Albizia saman – Rain Tree leaves started yellowing

[mike020]

Hi,
I’m looking for advice from people who have experience growing tropical trees.

While on vacation in Thailand, I came across a tree that I really liked (most likely Albizia saman – Rain Tree). I decided to collect a few seeds and try to grow it at home, with some help from ChatGPT.
I planted the seed in June using Pokon Bio Palmen Potgrond Turfvrij. As of today, the tree is about 150 cm tall, growing in a large self-watering pot (approx. 20 L). I’m using a Spider Farmer SF2000 Pro grow light at about 20% power. Until recently, the plant was growing very well, but over the past few weeks some problems have appeared.

Symptoms:

• leaves started yellowing (first the lower ones, then gradually higher),

• some leaves dry out from the edges, others fall off completely,

• some leaves have lighter spots or short streaks, sometimes slightly raised and noticeable to the touch,

• some leaves feel wrinkled, while others are completely smooth,

• when one leaf on a branch starts to yellow, often more leaves from the same branch fall later.

Soil / watering:

• previously watered with tap water (quite hard); for the past few days I’ve been using filtered water / rainwater,

• Pokon fertilizers were used (fertilizer spikes + liquid bonsai fertilizer), but only occasionally,

• fungus gnats appeared – I treated the soil with beneficial nematodes (BTI / nematodes against larvae).

Additional info:

• I need to cut the top of the tree (it’s too tall and is touching the ceiling). One of the photos shows where I’m holding the stem – that’s where the main shoot was cut before. As you can see, two new main shoots appeared in its place.

• I’m planning to try rooting the cut top to grow a new plant.

My questions:

1. Do these symptoms point more toward salt buildup / root stress, or rather leaf diseases or pests?

2. Is it normal for Albizia to drop entire groups of leaves when stressed?

3. After conditions stabilize, will leaves regrow on the same branches, or only as new growth?

4. What should I pay special attention to when continuing to grow this tree indoors?

I’d really appreciate any advice, especially from people who have experience with Albizia or other tropical trees 🌿