United Kingdom 
We have just planted a mimosa and filled the bottom of the hole with ericacious soil and watered it after planting as advised by the nursery. Today has been a little wet and it has been a bit windy but some of the branches are now sagging on the ground and it looks pitiful. The central upright trunk is also sagging to the side, generally most of the branches are hanging down now. Is this normal. I am wondering if it has been over watered or under watered. We only planted it two days ago.
7 Aug, 2017
Answers
when you say mimosa, do you mean Acacia dealbata, or Albizia julibrissin? Mimosa is commonly used for either of those plants. Acacia dealbata prefers neutral to acidic soil, Albizia isn't so fussy.
How much ericaceous compost did you put in the planting hole? If it was too much on its own, that is, not mixed with proper soil, its probably too much to make a firm support for the roots of the tree. Did they tell you use to ericaceous compost because the soil alkaline where you live?
7 Aug, 2017
Did you stake it? It doesn't sound as though you did if its sagging over. Could you provide a photo? There is an option to do that at the top of the question form.
Good idea to add your county to your profile - sometimes it makes a difference to the advice you get.
7 Aug, 2017