In this special setting, the bulrushes symbolize the arrival of Moses who ushered in the Law of the Old Testament.
"Moses & the Law"
"The bulrush is a lowly, thickly clustered, common plant, growing near the water. Because of these characteristics, it has become a symbol for the multitude of the faithful who lead a humble life and abide by the teaching of the Church, the source of living waters. This explanation found support in Job 8:11, 'Can the rush grow up without mire? Can the flag grow without water?' Also, since the infant Moses ( and Moses is taken as the forerunner of Christ) was found in the bulrushes, they have come to be connected with the place whence salvation came."