Japanese Speakers: Would this be a proper haiku?

Grammatically speaking, the 花を咲く(はなをさく) should be 花が咲く(はながさく) since 花 is the subject of the verb 咲く. (が is the case particle as a subject marker, as you may know)

If you want to say "仏 lets/makes flowers bloom" then the verb 咲く needs to be changed to the causative form, i.e. 咲かせる(さかせる).

If this is the case, to make the last line (?dunno how you call it) 5 morae, I think you can say 花咲かせ(はなさかせ), leaving out the case particle を.

If you your mean,

"In a summer day, Buddha is walking and let the flowers bloom" 菫's answer is perfect.

If you mean "はなをさく" by "花を放く", an ancient Japanese word meaning "be distant from flowers", your original haiku is perfect.