It depends on the title of the book
Participles are words that are formed from verbs but act as adjectives. They are called <em>verbals</em>. 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.
I would say the answer is "harbor". This word is usually used as a noun; but in this case, it is a verb that means to provide shelter to someone or something, to keep someone safe, but also to have something in mind and think about it. This word is purposefully chosen and fits into Whitman's well-intended, philanthropic and inclusive worldview.
