Participles are words that are formed from verbs but act as adjectives. They are called verbals. What is more, phrases are a group of words without a subject and a verb that function as as single part of speech. Therefore, participles consist of the participle and its complements and modifiers.
Taking all this into account, it can be said that the participial phrase is "trimmed that morning" and that it modifies the noun "grass". This is a past participle which is further specified by the NP "that morning" which gives information about the time in which the grass was trimmed.
