I dug my yellow semp up today and its definitely root rot. We've had an awful lot of rain this summer and I guess some are fairing better than others. It could be that some of the roots were damaged when I transplanted them into the bed because they were all cramped up in a small pot and the roots were all tangled together. I've cut the root out as far as I can and have left it to dry out before re siting it, but I don't know if its too little too late.

Good job Kate and Chris. Hopefully all will be saved.

OK
I have removed the healthy plants and the main chick seems to go first the roots are either gone or soft. My only problem with this is that these beds have been there for years and no problem so I am a lil hesitant to think it is too much rain. The picture of the empty bed by the tree is the bed I lost last year which started the year before it took 15 years to get that bed that size and two to wipe it out. You will notice it is a lil shady in both the beds I am having problemsI have pics of what the roots look like
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Aww I'm sorry that you lost so many. Maybe it a combination of rain and shade. Has the tree grown significantly in that time? Or maybe the tree roots are very dense preventing water running away quickly enough? You could dig some grit into the bed to help improve drainage.

Well it does look like rot but nothing much has changed. I want to remove the dirt from the beds and replant what is a good replacement type of soil for them really bums me out it took years to grow them and also could anyone recommend how to go about fertilizing and what not to get these going again

It also looks like some of those could have bloomed.
I agree with Kate, with time the soil has compacted, too much shade and too much water. I think the combination has been causing them to rot.
The happy note is that you saved some to carry on. Once they are growing in healthier conditions they should multiply quickly for you.
I look forward to seeing more photos of them newly planted, and watching their progress.
By the way, the above photos are great.