Pushing The Boarders USA Style

[MartinNicklin]

I agree with Charlie that any hardiness zone guidance must be taken as a rough guide. For years I had grown a massive collection of rare plants outdoors suitable for my so called hardiness zone. Then came 2010 with a Siberian-like spell over several weeks (-18°C in my garden and Shrophire has gone down to -25°C and lower). All that hard work, effort and money destroyed in a weather event which may or may not be repeated in the future. Hardiness zones became irrelevant that year as much of the UK experienced temperatures say out of line with the zones. Of much more use to me, as someone tempted by the less common and less hardy plants, is the simple fact of how low can the plant stand and survive. Now for outdoors I don't buy anything which can't stand anything below -10°C or so which is a likely event here.

[LostInJapan]

How widely are the USA Sunset Climate Zones used?

Chad

I think it is more a an east coast/west coast thing, with the sunset zones used in the west due to greater variations in local conditions based on elevation changes, proximity to the ocean, and so on. Of course these are relevant in the east as well, but often over longer geographic distances.

[MartinGilmour]

I’d be interested to know people’s opinion on how rare a weather event needs to be for you to consider it irrelevant. For example, I imagine a winter like 2010 in the UK will occur again in the next 30 years, so I would grow some plants in the knowledge that they may get wiped out at some point in my lifetime. But as another example, we see great pictures of rhopalostylis growing away at Tresco but I believe these were wiped out by a bad winter in the 1980s. Such a winter may occur there again, but if that was a 50 or a 100 year event then would that stop you growing rhopies if you lived on Tresco?

[charlie]

I think it was the 1987 storm winds that ripped out the aging shelter belts on Tresco that made the following cold winter so bad rather than the extreme cold which wasn't actually that exceptional , I remember those palms shortly after they were replanted - they have done well!

[David F]

Chad, there is a national zone map for the USA based on the Sunset mapping system, but I don’t think it really ever caught on outside the West Coast, which is mapped(weather and topography), more consistently. Plus, microclimates don’t change every mile apart on the East Coast as they often do here in California.

There is also very much the additional concern of accumulated year-round heat to consider in a California zone 10a versus a Florida zone 10a, plus cool summer nights(given colder ocean temps), here in California versus Florida. In addition, degree of wind year round is also very much a factor to consider in choosing plants here, which the Sunset zones do factor in, along with proximity to the ocean, elevation, topography orientation, etc.

I appreciate that there has been warming, but I don’t see that 5°F lift pushing me into a more consistent zone 10b with winters that consistently stay above 35°F. Some squeak by, some don’t. I still have the cooler summers and consistently cooler summer nights, but summer night lows do seem to be a consistently 2-3°F warmer now than they were 35 years ago when I started my garden here in Berkeley.

As just one example, we do see some more adventurous plantings of things like Bismarckia nobilis here now, though they can grow here, they certainly look feeble compared to those in Florida. And just in my own neighborhood, a Jacaranda mimosifolia won’t bloom if improperly sited; they need shelter from chilly winds off the bay to have a chance of blooming, a north/south oriented street will be much better for blooms than me east/west oriented street where my front yard is a wind tunnel off the bay, directly in line with prevailing winds and fog through the Golden Gate opening to the bay.