Question

Why was it still winter in one corner of the garden

Answer 1

The isolated corner was to remind the Giant that he also needed to take some steps to keep the gloominess away. The presence of symbols of winter in that corner is to symbolise the past deeds of the Giant which led to overall unhappiness in the garden.

Answer 2

Answer:

It was still winter in one corner of the garden because a tree in one corner was covered with frost and snow

The north wind was blowing and roaring over it.

Due to this winter was still  there as winter represents the symbol of suffering.

Hope this helps....