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Why do lily of the valley seeds need two cold treatments separated by warmth?

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Cluster of bright red seeds emerging from the cone-shaped fruit of a magnolia grandiflora, surrounded by large, glossy green leaves. — Seed Fruit and seeds of the southern magnolia (*Magnolia grandiflora*). The fruit is an aggregate of follicles, and each follicle bears a single red seed.

Lily of the valley seeds exhibit a phenomenon known as "double dormancy," which necessitates two distinct cold treatments separated by a warm period to achieve complete development.1

This complex process occurs in stages:

First cold treatment: This initial period of cold eliminates the dormancy of the root.1
Warm period: Following the first cold treatment, a warm period is required to permit the outgrowth of the root.1
Second cold treatment: This final stage eliminates epicotyl or leaf dormancy, allowing the plant to complete its development.1

Because of these specific requirements, it can take nearly two years for a lily of the valley seed to produce a complete plant. This strategy is part of a broader process called afterripening, which ensures that seeds only germinate when environmental conditions are favorable for survival.1