A Prairie Garden Journal   
                                              by Dick Meyer (Not Ernest Hemmingway)

Urban Legends
of
Trees

(or The Sap Also Rises)

Urban legends are contemporary cultural myths that seem believable, and which continue to be passed on as truth, without ever being proven to be true—like the guy with the hook for a hand that haunts the “lover’s lane” of some nameless small town, or the “moon landing” that was actually filmed on a secret government movie lot, or the aliens being held on a secret government facility in Nevada. 

Like all fields of endeavor, tree care has its share of urban legends.  The urban legends of trees are myths about trees and tree care that are a mixture of fact and fiction.  Many of these urban legends have been haunting landscapes, garden centers, and gardening classes for decades, if not centuries.  And like the villains in a slasher movie, these urban legends of trees are almost impossible to kill. 

The various solutions of plant sugars and starches and water that we call sap, likewise move throughout the tree, primarily during the growing season.  They are carried from the leaves, where the sugars are manufactured and refined, to branch, bud, and root system tissues where they are stored (year around) until needed as the raw materials for growth and other vital functions. 

  One of those vital functions, by the way, is the feeding of a diverse group of microbes that live right next to the roots of trees and shrubs.  Essentially, trees and shrubs “trade” some of their valuable sugars to soil bacteria and fungi for various chemical molecules that the plants need, but which their roots are unable to pull directly from the soil. 

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Fractions March 15
Yardner March 8

2006 Articles

One particularly well-entrenched urban legend of trees is that trees send their sap down into the roots in the fall to protect it from freezing over the winter and then send it back up into the branches in the spring.  Trees do appear to significantly reduce the amount of water held or stored in trunk and branch tissues in the fall of the year—probably as a way to prevent damage to tissues from the freezing of excess water in those tissues through the winter.  However, it is now known that some water does remain in the branch tissues of trees over the winter and that trees appear to create room in and between cells for that water to freeze without harming its tissues.

But the sugars and starches that a tree manufactures during the summer are not moved down in the root system in the fall in order to protect them through the winter.  In fact, most of the basic sugars made by trees through the process of photosynthesis are chemically converted into starches and other more complex sugars and stored above ground in branch and bud tissues.  Only those sugars actually needed by the root system for growth and functioning are transported down to the root system.

So where did the urban legend come from that “sap rises in the spring?’

That said, the movement of water and other fluids within plants is still not completely understood.  Plants appear to push or pull water from the ground and move it up into their above ground tissues by using a variety of methods.   But the ability of very tall trees like giant redwoods to move water 300 to 400 feet into their very topmost branches still defies explanation and appears to violate several laws of physics. 

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 “Hey buddy, how about I trade you a molecule of phosphorus, for a little bit of your sugar?”  “Can you spare a little iron, there Mac, I’ve got some particularly tasty sugar today?”  This function is so important that most plants “spend” from 10 to 20% of their manufactured sugar “buying” soil elements. 

In case you’re wondering, this column does have a point.  And the point is this.  While the sap may not rise in the spring, the water does.  Trees are actively refilling their above ground reservoirs of water throughout the spring, and if those trees happen to be growing in a semi-arid climate that is experiencing a prolonged drought, they might some help in getting the springtime drink that is such an important part of their annual growth cycle.

Trees in Wyobraska are already beginning this process.  You can help them by watering your trees, especially if our spring weather remains warm and dry.  One or two supplemental waterings before the sprinkler system is turned on can provide a big boost to thirsty trees and shrubs.  By the way, it’s another urban legend that watering trees in the early spring will bring them out of dormancy earlier.  If anything, just the opposite is true. 
 

Another way to help your trees take a full spring drink is to make sure that they are adequately mulched.  One of the benefits of mulch is that it reduces the evaporation of water from the soil.  During a dry, warm, windy spring mulch can significantly increase the amount of water retained in the soil around your trees—making more water available to them for their spring drinking binge.