Phenology

• Phenology is the study of critical moments in nature, usually the first emergence of something. In the case of cacti, phenology studies the first growth after spring, the first bud formation, the beginning of root or spine development, and other such beginnings. For cacti with multiple segments such as prickly pear cactus, phenology might also study when new segments are formed.
  
  

Water Dependency

• Many cacti live in areas where water resources are severely limited, such as deserts. So even if it is springtime or otherwise a time when many plants begin forming new leaves, begin budding or begin root development, cactus might still be dormant. Instead, many begin new development after a rain, or when a nearby river floods the soil with water. This decreases the chances of the cacti using up all their water reserves before the next rain comes.
  
  

Temperature Dependency

• Even in situations with ample water, many cacti have low temperature thresholds. They stay dormant until the temperature is warm enough for them to begin growing, budding or spreading new roots until they will not be in danger of damage from cold temperatures. So if a rain or flood comes during the winter, this will not necessarily kick-start the phenological cycle -- the cycle when the cacti begins forming new plant parts.
  
  

Considerations

• Cacti are often considered slow growers when compared to other plants, because they do not accumulate biomass, or grow and add to their mass, as fast as many other plants. This is due in part to their phenological restrictions of temperature and water availability. Even when both restrictions are met (which does not occur often during the year in most natural environments of cacti), the harsh environment demands that new growth be cautious and slow to prevent harm from predators, excessive heat or disease as well as to allow the cacti to store as much excess water as possible for dry times.