Water is, of course, vital to the survival and health of a tree.  You can provide a tree with the moisture it needs.

How

A tree takes moisture in through its root system.  At the tip of the root, there are tiny structures called root hairs which absorb moisture from the surrounding soil.  This is where you want to target your watering.  

In an elm, the tips of the roots are usually located outside of the weeping or drip line.  This is the outermost extent of the crown of the tree.  Therefore, sprinkling water onto the trunk will have little or adverse effect on the tree's health.

It is important to remember that grass and other vegetation compete with a tree for moisture.  As a result, sprinkling is not a very efficient method of watering.  Firstly, there is extensive evaporation, and secondly the grass absorbs a large percentage of this water.

A preferable method of watering is to soak the ground outside the weeping line with a hose.  There are also special devices that allow you to apply moisture directly into the ground near the roots.

It is important to allow the soil to dry in between waterings.  If the soil is constantly wet, it can easily become compacted, hindering gas exchange with the air.

When

Often nature provides all the moisture a tree needs.  It is times of drought or in certain urban environments when watering is required.  

• Examine the leaves:  wilting, browning, discoloured margins - these are all signs your tree needs moisture

• Examine the grass:  if your lawn is yellowing, there is good chance that your tree lacks moisture even if it has yet to show signs.

• Examine nearby trees:  If other trees in your area show signs of drought, your tree may be next.  Often other species, such as maples, show signs earlier than elms giving an early warning signal.