Despite its title, this poem isn't really about gardening. Two blades of grass stand in for something closer to two people, or two desires, swaying together in the hour before dawn. The strange image of them "etching" outlines of lips and gin suggests kissing and drinking, a night of pleasure that is almost over as the sun comes up. Pleasure is what they're reaching for, but the ground around their roots is lonely, so what they really want seems to be comfort.
The final lines turn darker. The soft undergrowth offers some relief, yet it also carries "reminders" of being mowed down year after year. The speaker seems to feel that closeness is tender but fragile, and that old hurt never fully leaves. The mood is hushed, wistful and slightly tired, like the quiet after a long night when you're still wishing for company but remember how easily things get cut back.