The bloom time listed for each variety is for horticultural zone 5 in ‘normal’ spring conditions. The warmer the horticultural zone, the earlier Narcissi will bloom in the spring. The colder the horticultural zone, the later Narcissi will bloom in the spring.
Flower bulbs do everything in response to temperature, sunlight and site conditions. Bloom times, heights and colors are approximations affected by temperature and site conditions regardless of the calendar date. If it is a warm spring, bulbs will bloom earlier. If it is a cold spring, bulbs will bloom later. If it is a long cool spring, followed by rapid warming, you may find odd bedfellows: earlier blooming Narcissi flowering right along side later blooming varieties. Each spring can offer a different sort of garden surprise party.
In the event of a mild winter or a warmer-than-usual spring, Narcissi that have emergent stalks with set buds may bloom early, small and short, although they will likely grow taller and larger as long as temperatures moderate. Temperature spikes can also affect mature root development, the actual form of the flower or the process of flower color maturation. An example would be Large Cupped NarcissusFragrant Breeze. In normal or warm temperatures, the flower has a flattened cup, whereas in a cooler spring, the cup would be more bowl-shaped. Generally, cooler weather inhibits fragrance, whereas warmer weather incites fragrance.