Sedums need good drainage, so I'm guessing the compacted clay is the main issue. I don't think root competition is an issue at all, but the fertilizer might be. Sedums tend to like lean soil. FWIW I don't fertilize my drift roses at all and they bloom their heads off anyway.

Thank you, SYinUSA!

To be honest I don't think that is the problem since the bed is kind of on a slope, the drainage there is not a problem and most of the time the soil looks too dry, certainly not too wet. I think they are getting too little water in that dry compacted clay but I am not sure. Maybe I should move them to see what happens.

The Drifts are amazing indeed. I do fertilize them about once a season and they do great. They are fantastic workhorse plants.

Should I maybe dig them up and replant them with soil conditioner, so I can loosen up their roots?

I'm kind of a lazy gardener, so if the plant is growing ok (just not thriving and flowering well), I'd probably just top-dress and let the worms do the work. I'd be more afraid of losing a plant by moving it and disturbing the roots

Most people understandably want quicker results. In that case, remove the plant; dig a shallow but wide hole; add conditioner (or compost); work it in with the existing clay, breaking up clumps; and replant. Water well, of course. Good luck and let us know how it goes!

Which Sedum is this?

I too wonder what kind of sedum it is please.

@ SYinUSA - When you say soil conditioner, what exactly are you using to topdress with? And is that generic, to be used in all conditions?

Sedums - in my experience the tall sedums like Autumn Joy, are bullet proof. Once, I was transplanting and dug one out and put it on top of a leaf pile, forgot about it and discovered it the next spring....still living. lol My property is all clay but not really all that difficult to dig. In full all day sun, AJ sedum thrives and only has an issue wtih splitting and flopping when it gets too large and needs dividing. But I do water the whole bed it is planted in on a regular basis too.

I have grown some of the creeping sedums and tried them in part sun situations and I haven't been thrilled with those at all. I bought a couple and never bought more. Actually though, I've not tried them in full sun.

I have a lot of experience growing under trees. The competition from the tree roots is real and entirely noticeable. I have tried so many plants with good reputations close to trees where they didn't thrive and once moved to a better location, did great. The tree roots suck out all the moisture and nutrition.

The stuff I used is just labeled Soil Conditioner - it's finely chipped pine bark. Because my soil was in such poor condition - no topsoil to speak of whatsoever - I figured it would break down and improve tilth faster than coarser mulch. Compost would probably work too, but it may tend to wash away on the OP's slope, and it's not as "finished" a look as the conditioner.

Without knowing what sort of Sedum we are talking about it's all just speculation. The creeping little Sedums like acre, album etc grow naturally on poor, gritty, fast draining surfaces in full sun. They don't need a fine tilth or soil conditioner. If it's Sedum spectabile, now correctly Hylotelephium spectabile, then they can cope with better soil and more moisture. But they shouldn't be difficult. Bits sprout from stems on the compost heap. Perhaps the OP can provide pictures of the plant and the situation so we can identify the issue with better information.

how about a pic, more than once folks putting questions here did not mention very basic problematic factors which were obvious in a pic

if soil amending, in case of a Sedum, I would go for sand/gritt/ stony stuff, Flora makes a good point

at this point the rest is just educated speculation from the other members (me included), without more information = a pic or two

Not to argue with my betters here, but I would absolutely not use sand (even sharp sand) in Georgia clay. There are few plants that can deal with this very well (roses being one) without improvement. Even weeds struggle where the clay is at the surface.

Sedums probably aren't the best companion for roses since they like lean, free-draining soil, but that wasn't the question being asked.

Thank you all so much for your responses! I will post a few pictures tomorrow and a few more details, as I checked the site quite late today.  

The sedum is the light purple type like the first picture in this article:

https://www.thespruce.com/growing-sedum-showy-stonecrop-1402861

Right. So we're talking about what used to be called Sedum spectabile, now renamed Hylotelephium spectabile. Hard to know what the problem is since this is a really tough species. Bits root anywhere they're dropped ime. Looking forward to pix.

I am back with pictures, which I will post separately from my phone after I type this message.

I wanted to add that when the plants come out in the spring, they look just fine, as they are now in the pictures below. No complaints at this stage.

The trouble is that by the time they reach summer, the leaves turn yellow-ish and the bloom is minimal, on one, two stems max, but usually just one and stunted. The plant looks like it quits growing and stays pretty close to the ground. Clearly not thriving like in Spring.

My suspicion is that despite being a drought tolerant plant, it is not getting enough water, as I basically almost never watered it, only what it gots from the rain.

It may struggle too much in that sun. The soil is GA clay, just as SYinUSA showed, and it probably gets brick hard on these poor plants but I am not sure.

The only other explanation I have is that whatever fertilizer transfers from the roses they don't like and turn yellow. I just don't think a bit of fertilizer would do that.

Unfortunately, I do not have pictures of them in distress from the summer.

As they look now - all happy. But come summer, it's a different story.

I should also add this seems to be a difficult bed in general. Other than my trooper Drift Roses planted on the edge along the driveway and which perform beautifully, any perennial I tried in that area behind them just never seems to do well and eventually they all die. I suspect it is because of the large tall tree in the back and all the nutrients and water it hordes away from others.

This is why I eventually added two large containers there for some height in the back because nothing else seems to do well. Originally I thought the Sedum would grow to at least the level of the drifts and look OK in the back but mine clearly stay much shorter. Problem is have limited sunny spots and this is the main one.

I plan to focus more on containers there (want to add Russian Sage in two large containers) but in the meantime, I would love to figure out this Sedum situationship.

My stems tend to stay on the short side and never get as tall as these here, plus few stems with flowers (often just one) and weak, sparse. Nothing like the lush and abundant flowers pictures usually show.

https://www.whiteflowerfarm.com/hylotelephium-spectabile-brilliant

Google Allelopathic trees. You didn’t say what tree.

I think it's an oak. It drops nuts all over the place.

You say they have 'plenty of sun' but how much is that exactly after the tree leafs out? I think they're just a poor choice for the location. Perhaps look in to more dry shade tolerant plants and put the Hylotelephium elsewhere.

Try liriope. There is a varigated and the usual solid green. Mine have purple flowers once a year.

Hi floral_uk,

In fact, this bed receives sun virtually all day because the tree is very tall and the sun is on that bed at a slight angle almost all the time. So sun is not an issue there, but compacted soil, lots of root competition and possibly too much fertilizer form the nearby roses may be. I am not sure.

Thank you for the recommendation, Sherry! They are so pretty - definitely something I would like to incorporate in the garden in the future. Great reminder.

I have a few that bloom white. Mine are all pass-a-long plants. There are clumping and running green, I prefer the clumping.