Based on the Philip Morin Freneau poems available, I would actually hesitate to argue that Freneau used imagery or any other devices in his poems to present nature as having overwhelming powers. In fact, in both his available nature poems "On a Honey Bee" and "The Wild Honey-Suckle," Freneau is very honest in presenting nature as being very feeble, just as mankind is feeble.
We first see nature as being presented as feeble in "On a Honey Bee" when the bee is presented as having lost its way. We know the bee is presented as having lost its way because the poem's speaker first describes the bee as being "born to sip the lake or spring" and then asks the bee, "Why hither come on vagrant wing," meaning lost or wondering wing. Freneau later presents the bee as being even more feeble when the speaker invites the bee to drink from the speaker's wine glass but warns the bee not to take too big of a drink, or the bee will pass out and "in the ocean die," meaning the ocean of his glass of wine. He further warns the bee to drink as it pleases, and the speaker will tell the hive that the bee has died. Hence, using imagery to refer to the bee's possible death from drinking wine certainly portrays nature as feeble rather than overwhelmingly powerful.
The same can be seen in "The Wild Honey-Suckle." At first, the speaker praises the flower for having found a place to grow where it will neither be stepped on nor plucked, as we see in the lines of the opening stanza, " ... No roving foot shall crush thee here, / ... No busy hand provoke a tear." However, as the speaker continues to describe the honey suckle flower, the speaker begins to reflect on the fact that the flower must eventually die, as we see in the lines, "Smit with those charms, that must decay, / I grieve to see your future doom." Hence, since even in this poem Freneau described the flower's eminent death, it's clear he is portraying nature as feeble, just like man is feeble, rather than overwhelmingly powerful.
In terms of Romanticism, Freneau's poems are definitely in keeping with Romantic literature that developed in the United States. The Romantic period developed later in the US than in England, around the mid-1800s, the same time that political debates were giving rise to the Civil War. Due to political influences, American Romantic literature developed a much darker tone than English Romanticism. Specifically, rather than focusing on the individual in an optimistic and even spiritual way as seen in English Romanticism, American Romanticism was much more cynical and instead revealed the "underside of humanity" (Rahn, "Romanticism"). We can certainly see through Freneau's death references the cynical, darker tone of American Romanticism. Even his reference to the bee dying from getting drunk can be seen as an analogy for human beings and their own dark, drunken behavior.