larger banyaning ficus trees and seeing them self propagate. I always love seeing the other trees getting swallowed by a ficus tree . Large royal poinciana trees also help define zone 10+ as well. Can’t always go by seeing large, mature palms in a landscape unless you know they were planted as small plants and have survived there for at least 10 years in my opinion. Royal palms and foxtail palms are pretty solid for warmer 9B areas when they are planted as larger trunking specimens so seeing those around doesn’t help define an area as zone 10. Large, long term coconuts do but that really doesn’t help for out west. I guess there would likely be other indicator palms for out west to define solid zone 10 but I think the ficus trees as indicators still stands for Florida and out west.  

 

 

This is a pretty accurate thought, though there is some wiggle room here..  Sans Coconuts, there are at least a few Royal Poinciana around town, two in my neighborhood for sure, Same w/ Cassia fistula, another tree id associate with zone 10..  and, by desert standards, theyre pretty big. Apparently were planted from seed the homeowner obtained from another neighbor who had colected them in Thailand. Same yard has a large Tamarind that has been there at least 15 years, per what she had told me. 

There are also plenty of Ficus nitida all over the valley, some huge ones in Tempe for sure. There's also a yard in my own neighborhood w/ several Ficus benjamina though they're kept small..  Don't forget the massive Ficus planted in Scottsdale, where one of the largest ( I've seen here, thus far) Ficus petiolaris specimens is also growing.. and a couple other Ficus at that park I'm not sure on the Id of.. 

One of the big reasons you don't see as many tropical, standard zone 10 type things is availability ...though a couple nurseries, one in South Phoenix in particular, is working on changing. His availability list reads like what you'd find in Florida.  Him, and another place in Apache Junction, a fairly cold outskirts area of town, also supply a good amount of the less common Bananas, Mangoes, Jamaican Cherry ( Muntingia ) ,  Guava, etc that are becoming more commonly seen around town.  

The other big factor regarding tropical here is getting them through the heat during the summers..  Trust me,  our yearly nuke fests kill or badly burn more stuff than the cold.. at least what I have experienced in my own yard/  seen in the neighborhood / general area of town seen since living here.  Florida enjoys a lot more humidity thru the summer that buffers the effect of the heat on things. This is one reason as the saying goes ..." Shade is your best friend here"...  You want it until X palm/ tree/ etc is big enough to take on our sun.  Stuff that has been damaged over the summer is going to be a lot more susceptible to damage if the following winter is cold, esp. if smaller.. 

As far as where the 9b/ 10a line exists.. I'd stretch it south in anywhere generally  inside the 202 loop and west of the 87 / Arizona Blvd, maybe Gilbert Ave.  That said, as fast as Mesa, south Gilbert and Chandler, ..and Queen Creek are filling in, the local heat island is set to expand quite a bit in short order.  Combined with a changing climate, that's only getting warmer, for now atmleast..  I have no doubt most of the area will bump up to solid 10a, with areas closer to downtown extending into 10b

Can't imagine what the area will look like in another 30- 70 years, esp.  as the area between San Tan Valley and Tucson  starts getting built up. Same idea headed west along the 10. Lots of talk of that area seeing major development in the coming years.  Availability of water will be the only limiting factor, unless they start piping it north from the Gulf of CA. 

[wrigjef]

Great info everyone,  thank you. 
 

I visited Mark (owner) of Pacific Palms today as I have the week off from work. He is located off I-17 in North Phoenix.  What a great collection of trees. I spent close to an hour wondering through his amazing garden. 
 I had been meaning to get up there for awhile because he has several Beccariophoenix Alfredii growing in pots for the last several years and I wanted to see how they were doing.  Well when I got there I had to get one so he delivered it to my house an hour later.  The Mules are beautiful and his Foxtails looked nice also but appeared to look at me and say please don’t plant me in the ground I don’t deserve a death sentence. Here are a few pics  

The last pic is the Alfredii I bought today, I put it in my screened patio for now.  It’s about 7 feet in a pot,  I will probably wait to plant until February   

 

 

[sonoranfans]

wow those are in tiny pots!  They must be dying to go in the ground.   Initially they will not have big root systems of course, and that means either cut off 4-5 older leaves to reduce transpiration losses or(preferred) put some overhead netting over then for a growing season.  They are great addition in PHX, just spray them down at the end of the day in the super dry heat(5-10% RH, 110).  

[OC2Texaspalmlvr]

Nice buy can't wait to see its progress once in the ground =) Very jealous after doing research I'm very doubtful they would be long term in my area =/ 

[Walt]

On 11/25/2019 at 4:27 PM, ruskinPalms said:

Walt has it right. Look for indicator plants, preferably things like long lived Zone 10+ palms and trees. Small zone 10 plants do not count. I tend to know I’m in a solid zone 10 when I see larger banyaning ficus trees and seeing them self propagate. I always love seeing the other trees getting swallowed by a ficus tree . Large royal poinciana trees also help define zone 10+ as well. Can’t always go by seeing large, mature palms in a landscape unless you know they were planted as small plants and have survived there for at least 10 years in my opinion. Royal palms and foxtail palms are pretty solid for warmer 9B areas when they are planted as larger trunking specimens so seeing those around doesn’t help define an area as zone 10. Large, long term coconuts do but that really doesn’t help for out west. I guess there would likely be other indicator palms for out west to define solid zone 10 but I think the ficus trees as indicators still stands for Florida and out west.  

 

 

I want to make it clear (not to you, but maybe others so that's there's no misunderstanding) that in my above post I was only referring to USDA zone 10a in my area (south central Florida inland). S.W. Arizona's zone 10a is different and not comparable to my area. Certainly,  California's zone 10a is far different than my area (much less wintertime heat). Even zone 11a in California can't support coconut palm (not enough wintertime heat, notwithstanding no freezes).

But tonight I made a confirmation (explanation below) observation. My wife and I had to drive up to Davenport, Florida, which is about 1-1/2 hour's drive north from Lake Placid, Florida (where I live). 

One factor that is different in my area compared to coastal Florida is relative elevation (that can elevate a USDA zone number). I'm speaking of the Lake Wales Ridge that runs down the spine of central Florida. 95% of the coldest nights we get each winter are on radiational cooling nights (no wind and clear sky). In past winters I've recorded (with my car thermometer driving from my house up to the top of the hill (on the ridge) an 8 degree temperature difference.

But early tonight I made a confirmation observation. On my drive home from Davenport I had to stop at the Lake Placid CVS. When I pulled out of the parking lot around 7:45 p.m. my car thermometer read 67 degrees. In just a short 7 minute drive down the hill and through the subdivision to my house the temperature dropped to 55 degrees! I could hardly believe it but I watched the descending temperature numbers fall one by one.  That's a 12 degree spread in about 2 miles distance and elevation difference.

However, I don't expect that spread to hold over night, and I believe by sun up the spread will be less than 5 degrees. It was in the high 70s and sunny here today with good ground heating. I think the ground heat in the lower elevations  rose up to higher levels this evening; hence, the 12 degree higher temperatures. I don't know what else would explain such a big difference in temperatures. 

And before I forget, the two coconut palms in Sebring (I  posted above) are located on higher ground (compared to my place), and I think that's one factor why it's warmer at night there, and makes the area zone 10a as opposed to my 9b.

And of course,  water (ocean, gulf, bay, lake, etc.) is probably the biggest factor in keeping night. Again, driving home from Davenport on US 27 in an underdeveloped area between Avon Park and Sebring the temperature had dropped to 58 degrees. As I got to Sebring and abreast with Lake Jackson the temperature rose to 69 degrees. As I drove farther away from the lake the temperature started to drop many degrees.

US 27 runs along the Lake Wales Ridge, and the ridge has high areas and lower areas (hills and dales) like a roller coaster. I would watch the car's thermometer temperature rise and fall depending if I was on a hill or down in a lower area.

The bottom line in my area is,  if you want a zone 10 climate you have to live on a large lake or on a high hill.  Otherwise you will be relegated to a zone 9b or even 9a climate.