They are only guidelines at best and only as relates to average minimum winter temperatures. I would ignore any further distinctions into 'a' or 'b' - this is such a subtle deviation as to be easily absorbed by any local microclimate....and then some!

I don't know how much "book information" is necessarily outdated by these revisions - 99% of it was incorrect anyway. Again, USDA hardiness zones are only guidelines for how well a plant may be expected to survive winter.

If you want something more detailed, use the Sunset zones. For one, they were developed specifically in and for your neck of the woods and they do take into consideration altitude, rainfall, summer temps and humidity, etc. OTOH, the only source you'll find with plants with these zonal designations is in Sunset (and I guess Monrovia tags also) but few of their listings are correct as well.

btw, you should consider yourself zone 9 or average winter lows of 20-30F :-)

winter lows of 20-30F

???

most of us consider that.. spring or fall lows .. its when i bulk up from a T shirt to a sweatshirt ... heck.. maybe even a windbreaker ...

let me clue you in.. you dont know what winter is.. lol ..

w/o really knowing why you bring up the subject.. its hard to offer you any concrete suggestions.. on whatever you are contemplating ...

as gal noted.. MIN winter lows .. just bout sums it up ... and when you suggest.. some type of 'universal acceptance' .. i refer you to mother nature.. who scoffs at anything that is suggested as universally accepted .... really on man made charts.. and she will show you who is really in charge ...

ken

Your stated winter low is a little too low for us, it rarely goes below 35F (hate to rub it in Ken!). It never snows here, maybe once every 50 yrs. We have real seasons (as opposed to San Francisco's microclimate where it's chillier at night in August than in November!) but the winters are mild. If it really hit 20F all our citrus trees would die. It doesn't freeze except at the high elevations, but even there you see skiers wearing t-shirts in the sun so it's a completely different kind of "winter" than I experienced living in Michigan for example!

Here are the average yearly temps in Davis given in fahrenheit:

Ja Fe Ma Ap Ma Ju Ju Au Se Oc No De

53 60 65 72/ 80/ 88 93 92 88 79/ 64/ 54
/
37 40 42 45/ 50/ 55 56 55 53 48/ 41/ 36

Top number row is average high and bottom is average low, hope the columns make sense.

Climate zones work best in wide areas of similar goegraphy.

Out west things are different. Elevation differences just throw things for a loop. Too hot for you in Vegas? Drive to Red Rock. Same in Denver and its Red Rock. Climate zones are tighter than census districts!

Now lets throw in a bit of urban heat islands and what do ya know, the map is further stressed.

So yeah, use them for a guideline but also look around your neighborhood and see what lives there.

OK, the USDA zones are based on average (as in 30 years, sometimes more) winter minimum temperatures. You are lumped right there with Sacramento and most of the rest of the Central Valley. Most hardiness zone maps are researched by zip codes - yours (all three) come up as zone 9.

But being too specific about zonal designation is like being too specific about soil pH. Remember these are not permission slips but only guidelines that estimate what might be winter hardy in your area. There is often a large fudge factor involved, both with regards to the plant(s) in question and your specific situation.

In your area, I would be much less concerned about hardiness zones than I would be about heat and drought tolerance. That will be much more of a limiting factor on what you can grow and grow well. The Sunset zones will help to some extent with this data but local sources - parks and public gardens, botanical gardens, better garden centers - will help to fill in the blanks.

What matters in regard to cold is how hardy a plant you want to grow is, and what the likelihood is of it getting below that level on the planting site. Killing cold being infrequent does not matter, if you want the planting to live through it - look at the hassle they go through with citrus orchards in some regions, having to put out smudge pots because it usually doesn't freeze - but does on occasion, and the plantings will be lost if the cold is not beaten back somehow.

Being a populous state California has public and private arboreta and botanical gardens etc. peppered around that can be good sources of guidance. In addition to the UC Davis arboretum have you walked the Capital grounds in Sacramento, taken in the tree plantings there? It's quite a display, dating back a long time.

The revisions put us in zone 5b - It was -17F here two years ago. I would regard that as zone 5a - Maybe I'm wrong. (wouldn't be the first time)

ps. You can grow citrus in San Francisco but since the average neighborhood save a couple of lucky ones like the Mission, Potrero Hill and the Castro never gets much above 70 your yields will be tiny and ripening is a problem. There are wild swings in temps depending on the neighborhood, what side of Twin Peaks you're on (ocean or bay) and where the fog streams, a difference of over 20 degrees in just 6 blocks on some days, talk about tiny microclimates!

The summer heat of Davis produces huge yields, many orange and lemon trees are covered with fruit most of the year. Right now my Washington Navel Orange tree's branches are getting heavier each day with growing green fruits. I've had to sacrifice keeping Norfolk Island Pines outside and the other unusual cool wet weather species, but the amount of things I can grow successfully outdoors has grown dramatically, I just had to adjust to the fact that if you don't water regularly during the summer almost everything will dry out and die, including your lawn! This valley is full of orchards and farmland, all of them serviced by thousands of beehives. In San Francisco I'd see one or two bees visit my flowers on good days, here they come by the thousands. Plus the hummingbirds and butterflies and the moths at night, if your plant needs a pollinator you couldn't pick a better place to grow them. I also benefit from all the beneficial insects they release on the crops, we have a huge population of mantids, ladybugs and lacewings along with parasitic wasps, there's never a need to buy them they're everywhere. I've never had a healthier and more productive vegetable garden. Some neighbors have gone totally for cacti and have replaced all their lawns with rocks, I love and have cacti too but unless I live in Arizona I don't want a yard that looks like it belongs there, I keep them in their own succulent garden in the backyard. Some people have pines and broadleaf deciduous (maples, birch etc.) mixed in the same flowerbeds with bamboo, citrus, Hawaiian tropicals and cacti, it looks as if a tornado hit a nursery. I love everything, but when I plant outside I want the plants in the beds to look like they all belong together, it also makes care so much easier. Cheers.

BBoy the first time I visited Sacramento during the summer I thought I'd melt! Pure microwave heat, not a bit of moisture unlike the hot humid summers I was used to (I also lived in Puerto Rico for a time with humidity so high and air so thick you could feel it press against you when you left the airport sliding doors). I've also spent many summers on the East Coast from Maine (very San Francisco-like summer weather) to Florida. For pure human comfort outdoors nothing beats SF, it's never too hot or too cold, I got really spoiled. But others find it freezing and are desperate for hot weather, your bones never get a chance to warm up there and that can make arthritis worse if you suffer from it. Don't let the half-naked (or sometimes completely naked) people on the streets fool you, San Francisco is chilly! You can always tell a tourist in San Francisco by their stereotypical outfit. They come to the city expecting a typical hot August summer in CA and wear t-shirts with shorts. Soon after arriving they start freezing to death and run into the nearest shop to buy a thick sweater. So if you see anyone in San Fran wearing shorts and a sweater 9 out of 10 it's a tourist...;-)

Thanks everyone for the advice. Gardengal so I'm a 9, only need one more to match Bo Derek! LOL! In California we need a chart for each side of our houses let alone different regions. I've never experienced a more fluid and geographically changing climate than this state has. You're right, we measure climate in blocks not miles here. Take care.

East Coast from Maine (very San Francisco-like summer weather)

Not quite: last I checked it does rain in the summer in Maine. Not that you don't know this, but I'm pointing it out for anyhow who doesn't. Nights are cooler in San Francisco but record highs are somewhat comparable; and in fact if you look at a place like Provincetown, MA, they actually slightly favor the east! 98F in Ptown versus 103F in San Francisco. This helps explains the report about a decade ago of an Austrocedrus chilensis being found on Cape Cod, a plant which would not otherwise grow in the hot & humid eas. (In the sense that maximum temperature indexes to average fairly consistently, at least on a big continent like ours - of course other factors play a role too. Cape Cod is protected from excessive summer rainfall compared to much of the eastern US, so that probably helped prevent root rot. The cold water quenches mesoscale convective systems so that they don't dump 2 inches in a single storm, but the particular location along the jet stream pattern means drought tends to be rare, too. I cannot say how many systems I see split over the Ohio & Tennessee Valleys with a good chunk going north to NY & MA, and a southern branch going to NC & SE VA. We in DC & Baltimore are in a kind of "horse latitudes".)

If you live on the West Coast, the Sunset system obviously remains the preferred one. You have more crazy microclimates in the Bay Area than the entire east coast of the US does. The being said most gardeners I know think the new USDA map is a vast improvement. It's just less useful in a highly terrain influenced marine/west coast climate like CA/OR/WA where almost all of the "action" is happening in zones 10, 9 and 8.

You're right I knew, it rained when I was there, I was only referring to the day and night temps. I wouldn't trade the winters! It's so damp in SF's Sunset and Richmond districts that the grass is wet each morning and on the campus of City college at night the pine needles drip constant water drops from moisture obtained from the fog, it's like a nightly misty rain that covers most of the city. You know how they spray all the streets for night filming in L.A. so they look all shiny for the cameras? In S.F. it happens naturally each night. On paper you'd expect San Francisco to be much drier and more arid in climate with the months of no rain but what most people don't realize is the moisture comes in other forms in quantities that more than make up for it. It's the only place I've lived where rain wasn't the most important natural distributor of moisture from the atmosphere (atmos being Greek for vapor, how appropriate!), it's fog, and if it wasn't for our coastal fog the redwoods wouldn't be here, pretty amazing...

We are classified as a humid continental climate - We usually have warm, moist summers and cold, moist winters. Unlike the Mediterranean climate of California, our highest rainfall comes in the summer. We get around 40 inches of rain a year. (We've made up for the dry summer with our autumn rains).

As I'm typing this, I'm looking out the window at a sweep of white - It's 22F out right now.

Flora there are lots of folks at the UC Davis arboretum that I bet would be happy to discuss local conditions with you. It's a great spot with examples of many types of flora that flourish in the local climate (which gives one ideas of how to push the envelope by adding irrigation, etc). Another way to find people knowledgeable about local growing conditions is the Master Gardeners, who have a close relationship with the folks at Davis. You could contact them as well (see link below). As noted, some of this will come down to trial and error and specific plant placement on your property...and sometimes planting things that will bite the dust if, in fact, we have rare low temps. I actually have a Bougainvillea that has no business growing in my zone but due to its specific placement (and luck) it has been here for over 12 years.

Here is a link that might be useful: Yolo County Master Gardeners

>I would regard that as zone 5aThe USDA temperatures indicated are averages, based on years of records. Any given winter may have a lowest temperature warmer or colder than the range associated with that zone. You won't be in a lower zone unless there is a series of annual lows that pull your average down.

In that case, our "average low" in January is 19 degrees. Last year was the mildest winter we've ever had - It only got down to -3F for a low...

This map is the oddball of them all. Dax

Here is a link that might be useful: 2006 Arbor day Foundation Zone Map

I used to be zone 5b, the new map says 6a.

However, we seem to have strings of 4-5 years of zone 7ish winters, followed by a SOLID cold z5 winter, so usually, I plant for zone 5. However, I do make exceptions for a few things, and some zone 6 rated plants seem to do well long-term in my area.

We shouldn't get too lax, IMHO, since despite climate change and all that garbage, as recently as 1994, most areas of the eastern US had a winter a full zone and a half to 2 zones colder than typical (we hit -22F in '94) that wiped out a LOT of plants!

Not average low, jimbo, but average LOWEST temperature for the year.

In other words, the lowest you got, -3F, last year, averaged with the lowest winter temperature for the last 20 years or so, not the 'average low' which is derived from taking every single nighttime low for the whole month of January or whatever, a far different number.

Gotcha! We can usually count on at least -15 for a winter minimum; once every 20 years or so, we'll get a really severe winter that does in the zone-deniers. The last major blast was in 1992, when it was 28 below here.