Have you considered succulents? They can be tall, mounding, or draping. Purslane or portulaca come to mind for being particularly like a typical blooming flower yet also a drought-tolerant succulent.

Another idea is to try different soil mixes.

Currently I'm trying out these polymer/gel balls that absorb water. They grow 50-500x their size, depending on which brands you get, and release water back into the soil when needed. The soil where I'm trying these out would look dry every day, but since using these, the soil looks moist every day instead.

I mix them in with the soil when they are wet, so that the soil won't shift unexpectedly by having small balls that expand. Unfortunately, while they are wet, they can also break easily, but as far as I can tell, still retain their water-absorbing and releasing properties. However, it just looks better if it retained its spherical shape.

I'm not sure how many seasons they work for but they are doing OK in these last few weeks.

Here's a sample link but you could investigate other sources: Polymer Water-absorbing balls I actually picked mine up from a dollar store (wasn't willing to try them out when I saw them at other places for higher price but for $1, it was worth a try, and so far, it looks OK).

In order to get the gutters to work, you are going to need irrigation. Because it is shallow, you're going to want something that drains really well but holds a lot of water like 50:50 pumice:diatomaceous earth, or maybe straight coir chunks. Then just water as necessary, which depending on species could be daily. i tend to use adjustable microsprays instead of drip. The wetting seems more even.

The best bet for plants are succulents. They will do well with less water. Try sedums or senecio. Spider plants, though common, could form a nice living wall.

Agree with the succulents idea. Sedums alone offer numerous options. One word about those polymer water absorbents: I tried them in large planters in our downtown back in say, 1990. The idea was, of course, to reduce the amount and frequency of watering, this being a logistics nightmare. Thing is, then as now, if we start out running on Monday morning, we can just get to everything in our system twice during the work week. Adding these polymers changed nothing. It's not as if now we could just water them once. Also, even though you mix them into the soil as well as you can, inevitably, some are going to end up right at the surface. No big deal I suppose, but it's somewhat jarring to see little blobs of jelly all over the growing medium surface following every rain or watering. Personally, I don't think these things work. Sure, they do absorb water and they do sell up to much larger than they started out. But we don't use them for those reasons. We use them to reduce our workload and our need to water. This never happened in my experience. Finally, as I said, this was quite a while back. Maybe they're better now, I don't know. But for the price of this material-used to be quite high-I never felt they were worth it. Of course, ymmv.

+oM

Thanks for your advice, everyone! This is very helpful. We've started by installing succulents as you suggested, mostly ice plants, and they seem to be doing well. One side benefit is that they're dense enough that they shade their own soil, so the soil is retaining its moisture much better than it was when the gutters were lightly planted.

I really like this concept. I am going to put one in the shade, put end caps on the end of the gutters and them drill a few small drainage holes. Plan to put some 5-1-1 mix in it with a smattering (probably 0.5 parts) floor dry, and then put, in separate small 2x2 and 4x4 containers, some small plants in each (hosta plugs/liners are what I am going to try). I figure the 511 mix will help keep them in place for the summer. I will put it at eye height so I can see when it needs watering, and will sink the pots in the ground in the fall. My gutters will go between my pergola posts, and I will likely place a support in the middle too so the gutter doesn't sag. We'll see. That's the plan anyway.

dave