We all know the adage “The more things change the more things stay the same,” right? Well, it rings true once again. I had an idea for this month’s column, so I went back to what I wrote this time last year, and I was discouraging you from starting your warm season seeds too soon, because it had been in the 70s at the end of March 2025. Sheesh, didn’t they just tell us this is the warmest March on record?
OK, that’s all I’m going to say about that (she says in her best Forrest Gump voice). What I want to talk about this month is more of a science lesson. You might have noticed a plethora of amazing blooms all over town and maybe even seen what I said in the Herald last week about why the Callery pear trees smell so terrible.
If not, just know that evolutionarily, stinky flowers attract detritivores (things that eat dead stuff), like flies and certain beetles. Well, it just so happens that these pear trees are one of the earlies bloomers in the spring and detritivores are typically the first on the scene to start looking for food, even before the bees and ants wake up. Not exclusively, but for certain “pollinators” (air quotes because I know we don’t all think of flies and beetles as common pollinators).
So, I decided I wanted to share more of the science behind when and why fruit trees bud. Quick show of hands: How many of you have a cherry, apricot, or apple tree that only gives you fruit every few years, rather than other apple or pear trees? That’s what I thought, just about all of you. There is a very important process going on deep inside the tree that influences the timing of when each starts to bud.
Actually, there are a lot of factors, but I am going to focus on two of them: endodormancy and ecodormancy. Endo is Greek for “inside,” letting us know that the dormancy process is happening deep within the tree and begins in the fall as the weather cools and days shorten. Endodormancy is so fascinating as a lot of us first learned it as “chill hours or chilling units,” but what’s important to understand is that all trees (including conifers) enter a state of dormancy where their metabolic, hormonal, energetic and even cellular systems slow or even stop.
While this dormancy begins with the onset of colder temperatures, that is when the clock begins to tick inside each tree. If you have ever heard of chill hours you might have thought it meant that this particular type of tree needs a certain number of hours at or below freezing. When in reality, it is actually tracking the number of hours between 32 and 45 degrees. Hence chill hours, not freezing hours. The challenge happens when a tree with a high chill requirement experiences winters that are below freezing or above 45 degrees, and they don’t get their much-needed window to chill.
For example, a Braeburn apple tree needs 700 chill hours before their internal clock will go off telling them that they have achieved the needed hours of dormancy before they can be affected by external factors. But a Royal Lee Cherry tree only needs 200 to 300 hours.
Those external factors trigger the second stage, known as ecodormancy. This is when the lengthening of the day and warmer, above 45 degrees temperatures can begin the process of awakening the tree buds to start to produce leaves and blossoms.
So, what could possibly go wrong? With the examples of trees mentioned above, the Braeburn might achieve the necessary hours by late January to early February in our region, then it will just enjoy the cold nights and live its best life as a cold-hardy tree waiting for warmer and longer days to break dormancy. But the Royal Lee cherry may achieve endodormancy by December, and if we have a warm winter, they will start to set buds very early.
In the grand scheme, it might not matter if our fruit trees start blooming right now, whether they needed 300 or 800 chill hours, but you and I both know that we live in a region that will likely experience freezing temperatures again anytime now, or maybe in May – it’s happened many times before. Then the blooms freeze, and we have a year without fruit from those little darlings.
So why does it matter? Well, if you want to plant fruit trees, look for labels that say 600-plus chill hours or “cold-hardy,” that’s a better bet that they can deal with our consistently unstable climate. But if you select something from a box-store that does not say anything about their hardiness, you might want to google the variety first and make sure it can handle life as a Colorado tree.
Heather Houk is the Horticulture and Agriculture specialist for the La Plata County Extension Office.
