The easiest way to tell if a tree or shrub needs water is to test the dirt.
- For a new tree, sample dirt at the base of the tree and for an established tree, test the root zone.
- Get a pinch of dirt (be sure that it is dirt and not mulch) and roll it around between your fingers.
- If it sticks together, there is enough moisture.
- If it is dry and crumbly, it is time to water.
- If it is muddy and hard to grasp, the area is receiving too much water.
Too much water is just as bad as not enough water.
This method, however, is not foolproof, especially in areas where there is a lot of clay in the soil or in urban environments where water can get trapped in pockets.
How else can you tell if a tree or shrub needs water?
The University of Illinois Agriculture Extension suggests a few things to look out for:
- Leaves may wilt, droop, turn yellow, show early fall color, turn brown at the tips or margins, curl or show all of these symptoms.
- Green leaves, stems, roots, and fruits may shrink.
- Shrinking can cause radial cracks in tree trunks.
- The leaves of some trees such as ash, linden, hickory and black locust can yellow and drop early.
- Severe water shortage in pines during the summer can cause needles to bend or droop near the needle base. Needles then either fade and turn brown or remain green and permanently bent.
Another way to track whether or not a plant needs water is to use a rain gauge. Place a rain gauge somewhere on your property where there is no overhead obstruction. Typically, installing one at the top of a fence in an open area is ideal.Check your gauge after each rain event to see how much rainfall you received. Is this enough for your plants? According to the City of Raleigh’s City Tree Manual, mature trees should consistently get 1 inch of water per week. A rain gauge will clearly show how many inches (or lack of inches) of rainfall your plants have received.
If a rain gauge is not an option or if you want to compile data just for fun, (because let’s be honest, we plant geeks love to gather statistics) you can collect rainfall data from your local airport or weather channel. Knowing how much rain has fallen in a given week or month is crucial information about whether or not your plants are receiving enough water.
If resources allow, you can buy fancy gadgets like this soil moisture meter.
According to Popular Science, in the future, we might even be able to hear stressed trees.
...researchers discovered that about half of the sounds made by a tree are due to cavitation, and that the process has its own unique acoustical signature. In the future, the researchers say, forest managers could use a hand-held acoustic device to identify water-stressed trees before permanent damage sets in.
What is cavitation?
Trees draw ground water up through specialized tubes called xylem, relying on intermolecular forces between water molecules and themselves, and water molecules and the sides of the tubes, to create a single column of unbroken water in each xylem tube. But as groundwater dries up, the trees must pull harder on the remaining water; if the pressure is greater than the strength of the intermolecular forces, the column of water breaks and an air bubble forms. This process is called cavitation.
Unfortunately, such a specialized tool is not available yet, so we still need to follow these tried and true methods. If the suggestions above seem too technical, we recommend you simply spend a little time observing your new plant. Within just a couple of days, your eye will become accustomed to seeing whether the plant is looking wilted, yellow, brown, dilapidated, or stunted. The power of observation can help you identify when it is time to give your tree the water it needs.