Several weeks ago, I noted that a number of landscape plants were showing signs of problems related to last year’s hot summer and other weather events. Now, hot weather has arrived too early, and local pines, arborvitae, juniper and other evergreens are dropping like flies. Old and young trees are turning brown and dying.
There are several factors that have led to browning and dieback. One is the watering habit of many tree owners. Despite my frequent urging of owners to deep water trees and shrubs weekly during hot weather, few actually do this. Instead, they rely on lawn watering during the growing season. This is typically shallow watering and has subjected trees to years of chronic drought stress. The cumulative effect has weakened the trees and made them more vulnerable to weather extremes we experienced last year.
Past winter drought stress is another likely factor in evergreen dieback. While this past winter was fairly wet for us, in recent years the winter weather has been mild with little precipitation. That is why I also urge tree owners to take on the burdensome task of deep watering trees, especially evergreens, in October and monthly during dry, mild winter weather.
At this point, we might assume that last summer’s heat was the proverbial “straw that broke the camel’s back,” but other factors could be contributing to the dieback problem:
1. Girdling roots restrict the uptake of water. They are caused by a failure to adequately loosen and spread the roots at planting time.
2. In addition to loosening roots at planting time, proper planting techniques are important. Planting too deeply smothers the roots. Leaving plastic pots, biodegradable containers and even treated burlap around the roots can delay or restrict root growth.
3. In many landscapes, soil becomes compacted from lawn use or was compacted during home construction. Soil compaction prevents air and water from getting to the roots.
4. Sandy and shallow soils are not capable of retaining as much water as heavier or deeper soils, limiting the availability of soil moisture.
5. Physical damage to tree trunks from weed trimmers or mowers impedes the transport of water from the roots to the top of the tree, causing water stress in the top of the tree.
6. I am a strong advocate of mulches, but rock mulches and close proximity to concrete walls put additional heat stress on a tree and increase its need for water. Bark or wood chip mulches are recommended, but when applied in layers thicker than 3 to 4 inches, they restrict air and water movement.
There are many lessons to be learned from our situation, but deep watering and keeping trees and shrubs in good health is one of the most important lessons. There is no one cause for the widespread browning and dieback of local evergreens, but the primary factors are watering, weather and root problems.
Next week, I’ll talk about wood boring insects that are exacerbating the dieback problem by attacking stressed trees and quickening their demise.