It is not possible to pinpoint a definite verse that mentions rush. It is likely, however, that one of the species of the rush genus Juncus might be included with some of the aquatic plant descriptions.
According to van der Kooij and Ibrahim (G. van der Kooij and M. M. Ibrahim, editors. 1989. Picking up the Threads . . . A continuing review of excavations at Deir Alla, Jordan. Leiden: University of Leiden Archaeological Centre) the "stalk," actually the culm, of the sea rush, Juncus maritimus was used as a kind of brush for writing (page 65) by cutting it at an angle. This followed a style of writing developed in Egypt in which the shape of the letters in Aramaic and Hebrew were determined by the writing angle. For Aramaic the tip was turned to enlarge the surface area, and for Hebrew it was turned to decrease the surface area.
While the plant is identified by van der Kooij and Ibrahim as J. maritimus I see no reason why the culms of J. arabicus could not be used. Juncus maritimus is more or less restricted to coastal regions while J. arabicus is found at many sites in the Jordan Valley as well as in the desert. The morphological features used to distinguish between these two species are minor and it may be that the two rushes were used without distinction.