Often. There was another big one out by itself in the eastern part of Twin Ponds Park, north of Seattle that could be seen driving by on the freeway but it's not there anymore. Others can still be seen here and there - both wild and cultivated.

"Tall Oregon-grape forms compact clumps or patches, which grow taller than its low cousin, 5-15'--or rarely more."

--Wild Plants of Greater Seattle

Here is a link that might be useful: Wild Plants of Greater Seattle

I just love that Genus. Mahonia is one of the most beautiful native plants we have. My favorite thing is when hiking, seeing thousands of Mahonia nervosa flowering. Looks almost like little palm leaves coming out of the forest floor with yellow flowers coming from the crown.

They really don't look like any other plant that I know of. In a good way. Nice pic by the way thane; thanks for posting.

To answer your question, I would prefer to respond that it's not uncommon that someone like a hiker or landscaping professional will see one, or some, during each year.

But I wouldn't describe them as "common".

Obviously, as you have seen, they can get that tall or a bit taller, but the average is not that big.

For example, if I took you hiking on Cook and Green trail down here near Rogue River National Forest, or up at Saddle Mountain, it would take a while to find one that tall to point-out; if there even was one 15' tall visible from the trail.

Other areas might have them that big in greater density, but that's not going to be the average.

M. D. Vaden of Oregon

That is one gorgeous mahonia! I would like to know if anyone has any ideas about what conditions are most conducive to such exceptional growth. I am also curious about the exposure and moisture in the area where this wonderfully neglected plant has prospered. Judy WWW

Mountains of southwest Oregon is not Bellevue, WA. I do live in the same ecosystem, and I have seen many similar specimens. Same reason Jacobson would give the typical size range as 5-15', with even taller being sometimes seen - he is talking about here.

I have one planted over the edge of the bluff that is 12 ft. tall. It's not quite blooming yet.

I think the reason many aren't seen very big is because they're usually planted where they have to be trimmed every year, like parking strips in a parking lot and small flower beds. Rarely are they left to grow to their potential.
{{gwi:1095659}}

I've seen lots of 'em, so I guess it's a matter of who's looking and where they're looking. So far we've already covered three specific examples.

Definitely true.

It just depends on where and who is looking.

bboy ...

You wrote...

"I've seen lots of 'em, so I guess it's a matter of who's looking and where they're looking. So far we've already covered three specific examples."

If you know of some really big ones, you may just stand a chance for the CHAMPION SHRUB CONTEST and the $50 dinner prize for Olive Garden!!

Seems there won't be many folks trying at all on this one. About 3 weeks, and my huge poison oak vine with the 3" diameter trunk over 100' high is the only entry so far. Most people will get distracted and forget. But the contest is for real.

Go to Google and search verbatim this line exactly - then look within the results: specific contest category posted ...

"Turf to Trees" champion shrub

You're in the Pacific NW - right? That's what the contest is for. You've been around more than most - why not give it a shot.