Kilayim, Chapter 2, Mishnah 5. This mishnah presents an exception to the halachah we learned in Mishnah 3: someone who has a field sown with one type of grain and wishes to sow another type in it must wait until the first crop takes root - at least a minimal rooting - and then uproot it and turn over the soil. Now we will learn a case in which this halachah does not apply.
The Exception: Hemp and Arum
"Haysah sadehu zeru'ah kanbus o luf - lo yehei zorea u'va al gaban" - someone whose field is sown with hemp (some read: karbus, a type of cumin or safflower) or with arum, which is a kind of onion, may not sow on top of them. Moreover, he does not even have the option offered in Mishnah 3, of waiting three days until the seed sends out its initial roots and then turning over the soil. Why not?
The mishnah explains: "she'einan osos ela l'shalosh shanim" - hemp and arum are exceptional, since up to three years can pass before they truly begin to grow. Normally, when a person turns over the soil the seed is ruined and there is no concern that it will still sprout; but here, since the growth is delayed for so long, waiting and turning over the soil accomplishes nothing, and the seed is liable to sprout three years later.
Plants the Farmer Has No Interest In
From here the mishnah moves on to other scenarios: even though additional plants actually do grow in the sown area, this does not constitute kilayim, because it is obvious to everyone that the farmer has no interest whatsoever in their growth, and he has no desire at all for the two things to grow together.
First we need to define what 'sefichim' are: plants that come up on their own, from seeds that remained in the ground from a previous year. When the seeds were being gathered, seeds of another type slipped out and fell randomly, and those seeds are buried in the earth - sometimes they need a year to sprout, and sometimes they only get lodged in the earth at a later point - and eventually they come up by themselves.
"Tevuah she'alu bah sefichei istis" - istis is a plant from which a bluish dye was produced, and it is very harmful to grain and has a negative effect on it. This is not something a person wants in his grain field, and everyone knows that he has no interest in it, and he certainly did not deliberately sow the two species together.
"V'chein mekom hagranos she'alu bo minim harbeh" - on the threshing floor area, where the threshing is done, a person wants a flat, smooth surface; once various species come up in it, this growth too disturbs him and he has no desire for it at all.
"V'chein tiltan she'he'elah minei asavim" - fenugreek served as animal feed, and when different kinds of weeds grow in it, they too are bad for it.
Bear in mind that most of the laws of kilayim are based on appearances, on how the matter looks, since the Torah forbade only a case where a person actually sows the seeds together, and not where they happened to grow together. In all of these cases we are dealing with plants in whose growth the farmer has no interest at all, and therefore "ein mechayvin oso l'nakeish" - we do not obligate him to uproot the foreign matter that came up in his field, because the situation speaks for itself: it is clear that they are not to his benefit, and therefore there is no kilayim in them.
"Im nikeish o kiseiach"
"Nikeish" - he began to uproot the foreign plants, taking a hoe and starting to pull them out.
"Kiseiach" - he cuts off the upper part of the plant, the leaves above it, and leaves its roots in the ground.
Once a person has begun this process of removal, even though he is not obligated to do so, we tell him: "akor hakol chutz mimin echad" - he must uproot everything except for one of the two species. In other words, under these circumstances he must be concerned about kilayim.
Leaving things as they are is one matter; but once he has begun the process of removal, by definition the parts he did not remove are the ones he is interested in, since he removed some of them and did not remove the rest. It looks as though he has an interest in what he left behind, and that creates a problem of kilayim. Therefore we say: once he has begun to remove some of them, he must remove all of them.
In summary: in this mishnah we learned an exception to the ruling of Mishnah 3 - with hemp and arum, waiting and turning over the soil is not enough, since they take up to three years to produce; and we learned that plants which came up on their own and in which the farmer has no interest - sefichei istis in grain, many species on the threshing floor, and weeds in fenugreek - he is not obligated to weed out, but once he has begun to weed or to trim, we tell him to uproot everything except for one species.