Avocado flowers are dichogamous (like pecans), which means that the male and female flowers on a tree mature at different times. This is a strategy that has evolved in avocados, pecans, and other plants to prevent self-fertilization and promote genetic diversity. In protogynous (female first) or Type A flowers, the pistils are receptive to pollination for a period in the morning, the flower closes, and then reopens to shed pollen in the afternoon in response to warmer temperatures. In protandrous (male first) or Type B flowers, pollen is shed in the morning, and the flowers reopen as female in the afternoon.
According to the University of California at Riverside, when daily temperature average (including night and day temperatures) is less than 70º F the daily switch between male and female becomes irregular, and one tree can have both male and female flowers at the same time. Houston’s average daily temperatures are usually around 70º F or less in early spring when the avocados are blooming, so under normal circumstances it isn’t necessary to plant both Type A and Type B trees. This is good considering that I have found very few nursery owners who were even aware that there are different types of avocados, and no one who could tell me the types of the different Mexican avocado varieties.
Speaking of nurseries, the same genetic clones are marketed under different names, and it can get confusing trying to keep them straight. I've even seen the same variety sold under two different names at the same nursery! Here's a key to the different varietal synonyms:
- Brazos Bell is a genetic clone of Wilma. It can be easily recognized because it's branches are 'wimpy'.
- Lila is a genetic clone of Opal (aka Holland, or Opal Holland). It is considered to be the second most cold hardy Mexican avocado.
- Poncho is the same as Pancho. As far as anyone can tell, it's just different spellings of the same name.
- Fantastic is Treesearch Farm's trade name for Pryor. This variety is considered to be the most cold hardy of all the Mexican avocados.