Practical Plants has the ability to store interactions between two plants. Polycultures of more than two plants are dealt with elsewhere.
An association can be beneficial or detrimental to one or both plants which make up the association. Members of the Juglans genus (to which Walnut belongs]], for example, are allelopathic, and have a detrimental affect on members of the Malus genus[Reference needed!], among many others, but some plants are unaffected[Reference needed!]. Thus it can be said that the association between Juglans regia (Walnut) and Malus domestica (Apple) is detrimental to Malus domestica (Apple). In some cases, what is detrimental to one plant can certainly be beneficial to another, and visa versa. A thirsty willow may hog too much water for one plant, while the other plant may appreciate the dry soil. In this way, associations are a product of the interactions between the plants.
Looking up associations
To view associations for a particular plant, search for it and scroll down to the Associations heading.