Is This an Early Spring?

Is This an Early Spring?

By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester


If I’ve heard it once, I’ve heard it a hundred times in the last couple of months – “This is really an early spring!” By “early spring” most folks are referring to a year when leaves and flowers appear earlier than “normal.” The conversation quickly moves to “How early?” and this is where some data would be handy. Welcome to phenology!


What in the world is “phenology?”

Phenology primarily deals with the relationship between climate and the timing of biological phenomena like flowering in plants or bird migration. The starting point of phenology is creating long-term records of when certain biological events occur. Using these records, one can then make statements about “early springs,” “late autumns,” and other periodic climate-related events.

I started keeping phenological records for Tryon Creek State Natural Area (TCSNA) in mid-spring of 2013. Since then I’ve made observations every 7 to 10 days on about 70 specific, individually tagged plants.


Cut to the chase. Is this an “early spring?”

Since the Tryon Creek Phenology study is new, the question I’m really answering is “Was 2015 an early spring compared to 2014?” After studying the records for the study plants, I found there are really three answers.

Answer #1: Yes, it was an early spring!

Based on 8 Indian plum (Oemleria cerasiformis) plants that produced flowers both years, in 2014, the average date for the opening of the first flower was March 10th. The average date for opening of the first flower in 2015 was February 18th, a difference of 20 days! For the flowering of Indian plum, 2015 was an “early spring” for sure.


Indian plum flower completely open


Vegetative bud break in red elderberry (Sambucus racemosa var. racemosa) was also earlier this year. Based on 3 plants, the average date of leaf emergence in 2014 was February 23rd, while this year it was February 14th, a difference of 9 days.

Finally, four Pacific waterleaf (Hydrophyllum tenuipes) plants produced visible flower buds on March 29, 2014, but this year, they all produced flower buds 11 days earlier, on March 18th.


Cluster of fuzzy young Pacific waterleaf flower buds


Answer #2: It was a normal spring!

Looking at the date of bud break of 10 Indian plum plants, a different story emerges. In 2014 the average date when leaf tissue extended beyond the bud scales (as seen in the photo below) was February 4th. This year, that date was February 1st, just three days earlier. This is approximately the amount of experimental error one might expect in this study. In other words, this difference might be real or it might be a consequence of the length of time between measurements.


Rolled leaves (blue arrow) of Indian plum extending just beyond bud scales (red arrows)


The particular vine maples (Acer circinatum) I monitored, rarely produced flowers, so I had to rely solely on the date of first bud break. I checked the date when the leaf tissue extended just 2 to 3 mm (about 2/10ths of an inch) beyond the red bud scales. In the picture below is a vine maple bud that is at this stage. The leaf tissue at this point has a whitish, hairy appearance.


Leaves of vine maple emerging from the red bud scales


On average, the six plants that were examined in both 2014 and 2015 reached this stage on March 23rd in 2014, and March 21st in 2015. This is a difference of just 2 days. In this study, that is within experimental error of being identical. So at least for vine maple, there was no difference in time of bud break between the two years.

Answer #3: It was a late spring!

Some of the plants seemed to be a little later this year than last. For example, based on 4 salmonberry (Rubus spectabilis) bushes, in 2014 the leaves first extended beyond the bud scales on February 11th. This year, they were a week later, with the leaves not extending beyond the bud scales until an average of February 18th.


Early, Normal & Late? What’s the deal?

For the vegetative and flower buds of native plants in temperate regions to open up (“break”) and begin growing, two things need to happen. First they need to experience cold (termed “chilling”), and second, they need to experience warmth (termed “forcing”). In that order! Why do the plants do this? While the buds of most plants are pretty tough, the young shoots are relatively delicate. So the plant faces two challenges in the timing of bud break.


Challenge #1: The plant doesn’t want the buds to break too early and start growing just before a big freeze hits. The plant “knows” that even after a short warm spell in December, it could turn fatally cold in January.

Challenge #2: The plant doesn’t want to start growing too late in the year, like mid-July! Starting growth too late means it would miss out on some really nice growing weather in late spring and early summer.


These challenges have been met by plants developing a requirement for a certain number of hours of chilling and then forcing. The definition of “chilling” depends upon which study you read, but generally temperatures in the range of approximately 32 to 50° F count as “chilling.”

“Forcing” is the amount of warmth a plant receives prior to bud break. Typically, any temperature over 50° F counts as forcing. Anytime the temperature is over 50° for an entire hour, the plant is credited with 1 forcing-hour.

The diagram below shows the basic relationship between “chilling,” “forcing,” and bud break. The black curve between the pink and green areas represents the combined amount of chilling and forcing that are needed before a plant can start to grow. The general rule is that the more chilling a plant gets, the less forcing it needs to bud break. Different species of plants have chilling/forcing curves with different shapes. Give yourself bonus points if you noticed that when a temperate region plant receives no chilling, there can never be enough forcing to cause the buds to break!


The “Cold Winter” Model

The diagram below represents a plant after a fairly cold winter. The blue line at the bottom of the chart shows how much chilling this example plant received. The red vertical line shows how much “forcing” it requires to start growing. In this diagram, the winter has been fairly cold. Therefore very little forcing is needed to allow bud break.


The “Warm Winter” Model

This diagram represents what happens in a fairly warm winter. Once again, the blue line represents the amount of chilling a plant received, and the red line represents the amount of forcing the plant needs for bud break. By comparing the two diagrams, you can see that with less chilling, the plant needs more forcing to achieve bud break.



How is this relevant to the question, “Is this an early spring?”

The differences observed in my study may have come about in several ways. One attractive explanation is this: A “warm winter” easily satisfies the chilling needs of those plants with a low chilling requirement, but not those with a high chilling requirement. Once it starts to warm up in the late winter, the plants with a low chilling requirement would only need a small amount of forcing prior to bud break. Plants with high chilling requirements would not have had all the chilling they really “wanted” and thus would have needed extra-large amounts of forcing prior to bud break. Thus these plants would have delayed bud break and blooming.


So the real answer is that 2015 compared to 2014 was an early spring, a normal spring and a late spring, depending upon what plants you were watching.

Nature is always simple, but sometimes it hides that simplicity under layers and layers of seeming complexity. Much of science is dedicated to burrowing through the complexity to get to the simplicity. At one meeting for scientists Dr. John Gordon, former head of the Oregon State University Forestry School, welcomed us by saying,

“You’re coming here confused, and when you leave, you’ll still be confused, but you’ll be confused at a higher level.”

Sometimes that’s the most we can hope for!



Posted on May 25, 2015, in Plants & Wildlife, Tryon Creek and tagged , , , , , , , , , , , , , , . Bookmark the permalink. 4 Comments.

  1. Carl Axelsen

    Over the years it seems as though even modest differences in altitude play a role disproportionate to the differences in elevation. It seems as though plants in the canyon floor activate ahead of those at the top and those activate ahead of the plants in my woods at nearly 500′. Perhaps altitude, in combination with varying chilling & forcing requirements, adds a bit more complexity.
    Thanks again Bruce for another wonderful piece, Carl

  2. Bruce Rottink

    Good observation and thinking, Carl. You’ve now stepped into the world of “microclimates.” Since I’ve already started collecting data on Tryon Creek’s microclimates for a separate Naturalist Note, I’ll keep this response brief. You are correct in that altitude can play an important role in plant development. Just as one tiny example, we know that cold air is heavier than warm air. So on non-windy days, the cold air tends to sink into low elevation areas. These areas are sometimes referred to as “frost pockets” These are facts.

    Now I will speculate about Tryon Creek Park without the benefit of actual data. What if we have a mild winter, which overall just barely meets the minimum chilling requirement of some species of plant. Now suppose some of these plants are growing in the bottom of the canyon, and on many winter days, cold air has sunk to the bottom of the canyon, giving those plants more chilling. When spring finally comes, and the weather starts to warm up, those plants (refer back to the charts in the Naturalist Note) need much less warmth to start growing. Depending upon where on the chilling/forcing curve you are, this could be true, even if the canyon bottom “spring” is a little cooler than the “spring” in other areas of the Park. Presto, earlier bud break in the bottom of the canyon.

    Thanks for asking! And thanks too for your constant support.

  3. How do these triggers work? Is it a chemical reaction? Is the cold weather causing a chemical reaction in the plant? Thank you for your explanation but now I have more confusion and questions.

  4. Bruce Rottink

    Thanks for your questions, Helen. Having questions is the sign of functioning brain!

    I will take you one layer deeper, at least. In many studies, several chemicals have been found to play very important roles in dormancy and bud break. These chemicals tend to work against each other. A chemical called abscissin (also known as “dormin” or “abscissic acid”) tends to be produced by the plant in the short days of autumn. This chemical plays a strong role in inducing dormancy in the fall. The counter-points to the abscissin are two classes of growth stimulating hormones called gibberellin and cytokinin (the latter sometimes referred to as “kinetin”). These two types of chemicals play many roles in the plant.

    As you may know, many chemical reactions are sensitive to temperature. That’s why in chemistry class you had a Bunsen burner, to accelerate the reactions. The plant ramps up production of cytokinens and gibberellins in the spring. These chemicals appear to play a critical role in bud break. So you are correct in thinking that the plant’s activity is regulated by chemical reactions. Please know that scientists are still unraveling the deeper layers of the mechanism of exactly how these chemicals work in the plant.

    As a side note, in the article you will notice that the lowest “chilling” temperature that is effective is about freezing. When the plant tissue is frozen solid, there are many chemical reactions that stop, since the plant is basically a water-based system.

    I hope this helps!

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