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pink Azalea

There are many plant species that normally bloom in the spring and do not bloom in the fall. So, it can be alarming when these plants start to bloom in early fall. Sometimes events that happen in summer may cause plants that normally bloom in the spring to also bloom in the fall.

Favorable Conditions

Indica azalea

Indica azalea blooming in Hale County, Alabama on September 11.

Most temperate zone plants go into dormancy in the fall and remain dormant during the winter when environmental conditions are unfavorable for growth. However, if the weather conditions are favorable for growth, the plant may extend its growing season.

Day length and the accumulation of certain plant growth regulators are involved. Dormant plants can only overcome this physiological condition after a period of short days and cold weather. Plants in the Prunus (peach, plum, and cherry) genus are particularly sensitive to the duration of cold temperatures and day length during dormancy. In fact peach trees are sold with a statement of the chill hours that a particular variety requires.

The shorter day-length in late summer, which is analogous to spring, may also play a role in inducing fall flowering.

Stress on Plants

Plants also sometimes experience a period of extreme stress during the summer growing season. Factors that can cause this stress include drought, very high temperatures, or a combination of environmental factors. These factors can lead to a condition that is referred to as quiescence. Quiescence in plants can be defined as a period of suspended growth or relative inactivity. Normal physiological activity still occurs but at a much slower rate. This is a response to the environmental stress.

When environmental factors that favor normal growth return, quiescence can cease and plants will resume normal growth. In other words, dormancy cannot be reversed but quiescence can be. Another term that explains this phenomenon is vernalization. If flower buds are formed during the summer at or just before the time that the environmental stress occurred, plants can sometimes bloom in response to the renewed activity.