Kilayim, Chapter 5, Mishnah 6. In the previous Mishnah we learned that even someone who does not plant kilayim in his vineyard with his own hands, but merely sees a foreign plant growing there and lets it remain - he too renders the vineyard forbidden. However, this is so only when he makes no active effort to uproot the kilayim. But if he has made clear that he wants to be rid of them, then even if he is busy at the moment and cannot do so immediately, there is no prohibition.
The words of the Mishnah:
"Ha'ro'eh yarak b'kerem v'amar: k'she'agia lo - alaktenu, mutar" - one who sees a vegetable growing in his vineyard, which constitutes kilayim, and says that when he reaches it he will uproot it - the vineyard is permitted and no prohibition of kilayim comes into being, since he has revealed his intent that he does not want it to remain and that he is not being lazy about uprooting it: the moment he is able to reach it, he will pull it out.
"K'she'echzor - alaktenu, im hosif b'masayim - asur" - one who says that he does not intend to deal with it now, but only when he comes back along his way, has already shown that in the meantime he is content for the vegetable to remain there, even if only for a short while. Therefore, if the growth that was added in the meantime amounts to one two-hundredth, the vineyard becomes forbidden.
What is the measure of one two-hundredth:
The reason for this measure is that kilayim are nullified in two hundred and one. Accordingly, once growth of one two-hundredth or more has been added, the matter becomes forbidden.
And how do we determine whether that measure of one two-hundredth has grown? The Rambam explains: we take a plant of this kind and cut it off from its source of nourishment, until it dries out, and we measure how long it takes to dry. If one two-hundredth of that time has passed, we know that this is the period of time in which it adds one two-hundredth of growth.
In summary: in this Mishnah we learned that the difference depends on how the owner of the vineyard reveals his intent. One who says "when I reach it I will uproot it" - it is permitted, since he has no desire whatsoever for the kilayim to remain. One who says "when I come back I will uproot it" - he is content for them to remain in the meantime, and therefore if they have grown by one two-hundredth, which is the measure at which kilayim are nullified, the vineyard becomes forbidden. This measure is calculated, according to the Rambam, by the time it takes such a plant to dry out once it has been cut from its source of nourishment.