Kilayim for fruit pits
Question
Are plums and peaches in the same halachic category so it’s not an issue of kilayim? Or is it an issue? I’ve been saving peach pits and someone in my house added a plum pit to the container by accident.
Answer
There is no issue of kil'ei zera'im (forbidden mixtures of seeds) when it comes to trees. You can store pits from the same type of fruit in a single container without any halachic concern. In fact, even planting them right next to each other is permissible—though it may not be advisable from an agricultural standpoint.
Kil'ei zera'im applies specifically to edible annuals and herbaceous perennials, which are halachically categorized as vegetables (this includes culinary herbs).
Even commercially-grown flowers, which may be planted near each other without restriction, must still be distanced from plants classified as vegetables.
The issue of kilayim that does relate to trees, however, is kilei ilan. This prohibition refers to grafting dissimilar species to together - that is, taking a branch (scion) of one type of tree whose fruit we want and connecting it to a branch of another type of tree that will be connected to the ground (rootstock).
For a list of kosher scion-rootstock pairs, see here.