There was a whole row growing and flowering in the ground against a wall in Seattle but when I drove by there earlier this year they were mostly gone, a few short shoots here and there that didn't look like the kind of sprouting one would expect to build into the display of the previous summer. The minimum temperature for these isn't very low, I only saw them in places like southern California and Hawaii before the Zonal Denial fad took off.

One approach you might try is planting out in the ground in a warm, sheltered nook with cuttings overwintered each year in the refrigerator as a backup. There was a web site on brugmansias that told how to do this, if it is still up searching "brugmansia" or "brugmansias" might get you there.

There is a brugmansia forum here with lots of suggestions. I grow some in pots, some in the ground. The ones in the ground that I don't lift, have sprouted but their growth is so slow that they won't be big enough to flower. I overwinter in an unheated garage or the basement, take cuttings and leave them in water and in early spring, pot up and put the pots out in May. They grow well in the ground if planted out like annuals, but of course need lots of fertilizer and water. They are growing them in Victoria in city-landscaped areas and they are gorgeous -- real show stoppers. I'd like to know how they get them to grow so big and bloom so well! Must be feeding them elephant poo.

Same with lots of other Zonal Denial plants, they may overwinter outside or in a minimally sheltered area but without some time in a greenhouse or other heated situation they don't come on fast enough in our cool summers to reach full development.

I used to have Dahlia imperialis overwintering without mulch but the flowers didn't start to open until November here--a clear signal that the plant was expecting a different climate regime. After repeated episodes of having it get frosted mere days after the first flowers opened I gave up and got rid of it. (Recently I heard of another, apparently more successful local planting, perhaps a different form).

Victoria BC has a warm microclimate. I've seen many plants there in the ground that I'm sure would freeze here.

Not just at Buchart Gardens where they have greenhouses and expert care.

I keep hearing about the mild climate in Victoria but when I go there the native and planted trees look like it is the same Fraser Outflow climate as throughout the area. It also feels like it. And map showing this weather pattern in Sunset WESTERN GARDEN BOOK has arrows representing Outflow pointing right at Victoria, weather data posted online for Victoria area stations also tells a differnt tale than those that claim it is a banana belt paradise.

Brugmansia don't do very well as permanent woody plants anywhere the winters will regularly get below 28F for more than just a night or two. They aren't just restricted to Hawaii and southern California, they can easily get 18 feet tall by across here in the San Francisco Bay Area,(but only with regular summer irrigation), but are typically a die back to the ground shrub if it freezes regularly every winter. I have had my large mature trees killed to the ground in the 1990 and 1998 December freezes, where it got down to 25F in my garden. They took their time sending up new shoots, it wasn't until mid June or July that they really got going again. In more normal winters here, they are still happily blooming away in mid winter in wind protected spots, and don't mind it getting down to just above freezing occasionally if it warms back up into the 50's during the day. So they are in fact pretty well adapted to our cool but mild winters, which is to be expected as many of the species used in the complex hybrids originate in highlands of the Andes, where the climate is very similar to coastal northern California for winter lows and summer night time temps. Very little fragrance to the blooms and usually paler color on winter blooms, while this week's heat wave is ramping up the fragrance quite abit.

I've never seen any substantial plantings in the PNW that weren't grown as container plants and taken under cover in winter, to preserve size and speed up bloom in spring. If you know your Sunset Western Garden zones, I'd say that zone 14 is the coldest winter climate that is reliably safe to plant them in the ground and expect them to perform well.

I find your comment about them being able to stand temps close to freezing interesting as long as it warms up during the day. My very best flush of blooms was in October with this exact weather. They lasted so well and looked absolutely lovely. They were much easier to look after at this time of the year than they were in the summertime. They sure like their food and water!