Kilayim, Chapter 6, Mishnah 8. This mishnah expands on what we learned earlier about the aris - the fence on which vines are trained.
The words of the Mishnah:
The Mishnah deals with "kanim hayotz'im min ha'aris" - reeds that protrude from the vine branch, from the lattice, "v'chas aleihem" - and he spares them. Taken simply, the expression means that a person spares them, and here it means that he has no wish to ruin them: he refrains from breaking them, because they are good reeds and he does not want to spoil them.
At the same time, he has no intention whatsoever that the vine should grow and spread onto those reeds that stick out. His refraining stems entirely from the fact that he does not want to break the reed and spoil it.
The practical ruling:
Where there is no intention for growth: One who sows opposite those reeds, or directly beneath them, is permitted, because he has no intention at all that the vine should eventually reach that place.
Where there is intention for growth: If he set up the reeds, or left them there, for the purpose that the new vines should grow and spread onto them, it is forbidden. Their status is like any other potential spreading of an aris, regarding which we said that there is a problem when it is designated for future growth.
In summary: in this mishnah everything depends on intention. Reeds that protrude from the aris, which a person spares and does not break so as not to spoil them, may be sown opposite and beneath, since he has no intention that the vine should spread onto them. But if they were set in place or left there so that the vine would spread onto them in the future, they are considered a continuation of the aris, and sowing opposite them is forbidden.