Why does Odysseus call Nausicaa a sapling?

Nausicaa is the daughter of King Alcinous, the king of the Phaeacians. Odysseus winds up on their island on his journey home to Ithaca. Nausicaa is a maiden and of an age to be married.
Nausicaa early on gets the idea that Odysseus would make a good husband for her. Here is what she says to her handmaidens:

“Listen, white-armed maidens, that I may say somewhat. [240] Not without the will of all the gods who hold Olympus does this man come among the godlike Phaeacians. Before he seemed to me uncouth, but now he is like the gods, who hold broad heaven. Would that a man such as he might be called my husband, [245] dwelling here, and that it might please him here to remain. But come, my maidens; give to the stranger food and drink.”

To defuse the sexual tension, Odysseus calls Nausicaa a sapling. This is likely for two reasons. One, in Greek culture, brides and grooms were often described using plant imagery. The groom was often described as a sturdy sapling or young tree, while the bride was described as a grapevine or a flower—imagery that would have been subservient to the groom. In calling Nausicaa a sapling, Odysseus is attributing strength to her, which will serve him well if she turns out to be Artemis in disguise!
Secondly, he calls her a sapling to put himself in a fatherly position to her. If she is a sapling, youthful and new in the world, he is an oak—sturdy and weather-worn. He is highlighting the age difference between them and respectfully pointing out that he is not a suitable choice of husband for her.