Geraniums 'Bikovo' or 'Karmina' might work if they're hardy for your zone. No trimming necessary.

Jules my friend has a similar area - in shade almost all day and then wham! That hot, relentless, late afternoon to evening sun. Full shade plants keel over and die. Full sun plants die a slow death because they don't get enough sun and then get blasted in their weak condition.

Over the years we tried several things that just didn't work, at least not for more than a season or two. Our latest attempt is sedum kamtschaticum. I have it on a very dry, hot, full-sun, southwest facing slope at the edge of my yard along the street (no sidewalks - in addition to sun the heat just radiates off the asphalt all day) and it does well there. In her yard, her situation was an island bed in the middle of the yard, with three large oaks, surrounded by grass, and as I said, in shade almost all day. There was just this southwest end of the bed that got hit with the sun in late afternoon. The soil is much richer than my poor lean, inch-of-soil-on-solid-ledge slope. So I was hesitant to try the sedum. But it's been about 5 years and it's doing quite well. The bloom might not be as prolific as mine but I happen to love sedum for the foliage anyway, and having a green groundcover is better than bare dirt, lol.

Good luck with whatever you choose!

:)
Dee

Oh I really like that Veronica and Dianthus! It's a little taller than I was hoping for, but I will try that on the sloping part! Do you know if it will handle occasional walking on? I have to step onto it to clean the pond weekly. How far apart can I space plants to get that hill covered this year? Do either of those spread quickly? Will the Dianthus do OK up against the rock wall? Any suggestions for something that stays low for right by the walking path? I found Veronica Tidal Pool when looking up Veronica, it says it stays under 3". Anyone tried it? My local nursery has it in stock! I was thinking Irish Moss, but can't find a source for it in stock for this spring/summer. I have some Phlox near the pond that isn't doing great, or I'd try more of that.

Both the Dianthis when not blooming and the Vernica Georgia Blue are quite flat. the Daainthus is perhaps 3” and this Veronica not more than 5”-6”. In both cases that is quite short. I would add stepping stones to avoid the walking on issue. One of the things I like about V. GB is that is stays in winter and turns a lovely dark reddish color.. Not sure if Tidal Pool does that, and it is somewhat coarser and fuzzy.

Moss phlox.

@Sigrid, is Moss Phlox different from Creeping Phlox? Do you have a scientific name? I have some Creeping Phlox in the area that's not doing well, I think it's too shady with over half a day of full shade.

@NHBabs z4b-5a NH, I see we're regionally close, do you happen to know a localish nursery that has those in stock now (Keene, Claremont, Lebanon are all reasonable distances from here)? My local one (in Rockingham VT) does not have any left.

My local nursery suggested Mazus reptans as they have it in stock. Anyone tried it? I really wanted Veronica but they don't have any left.

Jules, I live between Concord and Laconia, so don’t do any plant shopping that far west in NH. I would visit local big box stores if you are going out, since I have seen them there, though it is not my preference for plant shopping. I would also rummage around on the web to see what nurseries there are within whatever radius you are willing to drive. Then call and see whether they have the plants you want. Or check out mail order nurseries (Bluestone Perennials carries it) and order for fall, planting annuals for the summer along with mulch to keep moisture even and weeds down. Or see if your local nursery will order for you for fall planting.

My preference (which you don have to agree with) is to plant annuals for fill if I can’t get a plant I really want, and order what I want. I view gardens as a long term investment and one of my much needed life lessons in patience. ;>)