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Mandevilla - Corrective Actions Required

Ventura, CA

I bought a Mandevilla vine last Spring, and it was really happy until early December. I'm in Ventura County (central CA), and we had a really warm winter, so I don't think temperature is a factor. We had a huge fire, and a problem with toxic ash in the air for about a month. I could tell the ash seemed to bring all the plants down a noche. After the fire I tried to clean the leaves twice, and I have Jungle Rain foliage cleaner to use when I have time again. Other factors for why it may not be happy: a lot happened after the fire and my watering schedule got way off, so it has been chronically underwatered. I meant to fertilize it regularly, and sat on the fence for too long on which fertilizer to get, so it's only been fertilized once since I bought it--I read it needs regular feeding during spring and summer and a final boost in fall. Despite that, it seemed to do really well, and is still growing new tendrils and flowering at the top of the trellis. The top of the trellis receives more sun in the winter than the bottom portion of the plant, so the sun angle has changed too. I don't have a lot of options on where to place the plant. We're in an apartment building and most of the areas I have available are strong wind/strong sun combo or strong direct sun all day. Our front landing is the happy place for all my potted plants--sheltered from the wind and direct all day sun.
So, I have a Mandevilla where the leaves are yellowing and dropping from the bottom portion of the plant. The green leaves remaining have small yellow dots. The middle portion of the trellis is a bit thin, and the top is overgrown (I tried to train and prune, but didn't tend to it regularly enough).
Trying to figure out my best options to bring it back to happiness, and will do a better job with maintenance this coming spring/summer. What do I do?
1) I think it's also growing out of it's pot. I'd like to repot it and add some worm castings into well draining soil
2) Do I prune it way back? How far back...completely off the trellis, or selectively trim it back on the trellis? I was not letting it wrap around itself, but that got away from me too (that portion seems to be doing the best ironically)
3)Trying to stay organic, I bought some Roots Organics fertilizer: NPK / 0.5 - 2 - 1
I thought about supplementing that with more phosphorus? But I also thought the worm castings could help overall. In the summertime I bought some hydrolyzed fish fertilizer, but for potted plants on a landing it was too stinky. It got used once.

I don't want to shock the plant, and I'm wondering if all things are needed plus regular watering...and if so, which do I do first? Pictures included--it's indoors right now b/c we're having a cold snap.

Thumbnail by MaiMaisGarden Thumbnail by MaiMaisGarden Thumbnail by MaiMaisGarden Thumbnail by MaiMaisGarden

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