What happened here? I bought a Summer Chocolate Mimosa sapling a few years ago. The first summer after planting it was a beautiful deep chocolate color. The next summer, same thing, but 1 branch stayed bright green. Each year the green spread, until last year when half of the tree was green and half chocolate. Same thing this year.
Summer Chocolate Mimosa Tree
Certified GKH Gardening Expert
This is what's known as a "reversion". Summer Chocolate Mimosa is likely a hybrid which has the special characteristic of chocolate colored leaves, but it still has the genetic potential to produce green leaves. The branches with the green leaves have "reverted" and begun expressing the green-leaf genes. If you want to restore the chocolate color, prune out all the green-leaf sections, since reversions will typically grow more vigorously than the rest of the plant.