Why Evergreens are Green in Winter – Winter Photosynthesis

  When you look off the back patio at the arboretum during the winter months it’s such a different scene then during the spring, summer or fall because most of the leaves are off the trees.  Not only can you see farther down into the property from the same vantage point but what really stands out to me are the highlights of green trees scattered throughout the property. We all know that when the seasons change and it starts to get cold and the days get shorter deciduous trees automatically cut off connections to their leaves and stop performing photosynthesis in order to save energy for the upcoming winter months.  But some plants and trees keep their leaves all winter and they stay green.Trees like evergreens and conifers continue to stay green all winter because they have evolved special adaptions to survive the cold climate where they grow.

  First, these trees have needles rather than leaves.  The needles are a way of protecting their foliage all year round.  Snow easily falls off them.  The shape of the needle and its waxy coating is also designed to conserve water and minimize evaporation.  These trees feed themselves all winter by keeping chlorophyll in their needles by using smaller amounts of energy it produces during the winter.  The chlorophyll is the chemical that makes photosynthesis possible and what causes the needles to look green.  Plants that perform winter photosynthesis spend time during the spring and summer storing energy.  In simple terms, during the coldest months, evergreens convert these starch cells back to simple sugars that can be used to produce energy and food for the plant system, or to provide energy for new growth in the spring.  

  Also, evergreens are able to conduct photosynthesis in winter because the cells in the needles are protected by a type of self-made antifreeze that is carried in the tree sap.  The cold temperatures and lack of sunlight cause the photosynthesis process to occur more slowly and it is not as efficient as warm weather photosynthesis which is why some needles end of falling off the tree but many do not.

  If you want to see some winter photosynthesis in action visit the arboretum and see what nature has to offer.  The arboretum grounds are open all year round.  Happy New Year!

                                                                                  --- Sue Abbuhl