Criteria for plant classification and kila’im: is the botanical definition sufficient?

Answer

The definition of a species vis-à-vis the laws of kila’im are halachic parameters, which are not necessarily the same as the botanical ones. For this reason, two types of fruits might be classified as the same species botanically, but viewed as different species through the halachic lens and would therefore be subject to the laws of kila’im.

Below is an excerpt from Hilchot Ha’aretz, a publication of Torah VeHa’aretz Institute, on the topic (Laws of Kila’im, ch. 2):

1. The halachic definition is not the same as the botanical definition, so we follow the halachic parameters to determine whether or not kila’im is an issue.
2. There are three criteria that determine species according to halacha: (1) appearance of the leaves (2) appearance of the fruit (3) taste of the fruit.
3. Two plants that have similar fruit or leaves are considered the same species, as long as the taste of the fruit is similar.
4. It is permissible to mix different varieties of the same species when they are considered the same species by halacha, even if the plants are somewhat different in size or color. For instance: it is permissible to plant a red pepper next to a green pepper, and a radish next to a small radish.
5. Even when two plants are defined as two different species according to the accepted botanical standards, they may be considered the same species by halacha (vis-à-vis kila’im), when they are very similar in shape (as mentioned above). This is because appearance is a key factor in the laws of kila’im, and when there does not appear to be a mixture of species, kila’im does not apply.
6. If one is unsure if different plants are considered different species or not, a competent rabbi should be consulted, since there are a whole set of rationale and definitions that play a role in defining the species.

For a comprehensive list of permissible and forbidden rootstock and scion combinations for grafting fruit trees, see Grafting: a review.