For not having brains, plants are a very smart organism. One thing that they can sense that we cannot is day length.
Tomorrow we will experience exactly eleven hours of sunlight. For plants this is a magical number, when the sunlight is below 11 hours plants remain dormant. After 11 hours vegetable plants will start to grow again. For me this is when I will start growing transplants in the greenhouse; for gardeners with cold frames, their plants will begin to perk up.
There are many plants that have special day length requirements. Onions are one of these crops. Their bulb production begins while day length is shortening past a peak. The two types that are grown here in the Northeast are long-day and intermediate day. Long-day onions begin to bulb up after 14 hours of sunlight have been achieved, intermediate day begin to bulb up after 12 hours have been achieved.
I look forward to the increasing sunlight hours and the growth it will bring!