Most commercial plum varieties in California require pollen from another variety to set a full crop. Thus, a second variety needs to be planted in the orchard or grafted into trees of the main variety. Beehives also need to be placed in the orchard during bloom. Details on how to accomplish this are described in our UC stone fruit manual (Pollination Chapter in Stone Fruit Manual). Commercial nurseries also provide recommendations on variety combinations for good cross pollination. Links to their websites are found on our variety main page.
Commercial peach and nectarine varieties are all self fruitful and do not require cross pollination.