In the door-yard fronting an old farm-house, near the white-wash’d palings, Stands the lilac bush, tall-growing, with heart-shaped leaves of rich green, With many a pointed blossom, rising, delicate, with the perfume strong I love, With every leaf a miracle......and from this bush in the door-yard, With delicate-color’d blossoms, and heart-shaped leaves of rich green, A sprig, with its flower, I break.
The nature imagery here is showing the beauty in nature as things grow, and is also a symbol for death and life, because in nature plants have to die at some point in the winter to come back to life in the spring. This connects to lincolns death because although witman is mourning his loss, he tries to remember his life in a good and positive light.