I grow bonsai as a hobby, for almost ten years now, and to get new material I often just collect some seedlings or saplings in the neighborhood. They are from the side of the road or other "wild" and "rough" spots here in the city of Amsterdam (Netherlands).
Last year in summer I collected some small oaks, hawthorns, field maples and this hornbeam (Carpinus betulus), most of them yearlings I guess. All these saplings are doing great, they survived winter (was not that cold), and have gotten their leaves back since spring. However, the hornbeam is the only one that is not doing well, the leaves are turning brown quickly and shrivel (see image). What is wrong with it?
I have all these saplings in pots with drainage holes, with fresh potting soil, and they are placed next to a south facing fence, so enough sunlight for most of the day. The rest of my saplings are doing great, so it is only this Carpinus that is having problems, but I don't know what. I water them when I see that the top of the soil is dry (that is almost every day, except for rainy days). Can it be too much light? Does hornbeam needs shade as a sapling? Too much water? Or are there other factors that Carpinus needs or can't stand compared to the rest of the trees?
The temperature the last weeks were between 15-20 C at day time and not lower than 5 C at night.