Hardiness zones?

[Stevetoad]

I'm a little confused as to how to figure out my hardiness zone. I thought it was based off your record low ( in my case that would be 25) but now I'm not sure if it should be based off my avg coldest temp ( 40 in my case). Not to sure if it matters because odds are if I like something it's going in the ground anyway. What do you guys think?

[Xenon]

I believe hardiness zones are determined by your average annual minimum temperature. I'm in Zone 9a, but my record low is in the single digits, so that would leave me only with Trachycarpus and Sabals... Your average temperatures are important too, Zone 10 California isn't like Zone 10 Florida. I think it's better to experiment, then to rely on hardiness zones to determine what will grow in your area. Check out the recent North Texas recovery thread, USDA puts Dallas in Zone 7b, but based on what people are growing there, it seems closer to 8b.

Just my 2 cents,

Jonathan

[AggiePalms]

Yes, I agree, I believe it is based on a running average of the winter minimum temperature. I think if I recall correctly that the latest revisions use the last 30? years, to better reflect warmer climates recently, and decrease the importance of very cold years decades ago. That expands the areas of the warmer zones. Check the USDA website, they have a good description of how they determine zones.

Zone hardiness is great as one of many factors in determining suitability. Minimum temps matter, but so does duration of cold, necessary cold chill hours (especially for northern fruit trees), frost, relative humidity, duration of heat stress, seasonal rainfall patterns, etc. I saw some Astilbe and other Northern/Midwestern perennials in a nursery in Florida last weekend. The nursery's wholesale supplier was probably using the (poor) logic that since the plants could survive in Zone 4, they ought to be even more happy in Zone 10. True they won't freeze to death in FL...but they will succomb to heat stress. And if they were in South Texas as opposed to Florida, not only would the heat be too extreme, but the dry seasons would be equally deadly. So, use more than just the USDA zone. A neighbor's relatives brought a van load of plants from his nursery in the Upper Peninsula of Michigan, two years ago. Now? I'm not sure any are still alive.

Don't just go throwing plants in the ground, it is too much trouble to keep digging burial holes for the soon to die plants. If they won't grow there, don't bother. I moved to Zone 9A from 7B and brought lots of perennials. Most did not make it past 1-2 years of hot summer drought. I have kept a few alive for years by keeping them in part shade near a wetlands area in my yard. But you don't have to grow just in the ground. If the plant is good, but not hardy, it is worth growing in a pot. Pots also help in watering ease, and keeping great plants in a reasonable size to take inside to enjoy in the winter. I find it easier to grow more tropical plants, rather than those from too cold a climate.

[Kathryn]

Some palms won't care what your average minimum temperature is. If a palm normally dies at 28°F and your area experiences one of those crazy 5 to 10 year low freezes that last a few days, the palm will die.

I started planting my palm garden in 1997 and wish I would have planted more cold hardy palms. Even after some marginal palms died during a cold period in January 2001, I planted more of the same. Over the years I slowly changed my planting habits and I'm glad I did.

Too bad I didn't plant that Sabal uresana instead of trying to push the limits with a Bismarckia nobilis. A 14 year old uresana would look really great in that spot compared to the dead bismarck palm I dug up years ago.

[sarasota alex]

WMO recommends a period of 30 years for all weather averages. The last official USDA map only used 12 and it so happened that it was the coldest 12 year stretch of the century in the eastern part of North America. The reason they used 12 years was to maximize the number of weather stations to make the geographical aspect of the map more accurate. If I remember correctly only about 3000 stations in the US at the time had 30 years worth of data, compared to something like 7000+ with 12 years worth of data. So a lot of places have the zone wrong.

For example the average annual lowest temperature for Sarasota Airport is over 30 degrees no matter which 30 year stretch you use. But during that 12 winter period last used by the USDA it was 29.2 incorrectly making it zone 9b.

The biggest issue is mis-zoning of the plants. If a coconut can survive down to 30, people consider it a zone 10a plant. Well, in zone 10a it is normal for the temperatures to venture down to the 20s every now and then. Result - dead coconuts, and many people saying "we are really zone 9". No. Still zone 10a, but a coconut should be more accurately classified as a zone 10b plant.

I also read many post about the feeling that the zones change after a terrible winter. That's not true. Statistically speaking, for a single winter in Sarasota to shift the currently running 30-year average down to zone 9b, the temperature would need drop below 1 degree F (-17.5 degrees C).

[sonoranfans]

I wouldnt lean on hardiness zone too much, its not very reliable in determining survivability. A wet cold can be much worse than a dry cold for say a bismarckia, and the duration of cold and frost is not captured in the hardiness zone. Also if you have radiational freezes vs advective freezes at the same temperature, the results will be very different, especially under canopy. Radiational freezes under canopy can be very mild in comparison. Regardless, if you want to grow a 1/2 zone pushing palm, get some canopy and start out near the house. Sounds like you have it pretty easy and should be able to grow alot there if your average minimum is 40. If you got some canopy, it probably wont get much under 30 under it ever there.

[Takil-Explorer]

Dallas in zone 7b? That would say they are in the same zone as where I live. Well I know that alligators live near Dallas. So that means we can keep them here in Holland outside as well? Come on, what a nonesense!

Alexander

[Xenon]

Dallas in zone 7b? That would say they are in the same zone as where I live. Well I know that alligators live near Dallas. So that means we can keep them here in Holland outside as well? Come on, what a nonesense!

Alexander

Like sarasota alex said, the USDA only used 12 years of data during one of the coldest periods of the century...

I think this map is more accurate: USDA 2003 Hardiness Zone Map.

Jonathan

Edited June 4, 2011 by Xenon