ZZZ wrote:
Bambusa Chungii that I planted @6 months back that miraculously suffered no damage from being submerged in 100% salt water for many hours at a time over the course of 2 days.
I have had this species submerged for much longer periods under fresh water, but the fact that yours survived salt water so easily is quite interesting to me. I would have expected at least severe salt damage.
ZZZ wrote:
Is overwatering the cause? The fact that the landscapers buried the root balls under @8" of dirt? That I put organic potting soil in the bottom of the holes before transplanting? That temps got to the mid-40s for a few nights last week? Is this just standard transplanting shock, in which case these will bounce back and I have nothing to worry about?
It is hard to say for certain what is happening, but you can rule out temperature. You were way above the cold damage threshold for these plants.
The only way I would say it is "standard" transplant shock is if the plants lost a lot of roots right before or during planting. For instance, if a pot was heavily rooted into the ground, and then cut loose right before sale. This could cause the plant to drop some leaves, but is generally not of long term significance.
The other reasons (overwatering, burying the rootballs, organic matter under the rootball) are all possible problems. Where I live, if these plants were almost anything other than tropical bamboos, they would certainly all die. Deep planting alone will kill most woody plants, and many running bamboos, here. If the soil is very well drained, or in the case of some plants that can self-layer, you might get away with this. Buried organic matter and overwatering work synergistically with deep planting to create a very effective way to eliminate oxygen from the soil, and kill plant roots.
Tropical bamboos are about as resistant to the above problems as anything, but it does not sound like a great situation to me. It is possible that these plants will adapt and get some roots near the surface. They will then set their own depth for future shoots. However, I will say that I have been in your position in the past. After losing many plants, I have learned to generally avoid planting below grade. If these were at my house, I would not sleep well until I had dug them up and reset them at grade. The only way you will know for certain whether they will be alright is to leave them alone, but this risks more root death every day. Left alone, at some point they will either shoot or die. If you are losing roots, and you wait too long, it may be too late to save them. You can always dig the worst looking one. If the roots are obviously dying, or the soil smells anaerobic, you will know to dig the others.
If I understand you correctly, they are in a raised bed, but set deeply, so the roots are actually still above the level of the surrounding ground. This may afford you some measure of protection, but I would still be worried. You can back off on the watering for now, but if these plants really are drowning because of deep planting, what happens if you get a very rainy summer? In my case, many plants die

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