What’s wrong with my Oleander plants?

I grew up in Florida, and Oleander was one of those iconic plants that were in my neighbors yards, public gardens and even planted in the dividers between east and west sections of Interstate 4 (I-4) in an attempt to beautify the space. Last year I ran across some lovely white oleander topiaries at my local nursery, and just had to have them. A little voice in the back of my head whispered that they would most likely not over winter in D.C. and that I was a premeditated plant killer for my own greedy need to have a piece of home greet me each day as I came home. I got over that bit of guilt quickly, bought 2 topiaries and enjoyed their dark green leaves and beautiful white trumpet like flowers well into October.

November and December came and went, and the Oleanders were still alive, no longer flowering, but leaves a nice dark green. January, we’d had a few snows, and an ice storm uprooted one of the plants but I replanted it, and staked it so it would remain up right. February, they were still alive, a few brown leaves, but mostly all still green. March arrived and I added more soil to the planters, and the Oleanders began to sport some yellow leaves. I though it might have been winter drought related, so made sure to check the soil regularly and water when dry. In April more leaves started to turn yellow. I didn’t think much of it because there was enough new growth, and in early May, buds began to appear for the white trumpet flowers. Mid to late May, flowers were popping, simultaneously, the yellow leaves started appearing and dropping. I don’t know which has been more prolific – the flowers or the dropping leaves.

I know, I know, initially, I had no problem with premeditated plant murder, but they’ve been part of my front porch for the last year, and I’ve come to regard them as regular residents. I could replace them with something new, but it seems to be so much more fiscally responsible to try to save them. So here you have my sad Oleanders. What’s wrong with them?