False Spring

In the U.S., some people note the beginning of spring when cherry trees ringing the Jefferson Monument begin to bloom. But what if spring comes too early? Can trees and other flora be ‘tricked’ into ‘thinking’ that spring has begun and begin to bud too early?

Plants and trees can be tricked into waking up early by a premature bout of warm weather. But this “false spring” usually won’t affect native species. That’s because natives don’t initiate spring growth until the days become longer and until they’ve experienced a certain number of weeks of cold weather.

Some species are more susceptible to damage from a “false spring” than others. Magnolias, for example, are easily nipped by a frost — because the fleshy magnolia flower has a high water content that’s vulnerable to frost damage. On the other hand, most trees store starch in their wood and bark. If their early leaves are damaged, they can produce another flush of leaves by mobilizing these stored carbohydrates.

But in a very stressful year — with a drought or early storm that knocks down branches — producing that second flush of leaves can deplete a tree’s reserves. It’s thought that — with the rise in global temperatures in the coming century — temperature variability will also increase. There could be more bouts of early warmth followed by a return to cool conditions — with the potential to fool and even damage trees.

Camile Parmesan – Assistant Professor
University of Texas at Austin

It’s called a False Spring. And there has been an increase in false springs over the last 20 years of so. And it’s definitely been causing problems. It’s certainly caused extinction of some of the butterfly populations I’ve worked with because that’s exactly what happens. It’s not just the trees, but everything starts coming out of its dormancy and it’s not adapted. Once it does that it’s not adapted to deal with a cold spell. And so when that cold spell comes, when that freeze comes, it either dies, or is severely injured by the cold. Absolutely

Hermann Gucinski – Assistant Director
Research Southern Research Station
Asheville, NC

Plants do indeed get tricked into flowering early by such spells, and can thus lose, due to later frost, the potential to develop fruit. This is of significance not only in natural systems, but in crops (fruit trees etc.) and has often been seen with the famous cherry trees that ring the Tidal Basin and the Jefferson Monument in Washington, DC. In that case, there is not loss of fruit, since these trees, a gift from Japan, were bread not to produce cherries. In some cases trees hedge their bet, and will have some blossoms develop later, but most commonly such 2nd efforts lead to only meager crop of fruit. The blossoms that came early and were later frozen are all nonviable. I hope this answers your question, but feel free to probe further if you so desire. If I can’t answer your question, I will find people that can. best, always hermann

Terry Chapin – Institute of Arctic Biology
University of Alaska
Fairbanks, AK

This could happen in some species, but most species don’t initiate spring growth until the days become longer in spring and until they have experienced a certain number of weeks of cold weather. Species differ, but this is the general rule.

Evan H. De Lucia – Professor and Head
Department of Plant Biology
University of Illinois
Urbana, Illinois

The short answer is yes, plants can be tricked into waking up early by an bout of warm weather. We’ve probably all seen Irises and snowdrops coming up early when temperatures warm in the spring. Trees can be fooled into getting an early start too. Most trees respond to the combination of complex environmental signals to tell them when to initiate growth. First, the daylength has to be right, then they need to accumulate a certain number of cold days followed by so many warm days before they go. If warmth accumulates too early in the season, trees may break bud too early. This can be dangerous as the new foliage is vulnerable to subsequent frosts.

With the warming we are witnessing as a result of the accumulation of carbon dioxide in the atmosphere from cars and factories, we will not only experience greater average temperatures but the variability in temperature also will increase. We will see more bouts of early warmth followed by a return to cool conditions and consequently more bouts of trees being fooled with subsequent frost damage to their foliage.

Most species store enough starch in their wood and bark so that even if some of the early leaves are damaged they can produce another flush of leaves by mobilizing stored carbohydrates. So in a normal year early growth may not be too dangerous. But in a stressful year, say one with a drought or an early storm that knocks down branches, producing that second flush of leaves may leave the tree’s reserves depleted.

John Harte – Professor Energy and Resources Group
University of California
Berkeley, CA

Much depends on the physiology of the individual species. In the Bay Area, eucalyptus trees which were imported from warm Australia can be damaged by prolonged frosts which we occasionally get here, but native trees would not be damaged. Even the damaged eucalyptus will survive however, and the next year will grow fine.

In mountain meadows, where flowering perennials forbs are used to cold weather and go dormant in the winter, an early thaw will promote early sprouting and budding. If that is followed by a cold spell, the above ground part of the plant can die but the root system will survive and the plant will grow again the next year.

Some plants can be killed outright by thaw followed by frost, however. These would tend to be non-native shrubs and forbs (rather than trees) brought in from warm locations.

Robert DeFeo – Chief horticulturist
National Park Service

You can find information on the cherry trees at www.nps.gov/nacc/cherry. If you click on PEAK BLOOM the forecast and how it is conducted is clearly outlined and updated as necessary.

Mr. Gucinski’s response is for the most part correct. Any extended period of warm weather AFTER the trees chilling requirements are met can push a tree into bloom and/or growth. This is primarily a function of temperature but daylength also plays a small role. A point of interest is that this could not happen in the fall because trees need a certain amount of chilling before breaking dormancy. This is a survival mechanism that prevents trees from coming into growth if an extended warm period is experienced in early fall before winter. The chilling requirement varies with each species.

Another point is that 50 degree days with 45 degree nights might start things moving. If the temps are dropping back into the low thirties and 20’s at night you won’t see much happening – this is refereed to as the Dif – the difference between the night and day temperatures. It is used by commercial greenhouses to control growth in Easter Lilies to make sure they bloom before Easter (not worth much after easter). The larger the dif the less growth as the nights slow the plants down. I use this as well in measuring the effect of a warm day on the progression of the cherry buds.

On thing I’ve noticed with cherries is that three days + of temps in the 50’s with nights above 40 can get things moving a little – but – one night back in the 20’s will put the brakes on them and it will take a few more warm days to get them going again.

The effect is species related as well. Some species will respond quicker than others. And the temperature for damage varies with species as well. That is why magnolias are commonly nipped by a frost – they start up easily with an early spring and the flower has a high water continent (more fleshy) and thus subject to damage. I have seen both the cherries and the Magnolias push early with the temps dropping to mid twenties – the magnolias were wiped out – the cherries survived with no damage. And we see daffodils in flower survive sub 20 temps with no damage. So it is species related.

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