Kilayim, Chapter Six, Mishnah Eight
This mishnah deals with reed canes that protrude from an aris but that dont have vines hanging on them.
1) Canes which protrude from the aris and one is too concerned for them to cut them short, it is permitted to sow directly beneath them.
2) If he made them [long] so that the new [growth] might spread along them, it is forbidden [two sow underneath].
Section one: In this case the only reason that he hasnt cut the unused canes down is that he doesnt want to destroy them. He has no plans to train the vines over these canes. Therefore, he may sow seeds directly beneath them.
Section two: In this case, he does plan to use the canes as part of the aris. Hence, even before he does use them, it is forbidden to sow seeds underneath.