Disposing vegetable scraps and seeds near a vegetable garden for fertilization

Answer

Vegetable seeds are capable of sprouting, and if they are thrown near vegetables of another type, it may seem to constitute the prohibition of sowing kilei zera'im, forbidden seed mixtures, and one might need to be cautious.

Regarding the prohibition of planting on Shabbat, the Shulchan Aruch (OC §336:4) rules: “One must take care not to throw seeds in a place where rain falls, for they will eventually sprout.”¹

In Responsa Shevet HaLevi,² a dispute among the Acharonim is cited as to how this constitutes the act of planting. According to Tosefet Shabbat,³ the concern is that the seed will take root in the soil; whereas Nishmat Adam⁴ writes that the reason is not rooting in the soil, but that in a moist place the seeds will sprout, similar to the case of “one who soaks wheat in water—this is a derivative of zore’a (sowing).”⁵

Shevet HaLevi adds that regarding throwing seeds on the ground during shemitah, he heard that the Chazon Ish was stringent, due to the concern of taking root in the soil. However, Shevet HaLevi maintained that the prohibition on Shabbat is due to initial sprouting, and therefore he did not extend that concern to shemitah.”⁶

The practical implication regarding kilayim is: if the concern is planting and rooting in the soil, then one should also forbid throwing seeds on the ground near another vegetable. But if the rationale is merely that the seed from a plant detached from the soil will sprout in water, like soaking wheat, and not rooting in the soil, this would not be included in the kilayim prohibition, which applies only to sowing in a field or in the ground, as it says: “You shall not sow your field with kilayim” (Vayikra 19:19).

Furthermore, it seems that even if one assumes that throwing seeds raises a concern of planting and rooting, regarding kilayim there is an additional leniency known as hazore’a lehashchatah (“one who sows for destruction”). Rabbi Yisraeli explains this leniency (Eretz Chemdah, Kilayim I ch. 12):

The prohibition of sowing kilayim applies only when one performs the act in the normal manner of sowing. But if one was occupied with another activity and, incidentally, kilayim was sown as a result, this is permitted. Therefore, it is permissible to fertilize a field with manure that contains seeds of other species, even though it is certain that different plants will sprout in the field; but once they grow, they will be considered kilayim. The prohibition of sowing kilayim applies only when one plants in order to benefit later from the plants, but not when planting for destruction—for example, when the intention is to check if a certain plant will sprout in the soil. Whenever it is manifest from one’s actions that the intent is clearly for destruction, it is permitted even miderabbanan.

Conclusion

In light of all the above, it seems that vegetable scraps and seeds that are thrown on the ground do not actually take root but sprout on their own. Therefore, this is not included in the prohibition of sowing kilayim. Even if there is concern about rooting in the soil, since the intent is not to grow them but to fertilize the soil, this falls under the category of zore’a le-hashchatah, which is permitted and does not constitute the kilayim prohibition. However, if a forbidden kilayim mixture ends up growing together, one must uproot the plants, due to the prohibition of maintaining (kiyum) kilei zerai'im.

Notes

1. The Levush explains: “Even though there is no explicit intent, it is considered a pesik reisha (inevitable outcome).” See also Shevet HaLevi VII §180: “Even when, in the end, on Shabbat this is only a rabbinic prohibition; certainly it is not a definite pesik reisha, but is considered a safek pesik reisha or close to one, as explained in Eglei Tal, ibid. (and likewise in HaLevush, OC §336, where a pesik reisha will not necessarily result from the act).”
2. Responsa Shevet HaLevi VII §180.
3. Tosefet Shabbat §336:12.
4. Nishmat Adam, klal 11.
5. Shulchan Aruch, OC §336:11
6. Also written in Derech Emunah, Hilchot Shemitah ve-Yovel 1:16.
7. Demonstrated as follows: 
   

   In Hagahot Maimoniyot ch. 8, this law of not throwing seeds, is noted regarding the rulings in Zevachim 94 and Rambam, Hilchot Shabbat 8:2, where one may not throw wheat into water due to zore’a she-matzmi’ach (a seed that will sprout). Therefore, it is clear that in the case of shemitah this does not apply, because the other toladot (derivatives) are only rabbinic. Moreover, since the seeds are not in the ground and it says, ‘Your field you shall not sow, so it is not forbidden. Moreover, even if it would be prohibited, it would be forbidden only miderabanan as toladah; it is not certain that they will sprout from the rain there, this is only a rabbinic prohibition relating to Shabbat. If this is the case, why should we say it will be forbidden during shemitah?  There are three reasons for leniency. Shemitah is a rabbinic injunction in our days, and the toladot are rabbinic and not a pesik reisha in principle.

8. According to the Mishnah, Kilayim 5:7: “If a person passed through a vineyard and seeds fell from him, or they went out with manure or with the water that waters, and the wind scatters them afterward, it is permitted."
9. Rash explains (ibid.): “With the manure or with the water. When he fertilizes his vineyard or opens the irrigation channel to water the vineyard, sometimes seeds are with it, and all of these that were not planted intentionally are permitted; it is not as if he intended to sow his vineyard.”
10. The Mishnah permits this in the case of kil’ei hakerem. The Sifrei (Devarim §230) derives this teaching from the phrase “asher tizra” — “that you shall sow” — explaining that the prohibition of kil’ei hakerem applies only to active sowing, while it permits a situation in which the seeds fall on their own. Although the verse specifically refers to kil’ei hakerem, Rabbi Yisraeli extends this principle to kil’ei zera’im as well, since the verse containing that injunction uses similar wording: “You shall not sow your field with mixed species.” From this, he concludes that only intentional sowing is forbidden, whereas seeds that fall of their own accord are not included in the prohibition