1. The Gemara discusses whether one may plant a tree in a field if it will ruin someone else's water reservoir.
2. The issue centers on who is required to avoid damage: the one doing the damage, or the one being damaged.
3. The dispute also applies to planting mustard plants in proximity to a neighbor's beehive.
4. They argue similarly about a person who soaks flax near his neighbor's vegetable plants.
5. The Rabanan argue (in #3) that bees do not damage mustard plants.
A BIT MORE
1. Even though the tree is planted in his own yard and the water pit is in his neighbor's yard, the roots of the tree will end up invading his neighbor's water pit.
2. Rebbi Yosi: The one being damaged must move. Rabanan: The one doing the damage must move.
3. Rabanan: One may not plant a mustard plant near his neighbor's beehive. The bees will eat the mustard, and then they will consume their own honey to alleviate the bitter taste (Rashi). Rebbi Yosi: The one being damaged (the beehive owner) must move. Moreover, even according to those who maintain that the damager must move, in this case the beehive also damages the mustard (as the bees eat it). (See #5 below.)
4. Even though the Beraisa seems to say that they both require a person to move his flax away from his neighbor's vegetables because the water from the soaked flax seeps into the ground and ruins the vegetables, the Gemara concludes that Rebbi Yosi maintains that one does not need to distance his flax from his neighbor's vegetables.
5. Accordingly, Rebbi Yosi's claim -- that the Rabanan should agree with him because the bees and mustard plants damage each other -- is invalid.