As a landscape professional, one question I’ve always dreaded is, “How much should I water this tree?” Honestly, there are so many variables that giving a blanket answer is fanciful. More than likely, a part of the answer is, “More than you are now!” Trees are big plants with lots of leaves, and they can transpire a lot of water. That water is “lost” while the leaves are making sugars via photosynthesis, so trees have a high water demand.
If supply doesn’t keep up with demand, the tree gets stressed. How do you know if your tree needs more water? Learning to recognize signs of water stress can help you stay on top of your trees’ needs. There are some common symptoms and situations that may indicate that they are thirsty.
Water stress can be long-term (chronic) or sudden (acute). Chronic drought stress occurs commonly, as many irrigation systems are set up to be adequate for a newly planted tree. As that tree grows, though, it has more leaves and needs more water. Adding emitters and covering more ground is key to keeping up with the growth. Acute drought stress is less common; it happens when a tree suddenly loses most or all of its water supply, which up to that point was adequate. Finally, most rare of all, water stress can happen from too much water in the soil.
Chronic drought stress occurs when trees haven’t had enough water for some months or years. These trees will have smaller than normal leaves, and those leaves will be sparser in the canopy than normal for that species. They may be off-color, not as vibrantly green as normal. Often, there is dieback in the small, outer branches and twigs.
Annual growth will slow down. Evidence of this can be seen in small diameter branches if you can identify the ring scars that mark the annual growth segments. Ring scars are lines that circle all the way around the twig; they are not bud scars that form at each dormant bud and do not circle the twig. If annual growth segments are shorter last year than the year before, something bad is going on.
In conifers, we see similar evidence — smaller and fewer needles and shorter growth segments. Often, pine needles will brown out from their tips, maybe just a little or maybe halfway. Conifers keep their leaves for several years, so partially browned needles can really slow down growth potential as the brown part isn’t making sugars. Chronically drought stressed conifers abound in New Mexico forests. They are more susceptible to bark beetle attacks than unstressed trees. Combine beetle-killed trees with very dry trees and wildfire becomes a huge concern.
Acute drought stress might happen when a trench cuts through the root system, such as with a new sewer line is installed. Acutely stressed trees tend to have sudden yellowing and then drying of leaves, maybe just on one side of the tree (the trench side, for example). Acute stress is hard for the tree to recover from.
In either chronic or acute stress situations, leaves may develop brown and dead edges. Termed “marginal browning,” this happens when excess salt in soil water gets deposited at the furthest edge of the leaf, building up to toxic levels. Leaves will often turn pale and then yellow during mid-season, well before fall temperatures normally trigger that.
How about those few trees getting too much water? In some ways, symptoms can mimic too little water. When the soil is very wet, roots stop functioning due to lack of oxygen. This is of most concern for dryland conifers like juniper and pine. Usually, it is linked to an irrigation system that only has emitters at the base of the tree that run often. The tree doesn’t have the fine root mass near the trunk to take up the water, so the soil remains wet. Fungal diseases are likely to pop up, leading to root death and then tree death. Another symptom of overly wet soils shows up in leaves on deciduous trees like cottonwood — oedema. These are corky scars on the surface of the leaf.
You can reduce tree stress with proper irrigation. Trees seem to do well when irrigation is deep, broad and not too frequent. Watering every day is actually not helpful, especially when right at the base of the tree. Cover the soil with an organic mulch to keep the soil cool and the moisture available to your trees.
Learn more here:
How to Make a Tree Watering System.
Best Practices for Watering with a Drip Irrigation System
