Garden Variety: The many causes of tree decline, and what you can do about it
When a tree appears to be unhealthy or dying, the first reaction for many people is to suspect insects or disease. In many cases however, tree decline is the result of a combination of factors rather than a single or specific pest. This is especially true in urban and suburban settings where there are more predisposing factors.
Tree decline in early stages is characterized by limited tree growth (less than expected for the species) and increased die-back of branches (more than expected). This condition may go unnoticed until the tree displays greater signs of decline such as chlorotic leaves, early fall coloration and leaf drop, sprouts on the trunk, heavy seed crops and signs of decay.
This is usually the point when decline is noticed. Determining a specific cause is difficult. Decline may begin with a specific event such as a wound to the trunk of the tree, or it may start with other factors such as compacted soil, root competition or extended drought periods. Often, the decline has been progressing for years, and it is nearly impossible to pinpoint the beginning.
Once the tree is this far into the spiral of decline, there is little that can be done to reverse it. Consultation with a tree care professional (arborist or horticulturist) may offer options for alleviating contributing factors and extending the life of the tree. Each situation is different, but generally removing dead wood from the tree, watering over extended dry periods and reducing root competition and compaction will help.
When hiring a tree care professional, look for someone who is certified with the Kansas Arborists Association or International Society of Arboriculture, and/or someone with training in tree health and management. Get more than one opinion. If they want to treat the tree with insecticide or fungicide or fertilize it heavily, they may be more interested in selling products than helping the tree. Although in some cases these treatments may be warranted, they often have little benefit.
Pruning cuts should be made on the edge of the branch collar. Cuts that are flush with the trunk leave wounds that are larger than necessary and take a long time to heal. Cuts that leave long stubs will die back to the branch collar on their own but also take longer to heal.
When watering over extended dry periods, water deeply and infrequently. Set up a drip hose or sprinkler to water over a large portion of the tree’s roots. Check the soil moisture below the surface after a period of time to determine how far the water is soaking into the soil. Watering for a short period only moistens the soil surface and provides little benefit to trees and other plants.
Reducing compaction is more difficult. If the tree is growing in the lawn, core aeration of the lawn area can help. Mixing compost into the soil in the tree’s root zone can also help but must be done carefully to avoid additional damage to the roots.
To reduce the likelihood of premature decline in trees, select species that are appropriate for the region and soil. Plant trees at the proper depth. Avoid mulch volcanoes but use mulch to reduce water and temperature fluctuations over the tree’s roots. Keep fescue turf (which inhibits the growth of other plants) away from the base of the tree. Keep lawn equipment away from the base of the tree.
— Jennifer Smith works in regulatory horticulture and has worked as a horticulturist for various government entities. She has experience in landscape design and maintenance and as an educator.