Kim, tell us what plant types you have lost and what plants have survived. Is this a full sun bed? A bed around the house foundation? Watering?
What is on the other side of the bed that seems to be doing OK and why do you think it's doing better than the troubled side.

Also, the dead/dying plants..did they ever thrive at any point after planting? Are they dying abruptly or wilting/discoloring leaves and failing to put out new growth?

Hi Dottie (and everyone else),

I have two separate issues here. The first one is dying plants in my originally amended soil (rubber, topsoil, and some compost). I lost my Butterfly Blue Scabiosa, part of my Limerock Ruby, a Dahlia (top growth only, it is now sprouting from the tuber), and a Gaura. Right now my Geum Chilean Avens is looking sickly yellow in its leaves and is also probably on its way out. All of the plants (with the exception of the Dahlia) turned yellow and wilted before quickly dying - turning crispy. That Dahlia just wilted and died. I suspect stem rot perhaps due to placing mulch too close to the stem. They thrived after planting. The Butterfly Blue (planted last fall) bloomed almost all winter and was glorious in the early-mid spring before its quick demise. The Limerock Ruby overwintered and was thriving as well as was the Gaura (which was on its way to taking over the bed) and for its short life the Dahlia. The bed gets full sun, southern exposure, for close to 10 hours a day which is why I mulched well (using cocoa shells applied in the spring over a very thin layer of hardwood mulch) and close to the base of the plant. Plants that continue to do well in the "death row" bed are: hardy Hibiscus, Moonbeam and Sweet Dreams Coreopsis, the part of the Limerock that survived, Blue Mirror Delphinium (newly planted in the spring but currently sprouting some new growth), Surise Coneflower, Dianthus, and Joystick Armeria (gets some shade from other plants).

The bed is toward the front of the house, not near the foundation. The same plants (with the exception of the Avens, which is showing some yellow in its leaves) are doing well on the other side of the bed which is on the other side of the front walkway. We have a leaky gutter that pours water out on the "dying side" but not on the bed about 6' behind the bed and the yard slopes a little towards the front, so maybe the extra moisture is filtering towards the plants? The "non-dying" side also gets about 2-3 less hours of sun depending on the section of the bed. Beds are hand-watered overhead for the most part as needed depending on the individual plant (established, newly planted etc.) On the other side of each of these front beds is a dramatic slope to the street.

So, thinking it might be a drainage/mulch issue, I decided to rework the part of the beds with the dying flowers using the method I originally described mistakenly thinking that gypsum was an aeration agent like coarse sand or perlite. I then added mulch but left a nice open ring around the stem. So I am wondering if I need to offset all of the gypsum with a little lime which probably wouldn't hurt since my soil pH is so low which may have its own contribution to plant demise.

I apologize for the long post, but there is all of the information (I think :) ). Many thanks!

No..that's exactly what we needed to hear, that's why I asked so many questions.

I'd say, and I'm not an expert, that since it's a front yard issue (visibility) and you really should have the ability to plant whatever suits you..I'd take up and pot the living plants. Pull off all the mulch and set it aside.
Deeply till the remainder soil down to at least 12" and let it set for couple of rainfalls before taking soil samples for testing. Take them from 3" and 10".

Besides the pH being quite acidic, you probably have other mineral and soil chemistry issues that need resolving so you can finally plant what you want.
The best garden soil is quite a bit more clay and dense than what you've produced with all the amendments. Sometimes also there are aspects of compost that aren't as beneficial as we thought.
Might want to address that leaky gutter while you can get to it (sans plants) and think about putting a drip line for watering instead of overhead watering.
With all the mulch atop the soil you'd be surprised how much rain it takes to get through the mulch to the soil to the root area of the plants you want to water.
You may have inadvertently created a clay bowl when digging out the existing clay and refilling with bagged soil and amendments. That might be needing some attention too.

I'd point out that some of the plants she mentioned that died don't really do that well here in the first place. Armeria is hard, as are delphs. Scabiosas seem to prefer cooler weather in my experience. I finally got some limerock going well but it took about 3 tries- it seems to be a little finicky. Since many of those plant want good drainage i'd bet that your gutter issue has rotted out some of the dying ones. It will dry out less being that it gets a bit less sun, too, which isn't good if you're already water logged. It also sounds like the acidity could be a contributing factor. Good luck with your sleuthing- i hope you get to the bottom of it!

Many thanks for your replies. I am beginning to think that it is a pH issue more and more as I research and learn that the plants that aren't doing as well are plants that require close to a neutral pH. The thriving plants can tolerate acidic soil. The yellowing of my Geum and Gaura suggests perhaps nutrient deficiency due the unavailability of nutrients (I think). And I also think I had stem rot from placing mulch too closely to the stems. If anyone thinks of anything else to consider please let me know. Thanks again.

-Kim