What will sprout between grass?
Rashi: Converts who will be added [to Yisrael] from amidst Amalek. "Chatzir" is Amalek, about whom it says "v'Haysah Naveh Sanim Chatzir li'Vnos Ya'anah" (34:13).
Radak: The prefix Beis in "b'Vein" is for added clarity, just like the prefix in b'Soch. Yisrael will sprout with good, as if they were planted amidst grass, which sprouts quickly. For this they are compared to grass, but not for endurance; grass does not last - "Chatzir Meherah Yimalu" (Tehilim 37:2).
Malbim: Refer to 44:4:2:2.
What is the comparison to "Aravim Al Yivlei Mayim"?
Radak: They always remain in their moisture 1 .
Malbim: Even though they will grow amidst grass, which dries quickly and grows in a dry place, they will be like willows, which are strong and firm, and grow near water. The Nimshal is, Yisrael will sprout amidst Nochrim, who are like dry grass, and Hashem will pour His Ru'ach of spirituality and Kedushah on Yisrael.
I.e. even in winter, willows leaves are green, and stay on the tree. (PF)