IME, there are very few issues that bother hebes...........extreme winter cold (for some varieties) or inadequate drainage leading to root rot. And yours is displaying the classic symptoms of root rot. Very good drainage is an absolute requirement of these plants. I'd also wonder about the amount and frequency of your watering regime. While not excessively drought tolerant, they are also not very thirsty plants and I rarely ever water mine during our cool but usually very dry summers (except those grown in containers).

Knowing your geographic location would help as well.

Thanks gardengal! I’m in zone 9b/10a. I was worried it might be root rot, do you have any advice going forward? Can the affected ones be saved or do I need to dig them up and start over?

at a min.. you should dig one up.. and look at the roots ... why guess about it ...

either replant it.. or throw it out ... snap a pic if you dont understand what you see ..

rots are usually rather easy to smell ... it isnt pleasant ... but its a clue ...

you didnt answer about your soil type ... rots are usually associated with poor draining soil ...

big city name would be more helpful than just zone ...

ken

Recovery from root rot is rare. Dig up the dead one and examine. If a rot, the roots will be soft and mushy and off color. If you find that, then you need to remove all, fix the soil conditions and start over. I am assuming you are somewhere in NorCal :-) Hebes will not thrive in any east coast locations that carry that zonal designation.

Thanks for all your help Ken and gardengal,

I'll take a look at the roots. I'm close to San Jose, CA so it rarely goes below 40 degrees ferinheight here. The soil where these are planted is silty and a bit sandy. Perhaps I should add some peat moss?