Frost Damage to Trees

Magnolia flower showing browning from frost damage

This week’s cold nightly temperatures are causing some tree flowers that are already open to show frost damage. Particularly affected are magnolia flowers (see photo). Temperatures below freezing can cause this tender flower tissue to be killed and brown.  Magnolia and other early-flowering trees are always at risk for frost damage and losing their display early during the occasional cold snap. This spring’s above average temperatures has advanced the flowering of some trees by at least 2-3 weeks compared to normal. The resulting advancement in plant phenology has set the stage for the more seasonal nightly temperatures to wreak their havoc. After several hard frosts it will not be unusual for magnolia flowers to be completely brown.  Generally flowers are affected much more than leaf tissue although a timely hard frost to swollen vegetative buds or emerging leaf tissue may also cause leaf damage, browning or drop to occur. Trees will differ in sensitivity to frost damage. Partial leaf damage is usually exhibited as necrotic spots on leaves causing leaf distortion as the leaves unfurl. This will not usually have an adverse effect on trees unless all emerging foliage is killed. In this case, trees will re-foliate and put new growth back on but this will cause a further depletion of carbohydrate reserves due to the double flush of growth.  In this case it is important to protect this new foliage from further damage from insect damage to allow the new growth to return photosynthate back to the tree to rebuild its energy supply.