Hi Kadie and Willeke.
I think you got better answers than I have! I would have focused only on my own greatest fear for potted plants, too much fertilizer or salt build-up. When I heard "no drainage holes" I would have assumed that was the whole problem.
But keep an open mind!
When you transplant them, will they go into pots or into the ground? If you put them in pots, I hope the pots will have drainage holes and you will remove or flush the old mix to remove as many fertilizer salts as practical. But I always focus on drainage, and I blame bad drainage for every possible problem.
>> not all of the plants do have it, and also only the older leaves.
I think that's the most important symptom. I think it means the plants are fairly happy and are not in any danger yet.
