Not sure what you mean by hardy hibiscus. there's the shrubby hibiscus I remember from California - no, those won't do well here. then there are herbaceous hibiscus like H. moscheutos. I have seen those for sale here, but I can't say I've seen them growing in a garden. I think they need warmer summers than we get.

It's not just the cold in the winter that gets things here. It's the cold wet waterlogged soil that stays cold through spring, and it's the short cool summers that don't provide much if any heat.

Blue hibiscus (Alyogine?) will grow in a warm sunroom and go outside in summer.

Keep in mind that Olympia is significantly colder and wetter than most of western Washington. People in the Seattle area have a hard time understanding that and insist that we can grow everything here that they can there, and it just isn't so. Seattle summers are about a month longer than ours, and warmer (but still not warm), and their winters are not nearly so cold and wet. That said, there are climate differences within the Oly area - for instance the waterfront is very mild, almost like Seattle; south county, where the official weather station is, is much less temperate; and then there's the Oly urban area, which is in between both geographically and climatically. Oh and then there's the Black Hills, just high enough to be a bit different, and more snow in winter.

40s in July is not typical. June nights yes but not so much July. Right now it's just now fully dark at 10:30 and it's about 60 degrees, that's pretty typical of early July.

We are like England - she should try buying British gardening books. Gardening books meant for the US don't work here, and most books meant for the western US specifically expect a drier climate than we have.

I believe it is the H. moscheutos that moonie is asking about. Those are the ones I remember from living in Tn. They come out late and rely on consistent tropical warmth and wet to do well. Great post above reg, I agree with everything except the 40s; 45 -49 is a very common low July temp in East Olympia where I'm at - just reinforces your points though.

Brugmansia are not even close to hardy here in the Pacific Northwest. If your sister has moved to Olympia from North Carolina she has a big adjustment to make. I have a gardening penpal in NC and have had for a decade and the differences are huge even though USDA zones the same. They only take winter cold into account. We have very few days over 80* here and many more days of rain. My friend grows all sorts of semi tropical plants in the ground in NC that we can't here unless we have a greenhouse. My friend had waterlilies and roses blooming three months before I did if that helps understand the temperature and sunshine differences.