One day remembering her kernel-stone She set it by a wall that faced the south; Dew'd it with tears, hoped for a root, Watch'd for a waxing shoot, But there came none;
Finally, Laura remembered that she had saved a "kernel stone," or seed, from the goblin fruit she'd eaten.
She tries planting it in a nice, warm spot in their garden.
After all, she figures, if she can't find her dealer, she might as well try to grow her own.
But even though she waters it "with tears," it doesn't take root.
Laura's not much of a gardener if she thinks that salty tears are enough to start a plant from seed.
It never saw the sun, It never felt the trickling moisture run: While with sunk eyes and faded mouth She dream'd of melons, as a traveller sees False waves in desert drouth With shade of leaf-crown'd trees, And burns the thirstier in the sandful breeze.
Lines 286-287 literally describe the seed, which "never saw the sun," but they could also describe Laura herself, who is wasting away.
Laura can't stop dreaming about the fruit, but it's like a "traveler" who sees mirages of water in a "desert." None of it is real, but her dreams make her withdrawal symptoms even worse.