Kilei Zera'im: Mint spreading in the garden

Answer

Tammuz 5784

When planting two types of vegetables, if is necessary to maintain a distance of a tefach-and-a-half (12.5 cm). Both tomatoes and mint are considered vegetables and should be distanced from one another as such. It is true that Shulchan Aruch (YD 297:21) rules that if the vegetables were distanced properly at the time of planting, we need not worry about the plant's consequent spread:

If he distanced the two species with the appropriate separation, but one species subsequently leaned over the other—whether grain leaned over grain, vegetables over grain, or grain over vegetables—everything is permitted, since he [initially] distanced them by the required measure. Except for the Greek gourd, which spreads extensively; therefore, if it leans, he must uproot it from in front of it.

On the other hand, it is forbidden to sustain kilayim. Even if they were not sown or planted actively, but sprouted on their own, it is necessary to uproot them, as stated by Shulchan Aruch (YD 297:21). While the Sages disagreed with Rabbi Akiva (Moed Katan 2b), Tosafot explain (Bava Kama 81a, s.v. ein) that they only disagree regarding the biblical prohibition, but even the Sages agree that there is a rabbinic prohibition: "Alternatively, even according to the Sages it is necessary. And in cases where it is beneficial to him, the Sages require [uprooting] so he will not be suspected of having sown them."[1]
The Mishnah at the beginning of tractate Shekalim supports this as well: agents of the beit din would go out to fields to uproot kilayim.

For the case at hand, mint should be viewed differently than two vegetables sown at the required distance from one another and subsequently spread. In most cases, the leaves of plants that were sufficiently distanced at the time of planting grow closer to one another and mingle. In such cases, we need to ensure that they were distanced properly at the time of planting and need not worry about any further spreading.

The structure of mint differs[2] from other vegetables. Mint consistently produces runners underground and aboveground. A runner (a.k.a. stolon) is a short branch comprising many segments and nodes between each segment. Each node has the potential to take root deep in the ground and produce stems and leaves aboveground. The runner continuously elongates and produces new stems, thus spreading in all directions. Even when the runners are uprooted, parts of the runners remain in the ground, which subsequently produce new stems. For this reason, we can view each runner segment that produces stems as an independent plant. This phenomenon is somewhat similar to layering with grapevines. When we discuss the necessary distance between mint and other vegetables, both botanically and visually, each new stem that grows from an underground or aboveground node should be viewed as an independent plant with its own root system that functions as a separate plant.

Even when mint is planted at the required halachic distance from other vegetables, within a short time the plant will produce new runners, roots, and stems near other vegetables. For this reason, it is necessary to cordon off the mint and prevent it from creating patches of kilayim without our noticing.

Conclusion

In light of the above, mint should be considered a self-seeding plant, which, if left unchecked, the Sages would require uprooting when it grows too close to another type of vegetable. In the case at hand, mint should be cordoned off to prevent it from growing too close to the tomato plant.