* Every year - This is a prepositional phrase, with "every" as the preposition and "year" as the object of the preposition.
* Spring - This is not a prepositional phrase. It's a noun functioning as the subject of the sentence.
* We plant wildflowers seeds - This is a verb phrase and doesn't contain any prepositional phrases.
* In my garden - This is a prepositional phrase, with "in" as the preposition and "my garden" as the object of the preposition.
Therefore, there are two prepositional phrases in the sentence: every year and in my garden.