When the speaker of the poem says, "Being present means I accept its isness," I think this means that she accepts the reality of the present moment, its existence and significance, as valuable. Throughout the poem, there are many references to practicing mindfulness and achieving a sense of peacefulness by focusing on the present in this way. So much of mindfulness has to do with being in the present and focusing on what is happening in the moment, not worrying about something that has already happened in the past or what could happen in the future. In order to put the past behind us, so to speak, and refrain from dwelling on what ifs concerning the future, we must learn to accept the value and "isness" of the present.
When the speaker says, near the poem's end, "Plant your flower......" I think that she is still referring to living in this present moment: when we plant a flower, we should not think about the flowers that may have come before and died, and we should not think about whether or not the flower we are planting now will eventually prosper. We can exist in this moment of hope, the moment of planting of the seed and hoping for the best. The hopefulness of this moment exists independently of what happens in any other moment, and it has "isness" too.