This song reminds many of situations where they will be away for a time, and hope to return home.
Some of course think of the 'movie', but others think of leaving for the military, or perhaps for college.
I have wondered, too, when I will 'return home'. To me, it speaks of springtime when the plants first begin to bloom.
Below is some information on the 'Thorn Bush' that may help to shed some possible light:
Prunus spinosa (blackthorn or sloe) is a species of Prunus native to Europe, western Asia, and locally in northwest Africa.
Prunus spinosa is a deciduous large shrub or small tree growing to 5 m tall, with blackish bark and dense, stiff, spiny branches. The flowers are 1.5 cm diameter, with five slightly creamy-white petals; they are produced shortly before the leaves in early spring.
So, what is the answer to, "When does the thorn bush turn white?"
Apparently, the thorn bush turns white in Early Spring when the flowers appear on the plant.
"When the thorn bush turns white - that's when I'll come home..."