Oleander Leaves Black Green & Droop
I'm located in Zone 5/Ottawa, Eastern Ontario, and we have an Oleander shrub/tree lovingly grown from slip over 8 years ago into an over 6' shrub. We take it inside our house/large window foyer every winter and then repot it in the same south west, but partially shaded area every May/April. Because it had spider mites in late Feb. and it was unusually very warm out early April, we put the plant out on the porch to harden it off and hopefully kill off the mites that still existed after spraying it with soap for a couple of weeks. (We've done this around 3 times during the plant's lifetime with success.) The plant looked fine while on the porch for a week, except for the remaining mites and the odd yellowing leaf.
Then we dragged it out in 22 celcius deg weather in the yard towards its usual location and soaked the soil (clayish but with earthworms) in order to dig up the hole- and also put some compost assorted tree leaves in the bottom for conditioning. We hadn't fertilized the plant all winter, and still haven't, but now I'm wondering what to do.
The plant, out for a good 3 weeks now seems to have gone well beyond its usual transplant shock and I've never seen the leaves this dark and not rebounded from the droop. The only thing I can think of is that the weather turned cold at night again, but it remained pretty warm during the day: 15-20 celcius most days, and sunny, though with a few rain soakings now and again. We also haven't watered it since, since I know it hates sogginess, and it rained a few times since repotted.
Is it possible to save this plant if what harmed it was just the extreme cold weather shock (1-5 Celcius most nights), or is it something more sinister? What could I do to keep it going? I love the plant even more for its beautiful dark green palm tree like foliage than for the pretty pink flowers it gives 3 times a year, and we did get the original tiny clipping from an elederly Italian man's plant over 8 years ago and took great care to nuture it. Have we been careless and killed it now?
Thanks for any help