Why Aren't My Cacti Blooming?

Considerations

• Make adjustments in the house's environment. Once cacti are old enough to bloom, they may bloom annually or in a multiyear cycle. The desert cactus' thick skin protects its vascular system, but blooms require seasonal cues. Light levels are high in the desert, so even the light of a sunny window must be supplemented up to 16 hours a day. The only condition in the home that actually resembles those of the desert is its low humidity. Especially during the winter months, humidity in a house with central heating can approach the 10 to 30 percent averages in the desert.

Soil

• Desert cacti spread a complex system of fine roots susceptible to root rot in moist soil. They stay healthy in highly porous, alkaline soil. Plain potting soil is too heavy. Cactus soil needs leaf mold and sand to improve drainage. A gravel layer in the bottom of the soil and shallow, clay planting containers also aid drainage. High nitrogen levels encourage foliage growth, but blooms need phosphorus as cacti emerge from dormancy in spring. Bone meal, a 5-10-5 general plant food or 7-40-6 cactus food provide nutrition encouraging flowering. Withholding fertilizer beginning in late fall allows the plant to enter its normal dormancy, a necessity for spring blooms. Fertilization in early spring supports emergence from dormancy and blooming, and then continue through summer.

Temperature

• A cactus' thick skin protects it against desert temperatures that rise above 100 degrees. Once the desert sun sets, the arid air cannot hold the heat, and it radiates back into space, dropping the temperature. Desert winter temperatures are chilly, a necessary step in the plant's progression toward spring flowering. Plants respond best when with nighttime temperatures of 45 to 55 degrees and daytime temperatures up to 10 degrees higher during dormancy. Summer air conditioning might mimic winter temperatures and prompt premature dormancy.

Water

• Cacti kept in the house need regular watering because they have less soil than cacti in native environments. Water the plants deeply, encouraging deeper root growth, until water to runs out the bottom of the container. Water the cactus only when the top inch of soil dries. Cacti require more frequent watering if they grow outside during summer. Because the plant efficiently conserves water, less frequent watering during winter reinforces dormancy. When spring arrives, resumption of regular watering, fertilization and warmer temperatures signals the cactus to begin growth and bloom.