Is there USDA Zone 10b in California?

[Allen Repashy]

I was just browsing the Member List and noticed quite a few people think they are in Zone 10b..... 35-40 minimum average temperature.....

My thought is that this zone would be historically free of sub freezing temperatures. Possibly the Southern California coastal islands.... a few parts of Florida... very select Coastal areas...After our reality check a few years ago in SoCal, Is there really anyone in California that can say they are in 10b.. I can see 10a..... I guess the interpretation comes down to whether you believe 10b can get a frost once and a while.....

I think a lot of people are just in denial about where they live after looking at the towns they list and the zone they apply to it ... Any opinions?

[palmbob]

There are a lot of palm growers in zones 10b in San Diego county... I think that is the only place in California where there are 10b zones, but there are quite a lot of them, most along the coast and a few hilly areas inland a bit. Some of these folks have yet to see a freezing temperature in dozens of years. Some growers in Orange County claim to be in a zone 10b area, and some indeed have never seen a freezing temperature, so I guess it's possible. Some claim there are even zone 11s here in California but I am pretty sure they are mistaken. The tip of Florida is a zone 11 including the Keys, but a marginal one. Miami is considered zone 10b, but zones in Florida do not compare to zones in California, at least in terms of what can be grown in each state.... it's like comparing apples to oranges... just not that applicable.

[agavegreg]

I hate the simplicity of the USDA zones. Tucson and Houston are both in zone zone 9a, yet houston will get 5-6 times the amount of rainfall we get, rendering gardening in Houston much different than gardening in Tucson. Also, for marginally hardy agaves, what survives a dry 25 degrees F might not survive a wet 25 degrees F. The number of consecutive night at that temp also affect the "hardiness" of plants. I'm sure others can chime in with factors affecting hardiness as well.

Greg

[Matt Maggio]

I know that coastal areas close to downtown SD - Mission Beach, Coronado Island, Point Loma, etc. - never got to freezing in 2007. Corona del Mar and parts of Newport Beach did not as well.

Yeah I agree...USDA zones don't tell the whole story, especially with regards to microclimates...and keep in mind that they represent average minimum temperatures. Poway, CA should be 10a, but in '07 the portion of town I grow some of my plants plunged to 18F.

Keeping plants dry helps a great deal. This action lessens the damaging effects of ice crystals in cells because the cells are more flexible. Ironically though, overhead watering to create a film of ice over the plants can help insulate. In addition, drying raises the ion concentration within cells, thereby lowering the freezing point. Similarly, consecutive nights of near freezing temps can prime a plant for subfreezing conditions by triggering the plant to increase sugar content within cells. This is how our good friend A. utahensis can take 0F. The speed of freezing and thawing is also important.

[Tim Harvey]

Wet soil and a fully tugid plant will also increase the heat capacity of the 'system' and reduce its cooling rate. I can never work out which is better for my plants here in SoCal, though for prolonged cold spells (like in the UK) dry was definitely best.

T

[Stan]

There are parts of San Francisco that can be 10b..not going below 40 most years..the exceptions are not never though, or rare,as they can happen to have a real freeze every 10-20 years.
Even 10a's are regular for most of inner SF bay..but,10a's 32F limit is a much more limiting on the west coast and 37 north then any Florida 10a.
But 11 in California is not possible..ocean water temperature alone will create a nontropical winter. Or visit what some claim as z11 and swim in a unheated pool ,Dec-April!
It might be a nice exercise to argue for a z11 California for some...

[Zone10]

I live in Central San Diego, on a southwest facing hill about 10 miles from the coast. The majority of my plants are in my backyard, which in the 13 years I've lived here, have never experienced frost. The lowest temps I can remember were in the high 30's. The only reason I consider myself in zone 10a and not 10b, is because we've gotten frost a few times in our front yard. So, I believe there are several frost-free micro-climates througout San Diego County...even miles from the coast.

[thornforest]

My hillside garden is located on the northern slopes of a large hill in Spring Valley , California. This is near the site, some more seasoned San Diego plant growers may recall, of the old "Palms Nursery" begun first in Nestor, then moved to this hill sucessfully, after WW2. I would consider this a 10a/10b zone. Temps have fallen to lower 40's F. at ground level since I have been here. I did have the upper 10 feet of my +/- 35 foot high Mai Nung Bamboo have temporary leaf die - back when many coastal areas had frost but no frost visible on any plants or the ground here. No die back has ever been seen during this time of Mallosma laurina on this site which is a coastal sagebrush- habitat -native and sensitive to frost. I have observed Mallosma laurina die back closer to the coast following frost in East and Central San Diego. I moved here in 1992 from San Diego. I have never observed frost on this particular ridgetop. Fingers crossed. There seems to be some evidence for a micro climate associated with certain inland hills of San Diego County.