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Climate Change: Your Flowers Are Blooming Earlier

It’s not your imagination. A new study shows that for every 1°C (1.8°F) rise in average spring temperatures, plants produce flowers 3.2 days earlier.

tulip pollination
If pollinators aren’t available when flowers are fertile, reproduction will not take place

If you’re a gardener, you’ve no doubt uttered some incarnation of the phrase: “it seems like my crocus / tulips / daffodils / lilies / hyacinths are blooming earlier than they used to”. Rest easy, it’s not your imagination. It’s reality. Climate change is forcing flowers to bloom earlier.

In January of this year, a study entitled Record-Breaking Early Flowering in the Eastern United States, published in the peer-reviewed science and medicine journal PLOS ONE, showed that in 2010 and 2012, plants in the U.S. produced flowers earlier than at any point in history. Mathematical and controlled greenhouse experiments predicted this acceleration for years – warmer average temperatures would cause earlier blooming – but this real world data from natural settings shows that the acceleration is happening even faster than predicted.

The study was conducted by Boston University biology professor Richard Primack and University of Wisconsin emeritus professor of forest and wildlife ecology Stanley Temple. Primack and Temple studied the journals of two famous naturalists, Henry David Thoreau, author of Walden, and Aldo Leopold, author of A Sand County Almanac, who kept detailed notes on bloom times for their local flora.

Henry David Thoreau’s Walden Pond observations were used as a baseline

Thoreau, living on the banks of Walden Pond in Concord, Massachusetts, spent six years (1852-1858) recording in great detail the first bloom dates of dozens of species. “I often visited a particular plant four or five miles distant, half a dozen times within a fortnight, that I might know exactly when it opened,” Thoreau wrote in his journal. From 1935-1945, Leopold did much the same in Dane County and Sauk County, Wisconsin. In the first week of May 1940, Leopold wrote: “Pasque flowers are blooming. Lilac leaves at the shack are 1 inch long. Serviceberry, Chokeberry are breaking buds,” and added that two inches of snow had fallen that week.

Warming in the greater Boston area, which includes Concord, has been attributed to both global warming and the urban heat island. Within the past decade, two years have been record breakers in this region: mean spring temperature in 2010 was the warmest ever recorded… In these two years, plants flowered three weeks earlier than when Thoreau observed them in Concord.Record-Breaking Early Flowering in the Eastern United States

Using the historical information from the journals as a baseline, Primack and Temple compared recent bloom times of 32 of the same species in the same areas. On average, they found that these plants now bloom eleven days earlier than in Thoreau’s time. In a dramatic exhibition of the correlation between rising temperatures and earlier bloom times, during the record breaking warm spring of 2010, 13 of the 32 species had their earliest flowering date on record. In 2012, another record breaking warm spring, a different 14 species had their earliest flowering date. Of special note was highbush blueberry (Vaccinium corymbosum) which flowered on April 1, 2012, six weeks earlier than what Thoreau recorded (May 11-21).

The study shows a direct correlation between rising average temperatures and earlier bloom times:

  • From 1852–1858, the average spring temperature in Concord was 5.5°C (42°F) and the average first flowering date for the studied species was 15 May
  • From 1878–1902 the average spring temperature was 6.3°C (43.5°F) and the average first flowering date for the same species was 10 May
  • From 2002-2012 the average spring temperature was 8.8°C  (48°F) and the average first flowering date 4 May
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That’s a 3.3 degrees centigrade (6°F) shift in average spring temperature and an 11-day shift in average bloom time from the mid-1800s. Historical data like that provided by Thoreau and Leopold are essential in understanding the responses of plants to the warming which accompanies climate change.

Wisconsin’s spring temperatures have warmed dramatically as a result of climate change… In 2012, [warmest spring on record in Wisconsin] mean flowering was 13 April, the earliest date ever recorded, and over 3 weeks earlier than mean flowering in Leopold’s years.Record-Breaking Early Flowering in the Eastern United States

What do earlier bloom times mean for your garden?

The study observed that blueberries bloomed April 1 in Concord, which can be a very cold day in Massachusetts (when I lived there, the heat in my home was sometimes still needed the first week of May). If a frost or freeze follows blooming, flowers and leaves may be damaged, which will decrease the production of berries. But every gardener has seen late cold snaps, that’s nothing new.

But if local pollinators like honeybees haven’t adapted to the rising temperatures, that is, if they don’t venture out from their hives and nests while the flowers are fertile, the flowers won’t be pollinated – only a short fertility window exists. No pollination or reduced pollination means weak reproduction and little fruit. Warmer spring temperatures also make plants more susceptible to the effects of drought, which (as you’ve no doubt experienced) can weaken or kill the plant. But we also have to take into account a plant’s requirements for specific seasonal temperatures and light.

Winter chill and photoperiodism

Many plants have winter chilling requirements and if they don’t experience this dormancy, they won’t bloom. In warm temperate areas, winters are even now barely sufficient for winter chilling requirements. Throw in a couple of degrees rise in average temperature and the chilling won’t take place. Of equal importance is the length of day a plant requires to properly leaf out.

Plants keep time by a process known as photoperiodism, which is their response to changing lengths of day and night. The initiation of flowers is the process most closely linked to this. Many plants, but not all, time their reproduction to the duration of daylight in each 24 hour period.

Some plants are day-neutral and simply flower after they put on a specific amount of vegetative growth – day length doesn’t matter to them. Other plants have critical photoperiods and will only flower when day length is very specific. For instance, chrysanthemums will only flower when day lengths are less than 15 hours; violets, when day lengths are less than 11 hours, sedum and spinach when day lengths exceed 13 hours, etc.

Also, plants only flower when they have reached a stage called ripeness-to-flower, which is the minimum size a plant must be before it can bear the weight of blossoms and fruit, and when the plant has built up sufficient food reserves to supply the demands of reproduction. Only then will the plant be photoinduced (triggered by light) by the correct day length to flower.

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The problem is that the length of day in early spring is significantly shorter than it is 30 days later. For example, on April 1 of this year in New York, the sun shines for 12 hours 41 minutes but shines for 14 hours on May 1. Less daylight means less photosynthesis, which means less growth, less foliage, fewer flowers, less fruit, and smaller food reserves. The last of these is very important, as it takes an enormous amount of energy for the plant to support its reproductive cycle. That one hour and 19 minutes is very significant.

Every plant must follow a precise cycle

In order to successfully reproduce (including flowering and fruiting), each plant has a unique cycle which it must follow precisely, regardless of when it starts. There are many steps in the process, but when the reproduction cycle begins at the onset of warm weather, there also must be sufficient daylight to enable growth and the development of food reserves. An early start with less light results in smaller plants which may not be able to hold their flowers up, creating fewer opportunities for bees and other pollinators to find them.

There are an endless number of other ecosystem interactions that take place during flower bloom time. As mentioned, pollinators must be available when plants are flowering, and animals which eat and disperse seed must be available when the seeds are. Herbivores like deer and rabbits depend on plants growing leaves at precise times, and without this important food source, wildlife populations could dwindle. If bees don’t pollinate, not only will plants suffer, but so will the bees and the products the hives produce like honey, honeycomb, propolis, royal jelly, beeswax, pollen, etc.

Most plants have yet to reach these thresholds. But if the average temperatures continue to increase as predicted, and plants continue to bloom earlier and earlier, they may reach these physiological thresholds in less than 100 years. But plants may put the brakes on and stop flowering earlier, even as temperatures continue to rise, which will expose them to the dangers of drought and excessive heat they can’t tolerate.

Professor Primack says, “Over the coming decades, we’re likely to see a lot of significant changes caused by climate change—more and more extinctions, for example—but we can’t measure most of those things yet. Bird migrations and flowering times are the best indicators we have that natural communities are starting to change.”

Buy on Amazon:
Walden by Henry David Thoreau

A Sand County Almanac: With Other Essays on Conservation from Round River

Botany for Gardeners: Third Edition

2 thoughts on “Climate Change: Your Flowers Are Blooming Earlier”

  1. Hi Todd , Thanks for your great post. I think your article is very informative and helpful for every gardener. I appreciate your study. Keep it up

  2. This is a beautifully written, easy-to-grasp summary of the effect of climate warming on plant life. I hope you’ll submit it to your local newspapers and plant magazines!

    Tisa

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