why in the case of schach of a succah that was still attached to the ground do we say that you can just cut it to make it into schach kosher if it was placed on top of a succah(succah (11a).we should apply the rule of dichui and the schach should be possul.succah 11a.In that gemara there is also the case of tzitzis that was only completed after the knots were already made.We should say dichui.why dont we
Shmaya Ormonde, England
As we see in the Gemara here, in order for Dichuy to take effect there must first be an identifiable object that can be isolated as the object required for the Mitzvah (i.e. the "Cheftza Shel Mitzvah"). For this reason, a Hadas whose tip is cut before Sukos is subject to the question of Dichuy by Mitzvos. "Sechach" per se is not a "Cheftza Shel Mitzvah;" the Torah does not designate this branch to be used as Sechach, as it designates an Esrog or Hadas. Rather, the Sechach is merely an object that is valid to be used for creating a Sukah, and thus it is not subject to Dichuy.
The same applies for Tzitzis. Dichuy applies when we have the Cheftza of the Mitzvah but it is Pasul. It only applies where this is clearly the object that the Torah requires. This isthe case of Dam or Hadas; the Torah clearly requires that object to be used for the Mitzvah, but it has an invalidating factor preventing it from being used for the Mitzvah. Tzitzis which are not completed, however, have not as yet acquired the title of "Tzitzis," and therefore Dichuy does not apply to them.