How is chrysanthemum flowering scheduled?

The cultivated chrysanthemum is a short-day species, meaning that, in natural conditions and at least at our latitudes, it flowers in the autumn months, when the daily hours of darkness are greater.

Photoperiodism is the biological mechanism used by plants to regulate their vegetative phases, meaning growth, and their reproductive phases, meaning flowering.

Artificial darkness is the tool that enables growers to schedule the commercial maturity of the product at a specific time, both for cut flowers and for potted plants.

The critical photoperiod, meaning the maximum daily number of light hours under which the plant can move from the vegetative phase to the flowering phase, is approximately 14 hours. This means that more than 10 uninterrupted hours of darkness are needed to trigger flower induction.

Environmental factors inside the greenhouse are modified, influencing the biological cycle and forcing flowering. By managing light and darkness, the flowering period can be scheduled with great precision in order to meet commercial demand.

What method is used for forcing?

In technical terms, this is known as “blackout”: a technique in which growers completely cover the crop with a black, light-proof sheet, extending the natural hours of darkness. This causes the plant to move from the vegetative phase to the flowering phase.

When is it carried out?

For chrysanthemums scheduled for the All Saints’ Day period, artificial blackout begins in August, at a time when the continuous hours of darkness would not yet be sufficient, and continues until mid-September, when the natural photoperiod matches the conditions needed for flowering.

Each variety has its own “response time”, meaning the minimum number of short-photoperiod days required to flower. Response times range from 6 weeks for the earliest varieties to 15 weeks for the latest ones. The exact timing for starting this practice is essential in order to have the product ready for the desired period.