The designer chose all of the textures. I can't comment on exactly what she was thinking. But I can add some insight that might help.
First of all, dark mulch (dark hemlock barkdust) is very common around here. The most common way to fill planting beds. It goes with what the client has around the rest of their property. So I guess the use of this ties into the NW style of landscaping and the rest of the property.
Second, I think the general concept was to use a lot of different textures, each in their own area. We have 3 different varieties of river rock, some fine gravel, and the barkdust - each lending a different texture and defining a different area. So I guess the barkdust was just part of that plan of using different textures.
Third, sand doesn't do well in the NW. It gets very messy looking very quickly. Especially in this case, the neighbor has several big fir trees dropping needles regularly. In fact, we had to do a 2-hr. clean-up right before this photo shoot because so many needles had fallen between time we had finished the job and the time we took these photos (2 weeks). It's much easier to clean up needles from barkdust. Or at least we're used to doing that anyway. So our workers are fairly efficient at it. Trying to keep sand looking perfect - the way you see in real Japanese Gardens - would have been a logistical nightmare. And the barkdust at least blends in with the needles well enough that you wouldn't need to be constantly cleaning it, like you would with sand. Even without the needles, though, sand just doesn't stay looking nice here. I installed a nice little sandy beach in my cousin's back yard a few years back, as we were doing more of a tropical look in his yard. Palm trees, bananas, lots of flowering plants, etc. And we ended up removing the blond colored sandy beach a year or two later because it was impossible to keep it looking neat and clean. It always looked dirty. Sand, for the most part, isn't used in landscaping much at all up here.
So those are probably the reasons we didn't go with sand and used mulch instead. It's not 100% a real Japanese garden. It's the NW version of that, I guess.
Last edited by JimLewis; 11-12-2012 at 08:01 PM.