Seeing that it is only 18-20" across I can think of 2 different ways to add the white.
Use 8 - 45 degree triangles and applique the small trees onto those pieces.
Or just cut an octogon [8 sided] piece of your white background. The applique the trees at the bottom of each side. Let the quilting do the rest to make it look like 8 triangles sewed together.
ali