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   Morus rubra has winter buds that are 5-7 mm long.The buds are somewhat 
divergent to the stem and  the bud scales have dark margins.
   Albas buds are only 2-4 mm long and appressed.
   This being said when it comes to immature trees in that juvenile state 
measurements are not always accurate.
   I doubt that you would be dealing with Nigra.

Mark