Rashi: Does a reed grow if it is not in a moist marsh? Ibn Ezra - this is like "Bitzosav u'Geva'av."

Malbim: Reeds grow only amidst moisture. In their youth they are strong, but once the moisture ceases or the heat of summer comes, they dry and perish.

Ibn Ezra: It is vegetation that grows in a swamp, like "va'Tir'enah ba'Achu."

Malbim: It is grass.

Devar Eliyahu: Some grass grows tall, from mud. Other grass grows low, from water in a swamp. This is a parable for two kinds of success, wealth and honor - "bi'Smolah Osher v'Chavod." Ha'Yig'eh Gome refers to honor - it grows tall. Yisge is wealth - he has much property. Wealth and honor that come to a person for his few good deeds - there are two kinds of good deeds. Some are Emes and straight; others are mere flattery. The former deserve wealth, and the latter, only honor. "Ha'Yig'eh Gome b'Lo Vitzah" - honor comes for what grows in a marsh (water mixed with earth). "Yisgeh Achu Vli Mayim" - [wealth come from pure good deeds], clear and clean water.

Malbim: Grass does not need so much moisture - it lasts longer.