Welcome to the site! Would you please post a picture or two of your tree, including the berries? Seeing it is the best way to help diagnose the problem. Also, where do you live? How old is the tree? Does it have any visible damage to the trunk, branches or leaves? Press on the gray word "edit" and add your information into the question. That way, it will all be in the same place. Since you're new here, I recommend checking out our [help]. [Ask] is a good place to start. Leave us a note if you need help with anything!
– Sue Saddest Farewell TGO GLMay 28 '17 at 18:24
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What happens to them as time goes on (instead of ripening)? shrivel and dry up? eaten by insects/mammals? get moldy? drop off the tree?
– Lorel C.May 28 '17 at 20:15
First guess - you have a white mulberry but expect purple/red "ripe" mulberries...?
– EcnerwalMay 28 '17 at 21:25
I've only known one mulberry. Not very hardy in the Pacific Northwest. Not a flower nor fruit to be seen. If your flowering plants get some fertilizer because it is planted close to a lawn, that fertilizer is so high in Nitrogen, that imbalance will cause zip flowering and fruit. Could this be true ish? Have you fertilized? Is your Mulberry in the shade? You have to send pictures and as much information about that plant, its history, its management as you are able. Nothing is too insignificant. What zone are you and your plant in? What plants are nearby?
– stormyMay 28 '17 at 22:39
Could be that those berries are gobbled up so quickly by birds you've not been able to even see them. Pictures, information!
– stormyMay 28 '17 at 22:40