Almost all living creatures, from blue-green algae to gardeners, possess a powerful clock–known as the circadian clock–that controls many of life's processes. Though it mimics Earth's 24-hour rotation, it is internally generated within every cell. Its molecular biology is still being unraveled, but at the simplest level, the hands move around the clock because genes are activated to rhythmically produce proteins, which then degrade, activating the genes again 24 hours later. Studies with subjects that range from blind animals to plants locked in the dark prove that the clock is innate, and plants and animals don't actually have to experience day and night in order to have regular wake/sleep or stiff-leaf/relaxed-leaf cycles.
Sunlight is still crucial to our biological sense of time as it resets our internal clocks, harmonizing our daily rhythms with the environment. Otherwise we might live in a state of perpetual jet lag because our internal clock runs slightly long or short of a solar day.
As light lingering on a winter's evening tells a gardener that spring is near, this light also helps plants determine the season. "The [circadian] clock is not constantly sensitive to light," Imaizumi explains. "It has a light-sensitive window at the end of the day that acts as a gating mechanism."
In what are called long-day plants, light striking during this window begins the molecular process that produces flowering. These plants want to flower as we move toward the longest days of the year. In short-day plants, light striking during this window inhibits flowering. These plants want to flower earlier in spring or in fall. In day-neutral plants, it is not the photoperiod that prompts flowering, but another factor such as temperature or stage of maturity.
In my northern climate, where the snow often lingers on the ground into late April, long-day plants like mustards, radishes, and lettuces are generally difficult in spring. Because I have to plant them relatively late, as we race towards the summer solstice, they often bolt before they produce anything worth eating.
For other long-day plants, even my late planting date is too early in terms of temperature. Some long-day plants flower only after experiencing vernalization, a period of prolonged cold. This is what keeps delicious biennial root and stem crops like turnips, carrots, parsnips, and beets from going to seed in their first summer and using up their stored food before we can harvest them. But too much chill in early spring can fool turnips, for example, into thinking that they have already lived through a winter and now have permission to flower and set seed.
My answer? Avoid the summer solstice entirely for fast-maturing long-day crops like turnips, radishes, beets, and mustard. Planted in midsummer as fall crops, they are crisp and creamy, with long, luxuriant leaves–absolutely perfect.