This summer is a real scorcher, crisping leaves and wilting sprouts. Although the weather has baked us this summer, the symptoms of stress on our trees might not be due to the heat in the yard; instead, it might be lingering damage from a warm, dry winter.
During summer, trees show distress in several ways if proper care is not being given to them. Those that grab our attention aren't caused by conditions outside right now: mid-summer leaf drop, and/or scorching of leaves. These are both indicators of a lack of winter watering before this growing season.
Mid-summer leaf drop is characterized by trees leafing out normally in spring, then dropping leaves at this time for seemingly no reason. When trees don't develop enough fine roots to support summer's lush canopies, their leaves lose water faster than the stunted root system can replace it. Leaf tips brown and leaves drop from the tree.
Scorched leaves show up as evenly discolored areas on leaves that are dry and slightly curled, moving from the tips inward. These brown areas have no colored rings, called halos, or fruiting bodies of fungus on them. Fruiting bodies of fungi usually show on the leaf as small, pimple-like speckles and often have red or yellow halos around the diseased area. Bacterial disease on leaves will have this halo as well, along with a slimy or greasy looking center spot. If your tree develops these 'leaf zits', you should suspect a fungal or bacterial disease.
Being evergreen, pines are particularly susceptible to drying out from lack of winter water or heat/drought stress, both of which will cause needles to brown back with no rings or banding, and become brittle and dry. As odd as it sounds, gardeners should make winter watering of trees and shrubs a priority for a healthy landscape in summer.
High heat and drought cause other symptoms on trees as well. Many people notice the odd bright yellow discoloration of trees in our area caused by iron chlorosis. Although the nutrient iron is contained in our soils, our high pH makes it unavailable to plants and they suffer by being unable to make chlorophyll, which produces the green coloration in leaves. As a result, we have a landscape of trees that are a flaming lime-yellow in color.
In most cases leaves become pale yellow but veins remain green. Iron chlorosis can contribute to the death of the tree by reducing the chlorophyll to the extent that leaves develop brown spots between veins and die off, or leaves are stunted and branches die back over winter. Help can be given to the tree by applications of chelated iron to the soil, or by spraying iron on the leaves every two weeks. Helping our trees overcome their iron chlorosis is a life-long commitment, one that must be addressed several times each season, year after year.
For those who are not as enthusiastic about tending to their trees for the remainder of their days, choosing trees that are better suited to our climate and soils is a good solution. There are several fact sheets that list good trees species for our area, such as 7.421, Native Trees for Colorado Landscapes, 7.423 Trees and Shrubs for Mountain Areas, or 7.229, Xeriscaping: Trees and Shrubs, that can be found on our website at ext.colostate.edu/pubs/pubs.html#garden.