Rashi: His word will descend amidst Your nation and in their hearts, like rain that falls on detached vegetables. It needs rain after it is cut - "v'Hinei Lekesh Achar Gizei ha'Melech" (Amos 7:1).

Radak: This king will come on (to) the nation for their benefit and salvation, like rain descends on harvested grass and makes it sprout more. If it refers to Shlomo, so it was - all his days, a man sat under his vine and fig tree (Melachim I, 5:5). There was no opposition or mishap (ibid., 18). If it refers to Mashi'ach, Yisrael will be in great serenity and Shalom without comparison. In Galus, they were like harvested grass, and when the rain comes on them, they will sprout and grow greatly.

Malbim: The heart of the nation will be prepared for Yir'as Hashem. The king's Musar and rebuke will be like rain that makes grass grow. Since it compared them to the sun, it adopts also the metaphor of rain, for sun and rain together make all vegetation of the field sprout. The people's desire to hear his Musar will be like the grass' desire for rain - "Ya'arof ka'Matar Likchi" (Devarim 32:2), "v'Yichalu cha'Matar Li" (Iyov 29:23). Attached grass, its mouth is closed; the rain wets it from its roots. Cut grass, its mouth is open. So [the nation] is ready to accept his rebukes and make fear sprout in their hearts.

Rashi, Radak: It is an expression of drops in Arame'ic - 'Mata Zarzifi d'Maya a'Reishei' (Yuma 87a).

Malbim: When the drops of rain fall on Zarzif Aretz (a place that did not sprout yet), they make it sprout. So too, people who did not yet understand Musar and Yir'as Hashem (will become prepared for fear).