Spring bulbs obviously bloom for spring. Less obviously, they vernalize with winter chill. They are seasonable about now because this is when they should get into their gardens. Many prefer to be interred into their shallow graves by Halloween. Many can wait as late as New Year’s Day. If too much later, some may not experience enough chill to vernalize.

Planting spring bulbs can be ungratifying because there is nothing to see afterward. After all, they are dormant at the time. They lack foliage or any other parts to extend above the surface of the soil. They seem to remain inactive until they bloom as winter succumbs to spring. However, between now and then, they will secretly reset before resuming growth.

Vernalization is how many spring bulbs finish their previous season and begin their next. They know that it is winter after they experience a particular duration of a particular chill. By then, they should have finished all growth of the previous season. Afterwards, all new growth and bloom is for the next season. It is how they know to bloom on time for spring.

Dutch iris, freesia, ranunculus, anemone and common narcissus do not need much chill. Tulip, hyacinth, crocus and a few narcissus and daffodil do. Some bulbs that benefit from it get a prechill treatment before coming to nurseries. Prechilled tulip and hyacinth bulbs are sadly unlikely to bloom after their first year. They prefer more chill than they get here.

Otherwise, many spring bulbs are supposedly reliably perennial. Naturally, actual results may vary. Anemone can survive for many years, but might bloom only rarely. Ranunculus may be less perennial, but may also bloom better while it survives. Dutch iris and crocus are unpredictable. Freesia and simple varieties of narcissus are more reliably perennial.

Technically, some spring bulbs are not bulbs. Ranunculus and freesia, for example, grow from corms. Nonetheless, almost all spring bulbs bloom only once annually. Succession planting can prolong bloom. It provides subsequent phases to begin bloom as preceding phases finish. However, phases of reliably perennial bulbs synchronize after first bloom. Summer bulbs and bulb-like plants are seasonable later.