The gemorah brings a contradiction from the baraisa which says you can pick the grass and just can't feed it to your animals. The gemorah reconciles the Mishnah to be saying the same thing. How can this be - the Mishnah explicitly states it is so that people should see the unpicked weeds in order to inspire grief?
Sruli Bernstein, Baltimore, USA
1) The Rambam, in his commentary on the Mishnah, writes that what causes the heart-ache is the fact that the weeds are there on the site of the synagogue. He writes that it follows that it is permitted to pick the weeds and leave them there in their place. What one may not do is pick them and feed them to one's animal, or destroy them totally. We see that the Rambam understands that if the grass is left in its place, this also arouses grief. It is only if the weeds are totally removed that people forget the grief. (This is stated slightly more clearly by the Bartenura on the Mishnah, 28a.)
2) The Tiferes Yisrael (#17) goes a step futher. He writes that not only is it permitted to pluck them, but it is actually preferable to pluck them and leave them in their place. If one does not pick the weeds at all, people will think that one is leaving the weeds until one's animal needs them. Therefore, they will not be so saddened by the ruined synagogue. In contrast, if one plucks the weeds and then leaves them in their place, this is most upsetting and it will awaken people to rebuild the synagogue.
3) The Mishneh Berurah (OC 151:29, and Sha'ar ha'Tziyun #17) cites only the interpretation of the Rambam and does not mention the Tiferes Yisrael.