Since it's right by the barbecue I'd be tempted to fill it with hardy herbs.

I thought of herbs but I don't use enough leafy herbs in summer. I could sneak in a basil on the far end, which gets the most sun.

Most grasses have an arcing shape. Have you considered something more upright/spiky such as Salvias if they are suitable for your zone?

Your yard looks beautiful!

I don’t know what grows best where you are, but what about Cordyline? It stays spiky (but is soft) and I think the color would be so pretty there.

I plant them every year and overwinter them in my garage (D/FW, z8a).

I have them and Dracaena Spike with other plants in front, full sun:

And alone out back, afternoon shade:

Last year, I had a row of them in a long, narrow Talavera planter, but can’t find pics I took (I have way too many photos on my iPad).

Try May Knight salvia. It is a blue spiky salvia Nd stays rather small. It is a perenial here.

Calamagrostis Karl Foerster is a very narrowly vertical ornamental grass that would fit in that very narrow planter. It blooms early but holds its flowerheads well into fall.

Cordyline are fully hardy in the PNW but are a bit of a weed.......a very mess plant!!

Ahh, ok, thanks, gg. Not a good idea then! They stay so well-behaved here — maybe our brutal heat keeps them in check. :D

Sage comes in several variegations, as does thyme. It doesn't matter if you don't use much. They're ornamental in their own right. And they're evergreen. When we barbecue we use loads of herbs. Whole branches of rosemary, for example.

Thank you for your suggestions. It would be nice to have some color there because the rest of the garden is pretty shady most of the day, so I don't have a lot of flowering plants after spring is over. I try to use a variety of foliage colors though. The golden fuchsia behind it gets pretty big but the Japanese maple turns dull red and green in summer.

When the rush was new it was spiky. I could also grow sedge there.

Do you like blue?

You didn't mention what is the function of the grass in this area. Is it creating any privacy screen? Or is it only meant to be decorative?

In one of the photos you show, there are chairs right against the planter box. Many spiky grasses would grow 2 to 3 feet high and then 2 feet wide, which would put the grass right into the area of your head if you were sitting at that table.

Its function is to replace the 3-4 feet of decking that was removed when we had the air conditioner installed, so that we don't step off the edge. It's purely decorative.

I don't know if it could grow that high with its roots restricted.