Kilayim, Chapter 7, Mishnah 7. The Mishnah opens with a case in which the wind moved the plants from their place: "Haruach she'ilelah es hagefanim al gabei tevuah" - a wind that swept the vines up and laid them down on top of grain growing there, so that the two are intermingled in a way that constitutes kilei hakerem.
About this the Mishnah says: "Yigdor miyad" - and two explanations have been given for this word:
"Yigdor" - he must put up a fence immediately, the fence being the means of separating between the vines and the grain.
And some have the reading and explain it in the sense of cutting down - he must cut the vine down immediately.
"Im eira ones - mutar" - if a circumstance beyond his control arose so that he was unable to fence off or uproot, it is permitted in the meantime, since he has no interest in the existence of the kilayim. Therefore even if they grew more than one part in two hundred - the measure that ordinarily creates a problem - there is no prohibition, since in the meantime he does not want the kilayim.
Grain that leans under the vine:
Another case: "Tevuah shehi notah tachas hagefen, vechein bayarak - machzir ve'eino mekadesh" - leaning under the vine is not the same as growing in that spot: the root is not there, rather the upper part of the grain plant leans in under the vine, and the same law applies to a vegetable that leans under the vine. In such a case he must move the plant back so that it is no longer under the vine, but this does not create a prohibition, because the root of the plant is not actually under the vine.
From when do grain and grapes become forbidden:
From here the Mishnah moves on to discuss at what stage of growth the grain and the grapes become forbidden and kilei hakerem is created: "Me'eimasai tevuah miskadeshes?" - from what stage does the grain have the capacity to create kiddush, that is, a forbidden mixture of kilei hakerem:
For grain - "mishetashrish", from the time it takes root. And some have the reading "mishetashlish", from the time it has grown one third of its normal growth. According to both readings, only from that stage does the grain have the capacity to create kilei hakerem.
For grapes - "mishe'ya'asu kapol", from the time they reach the size of a white bean. Once the grapes have reached this small measure, from then on they can create kilei hakerem.
Grain and grapes that have fully ripened:
And what about the opposite situation? "Tevuah sheyavshah kol tzorkah ve'anavim shebishlu kol tzorkan - einan miskadshos" - grain that has dried out completely and ripened fully and grows no more, and likewise grapes that have ripened fully, do not create kiddush and do not create kilei hakerem, because at this stage there is no real growth.