Kilayim, Chapter Five, Mishnah Seven
This mishnah deals with various situations where seeds get into a vineyard without a person having intentionally planted them there.
1) If he was passing through a vineyard, and seeds fell from him, or [seeds] went [into the field] with manure, or with [irrigation] water, or if he was [in a grain field] scattering seed and the wind blew some behind him [into a vineyard], everything is permitted.
2) If the wind blew the seed in front of him [into a vineyard], Rabbi Akiba says:
a) If it has produced small shoots, he must turn the soil.
b) If it has reached the stage of green ears, he must beat them out.
c) If it has grown into grain, it must be burnt.
Section one: Since he didnt plant the seeds in the vineyard, rather they got there in ways that were unanticipated and perhaps even out of his control, they dont create kilayim in the vineyard. The vineyard will only become prohibited if he subsequently sees them there and doesnt uproot them.
Section two: In the previous section we learned that if the wind blew the seeds behind him into a vineyard, they dont cause the vineyard to become prohibited. Here we learn that if the wind was blowing in front of him, he will have to uproot the seeds and destroy them. If only small shoots, then all he needs to do is turn the soil in order to uproot them. If it has made green ears of grain, then he must uproot the ears and beat them out so that it will be unusable. Finally, if it ripened and produced grain then the grain must be burnt. The Rambam adds that if he noticed the grain growing there and didnt do anything about it, then the vines must be burnt as well, as we learned in yesterdays mishnah.