When does Spring begin, lower elevations, Interlaken ? I mean Spring flowers, daffodils, etc.
what about Alpine locations such as Murren?
I'm no flora expert so my comment is more general: in the lower areas around the lakes (Interlaken, Lucerne, Montreux), late April can already be nice and warm (20 °C/68 F). But higher up it will still be cold, with snow from around 2000 m. For Mürren I would say that early June is a better time. Right now, early April, there's still snow there. For high Alpine terrain, July to September are best. Of course things vary from year to year, but I hope this helps at least a bit to get a feel of what to expect.
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