1. The Flower Buds Are Made the Summer Before
In a four-year study in Spain, researchers followed flower buds of an apricot variety called 'Moniqui'. They sampled buds from the time the flower buds form, in August and September, until the time of flowering, in February and March. [1]
So the "spring" flower is really the result of about half a year of work inside the bud. Growers can use this fact too: because the buds already exist in winter, cutting off the wrong twigs then would also cut off flowers that have not opened yet.
One caution: this timing comes from a study in a warm Spanish climate. In a cooler place the dates could shift a little. What stays the same is the order: buds first, flowers many months later.
2. Why Apricot Buds Stop Growing and Wait for Winter Cold
In the Spanish study, the buds did not keep developing straight through to spring. In winter, the flower bud entered dormancy, and its development was apparently halted. [1] Dormancy is a kind of deep rest. Warm days in the middle of winter do not wake the bud up.
The bud needs to spend enough time in the cold first. In that study, the chilling requirement was met in mid-January in each of the four years, about five weeks before the buds opened. [1]
How much cold is "enough" depends on the variety. A 1981 Spanish study kept flower-bud samples from 15 fruit trees, including nine apricot varieties, below 7 °C (45 °F) for different lengths of time. The requirements across these trees ranged from 200 to 800 hours of cold. [3] That range covers all 15 trees, not only apricots, but it shows how widely a plant's need for winter can vary.
A simple way to picture it is an alarm clock that goes off only after enough cold has piled up. Plants do not literally count, though. The real change happens inside the bud. And if a winter is too mild, the alarm may go off late or unevenly.
3. Why Buds Are Easier to Freeze After They Wake Up
You might expect a bud to be safest once the cold winter has passed. A two-year study of apricot found the opposite. Flower buds were most at risk of frost damage after their chilling requirement had been met but before the buds started to swell. [4] Buds still in deep winter rest, while they were collecting their cold, showed no frost damage in that study. [4]
The damage was hidden. The buds looked normal from outside, but inside, ice had formed and left small gaps in the tissue, and the stamens (which make pollen), the pistil (which becomes the fruit), or both had turned brown. [4]
This is one study of buds from one region, so it does not describe every orchard. Still, it helps explain why a warm spell followed by a freeze is so hard on early bloomers.
4. Why Apricot Harvests Can Fail Even When the Tree Is Healthy
Apricots bloom early and are highly susceptible to spring frost. In Utah, a university extension service says a tree may produce fruit in only about 3 years out of 5. [2] Later-blooming varieties help, but they still face the risk of a freeze. [2]
Flowers also need pollen to become fruit. Most apricots are described as self-fruitful, meaning one tree can set fruit with its own pollen, but most do better with another variety of similar bloom time nearby. [2] That is one more thing that has to go right during a short, chilly flowering season.
If everything works, the fruit is ready in July and August depending on the variety. It turns yellow or gets a red blush and begins to soften. [2]
5. How to Follow an Apricot Tree's Year Yourself
If you have an apricot tree nearby, such as in a garden, a school or a park, look at its twigs in late autumn and winter. The plump buds you see are the flowers already waiting. Look only from public places, and do not pick buds or climb the tree.
Write down the day the first flowers open each year, and note whether the winter before was cold or mild. After a few years you may see a pattern. You can compare with a plum or cherry tree to see which one wakes up first.
If you want to read more, the Annals of Botany paper on stamen development and dormancy is a good place to start, and the university extension guide explains frost and pollination for home growers. [1][2] A question to carry away: how does a bud "know" it has had enough cold?