Avocado varieties are categorized as A and B flower types. All avocado flowers have both male and female components (stamens and pistil). But an unusual and unique facet of avocado flowering is that they open as a male flower at one time of the day and open again as female at another time. Here is the scenario: ‘A’ type flowers open female in the morning, close, and then reopen as male in the afternoon. ‘B‘ type flowers are male in the morning and female in the afternoon. This works out very well in a garden with both A and B type trees. In the morning, the female A type flower will be pollinated by the B type males. And in the afternoon, just the opposite will occur.
If there were only one type of avocado tree present in the garden, potentially little fruit would be produced. Yet the presence of bees, with their efficient distribution of pollen, will enable the single backyard tree to produce an abundance of fruit. In addition, there is usually, although not always, the opposite type tree somewhere close in the neighborhood. The bottom line for backyard growers is that in most cases one tree will provide ample fruit.
Commercially, much research has been conducted on mixed orchards, i.e., fields containing both A and B flower type avocado trees. Fruit production increased in the mixed orchards.
University of Hawaii CTAHR has an excellent publication available online entitled “What Makes a Good Avocado Cultivar Good?” Table 2, in this publication shows various avocado cultivars grown in Hawaii and their flower type. For example, the Sharwil variety, which has a bearing season of winter - spring, is a B flower type; Greengold, with a bearing season of winter – spring and Ohata, which bears in the summer are both A flower types. The CTAHR (College of Tropical Agriculture and Human Resources) website can be found at http://www.ctahr.hawaii.edu/site/info.aspx.
Other websites such as the University of California also have excellent information on avocado flowering types, http://ucanr.edu/.