back = the other way Here it means they went back into the ground.

"back" is being used to describe a period of time. The bushed may have died... when? Back in this cold autumn. In can mean "previous" or the "past". Some other expressions: back in the old days back in the last year back in the last century

"Died back" means the bushes are not covering as much land as they normally would. The bushes didn't die completely, but that the death of some bushes means there are less of them.

Ah, Helen! This is a great sentence! "Died back" in this case means that the leaves have fallen or the growth has stopped--but not completely dead. You know how Spring causes new growth yet sometimes things stop growing in Autumn (Fall), foliage can "die back." Incidentally, we will also use "cut back" on bushes/shrubs/trees when we are snipping away at limbs and leaves that cause the bush to look messy. Sometimes the bush gets too full, so we will "cut back." Ex: Yesterday I cut back my bushes in preparation for the wedding.