Summer drought

Photo credit header: Andrew Boone, South Carolina Forestry Commission. Bugwood.org

If you think a hot, dry summer is stressful to you, try being a tree, especially a city tree near hot pavement or next to a building that reflects the suns heat. Urban trees have it tough in the summer!  During drought, trees cannot obtain the water needed for photosynthesis (food production)  or replace the water lost to evaporation.

Photosynthesis – Water is needed for food production.

Insufficient water disrupts biological functions, reduces growth, makes the tree more susceptible to pests and disease,  and may even kill the tree.  The effects of drought may not be immediately apparent,  especially in mature trees.  If a tree does show symptoms of heat stress, this may include leaves with scorched, brown edges,  premature leaf drop, die back from the outer branches and top, or a thin canopy.

We encourage you to water trees during the summer when there is little rain and please remember the street trees near your home, especially young trees.  Use the guidelines below to ensure the best outcome for your trees.

Newly planted trees

Young trees need to be watered for the first two years after planting, regardless of drought.  They are very unlikely to survive a summer drought on their own.   Ideally, new trees should be given about 15 gallons a week, unless there is 1 inch of rain.   Most will survive with less frequent watering, but consistent watering will increase the chance of survival and result in a healthier tree.  At a minimum, water once a month between May and October, more often during hot weather, and weekly during drought.  Water can be applied using a gator bag or ooze tube, with several 5 gallon buckets with 1/4 inch holes drilled in the bottom or just slow flow from a hose.   Water must be applied directly to the root ball and the planting area.  Be sure the water soaks in and does not run off.   Apply mulch and keep the area around the tree free of weeds and grass.

Mature, established trees

It is not practical or possible to give a  large tree as much water as a good, soaking rain, but giving trees some water during severe drought can have significant benefits in the long term.

If possible, established trees should be watered about every 2-3 weeks during drought using drip emitters, soaker hoses, or just slow flow from a garden hose that is moved frequently. Water in a wide band under and just outside the canopy, but not right at the trunk of the tree.  The amount of water should be sufficient to soak deep into the soil.  A general guide is to use approximately 10 gallons of water per inch of trunk diameter at knee height.   The use of sprinklers is not recommended.

Dos, Don’ts and Tips

  • Do use organic mulches (wood chips, bark, etc)  to conserve soil moisture and moderate soil temperature.  Mulch should be 2 -4 inches deep and should never touch the trunk of the tree.
  • Do not use inorganic mulches such as stones.
  • Do pay more attention to trees near pavement or other hard surfaces since these trees experience more heat than those in  the middle of a yard or landscaped area.
  • Do water so that it soaks into the ground rather than running off.
  • Deep watering is essential! 
  • Do not give small amounts of water.  Even if this is done often, it does more harm than good. This encourages shallow roots that dry quickly.
  • The best time to water mature trees is between late evening, at night , or very early morning.   Trees replace the  water they have lost during the day at night.  Watering at night also reduces the amount of water lost to evaporation.  
  • Do not use fertilizers around trees during drought and be very cautious about the use of pesticides.
  • Do not prune unless absolutely necessary.
  • During drought and water restrictions, give trees priority over lawns.  It takes many more years to grow a tree than a lawn.  A lawn can become dormant and, if necessary, can be replaced in a season; it could take a lifetime to replace a large tree. 
  • To determine how long to water with a hose, simply place the hose in a 5 gallon bucket, turn it on low and see how long it takes to fill the bucket.   If the bucket fills in 5 minutes,  you will need to water for 15 minutes get the necessary 15 gallons for a new tree.  Remember that you will need to let the water run slowly or trickle so that is soaks in rather than running off.    
More information about this very dry subject can be found at:
http://www.coopext.colostate.edu/4dmg/Trees/caring.htm
http://extension.missouri.edu/publications/DisplayPub.aspx?P=G6879