The remaining two blue plumbagos–bought late last year at Mobile Botanical Gardens fall sale–are trying to die like the other two. I sprayed them last week with Sevin in case the problem was a pest. They seemed to look some better but still no new growth and a lot of dead near the base.
Sounds like it could be overwatering–as we first suspected–and now the root ball is rotting. A video we watched talked about how and overly wet root ball rots and the water prevents oxygen from getting to the roots. His solution was to pour diluted hydrogen peroxide to kill the rot and provide oxygen.
That sounded plausible and we had one 12 oz. full bottle of hydrogen peroxide. Pour it into a watering can and then two full bottles of water to make a 2:1 solution. Mary poured half on each plumbago. That was not nearly as much as he poured on per his statements but it was as much as he mixed up in the video.