Q. How small do the pieces of remaining pine schach or hoshanos leafs that fell on the floor have to be to be considered batel or not important, so it is permitted to step on them? (Pine schach especially on the outside is hard to completely remove from the ground where people walk).

A. Shulchan Aruch (O.H. 664: 8 and 21:1 and Shaarei Tzion ibid. 20) warn on not stepping on leftover leafs of Hoshanos or threads of tzitzis.
Horav Shlomo Miller’s Shlit’a opinion is that if the pine needles or the leafs are no more distinctly recognizable as such, as when they dried or the particles are small, they loose the status of tashmishei mitzva or objects used for that purpose. Otherwise one should indeed be careful to sweep, remove and correctly dispose of these mitzva remnants.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.