If you're in Alameda, it's a lot less likely to have frost because you're surrounded by water and are well inside the Bay. I live in Oakland, straight down High St. halfway up the Oakland hills, and our Meyers and Bearss are in the ground. Occasionally the fruit gets nipped by frost (interior dries out) but the issue is in-ground plants don't suffer because the ground NEVER freezes here. You may want to be more careful with container plants because most people use the smallest containers possible since large ones aren't easily moved....and smaller pots may be more susceptible to frost nipping. Keep them a foot or so off the ground; that will avoid the coldest air currents. Setting them against a south- or west-facing wall will help, too.

Where it is different from Britain here is there are very few examples of these of any serious age and size. Even the typical green parental tree is quite rare - although somewhat incongruously, large examples do exist here - so there is not an environmental condition that excludes the species entirely. Whereas the green ones in particular are ubiquitous south of here, in northern California coastal communities. Brown patches often appear on the recently planted yellow forms here. Recently I noticed that the browning of one near me was on the north side, implying winter injury. Otherwise last summer was hot (yellow foliage variants of conifers are often prone to sun damage), and the species is limited to outer coastal areas in the wild by disease pressure. Another yellow one I used to drive regularly got maybe as much as 8'-10' tall - and then fell open under a snow load, was cut down. An 'Aurea'? 'Donard Gold'? more than 24' tall (ca 2005) in Seattle (USDA 8) is quite exceptional for this area, the only other anywhere near comparable example reported (2006) from there being a 20' (ca 2005) 'Fine Gold' that was planted 1999 or earlier.

If I lived in Maine, I would definitely construct a winter shelter for extending the growing season and for getting a head start on tomatoes in the spring. Locally, you have one of the world's experts on this in Eliot Coleman, who helped pioneer the technique and published a few books on how to do this. We tried low tunnels but like the flexibility of the high tunnels. We built one in Utah (zone 6) and it has worked wonderfully to warm the soil and provide a sheltered climate for putting out tomatoes early. Here's this year's plants grown from seed indoors, transplanted in April a few weeks earlier than the last frost date:

Do you always bring the plants inside for the winter? Do they do well for you? (Just curious) Your coleus and hibiscus are very cold sensitive, resenting temperatures that approach 50F. I have already taken cuttings from my coleus, as here in northern Alabama the nights have dropped into the 40Fs. They are sitting happily in my sunniest windows right now! I don't know about the SPVine, I would consider it disposable myself. Don't they and the hibiscus erupt in whiteflies, aphids, and spider mites inside? I don't bring my showy containers inside for the winter. I used to store caladium corms (dry, cool, dark) but they never returned to anywhere near the quality of the first year. Better get organized, lol! That'll teach you to rely on the calender for weather related routines!