Our speaker's kind of a plant guy. In this poem, it seems that he hasn't met a plant that he doesn't like, or miss—save one. The glaring exception to his praise of all things green comes at the end of the poem, but we think that has more to do with where he is than the flower in question. Let's face it: some plants are powerful symbols that remind you of home, and some… well, aren't. After all, when's the last time you saw a Venus fly-trap and sighed with longing?
Lines 5-6: The elm tree is starting to bud its leaves, as are the smaller limbs ("brushwood sheaf") that surround it. The image here highlights the force of life emerging in spring, budding forth from its winter nap.
Lines 11-13: One month into spring, the pear tree is so filled with fruit blossoms that its limbs are sagging to the ground. That is one fertile symbol, reminding us of the life and energy that the speaker's memory ascribes to England.
Lines 17-19: Don't let the dew fool you—this field will be perking up with buttercups just as soon as the noon sun comes out. The speaker also uses the metaphor of "the little children's dower"—a kind of gift or inheritance—to explain how valuable these symbols of natural harmony truly are.
Line 20: There is, as it turns out, one plant that the speaker just isn't into: "the melon-flower." Boo, melons. It's too showy ("gaudy") for our speaker's taste, and we guess that we can see that. It is pretty big and yellow, after all.
At the same time, we think this has more to do with the speaker being in a hotter climate in spring. Weather that's hot enough to get the melons to bloom would be pretty different from the cool and delicate spring he's missing back in England. This flower, then, is more a symbol of his homesickness than an actual object of his dislike. Come on, man, don't blame the melons.