Not all plant deaths are caused by insect or diseases. Many are caused by humans and the environment. These causes are called abiotic and those attributed to insects and diseases are called biotic.
While biotic plant damage is often obvious, diagnosing abiotic damage is usually more difficult. Some common contributors include…
• Planting too deep. Trees and shrubs are often subjected to deep planting. The hole should be no deeper than the root collar (The point at which the trunk/stem joins the root). It’s better if the root collar is above rather than below grade. Planting too deep makes it difficult for water and air to penetrate down to the feeder roots.
• Planting in too small a hole. While the planting hole should be only as deep as the rootball, it should be two or three times bigger in diameter. Spread the roots out and make sure none are crossing each other before backfilling.
• Girdling roots. When one root crosses another, it chokes the root it’s crossing, causing that side of the tree to die back. If there are crossing roots all around a plant, they can kill the whole plant. Crossing, or girdling, roots are most common in plants that come from the nursery in pots. So be sure to spread out the roots and cut the offending root on either side of the root it’s crossing.
• Mulch volcanoes. Piling mulch up against a tree trunk can cause stress, and even death, to a tree. Mulch volcanoes can trap moisture between the mulch and the trunk. Trees don’t like this, and if there’s even the tiniest crack in the bark, that moisture can carry life threatening rot fungi into the tree. Also, rodents like field mice hide in the mulch while they chew on the bark, eventually girdling the trunk.
• Planting the wrong plant in the wrong place. Some plants like full sun while others prefer shade. Some like lots of water while others hate “wet feet.” Some will only flourish in acid soil while others will do just fine in our basic soil. You don’t have to be a plant expert to determine which plants go where. Just read the nursery tags or ask one of the horticulturists at your garden centers.
• Disturbing the roots. Raising or lowering the grade around the root zone of a tree will put it in imminent danger. Try to avoid this practice but if it’s unavoidable, plan to build a retaining wall if the grade must be lowered or a tree well if the grade needs to be raised. Avoid cutting the roots. If you must excavate near tree roots, hire an arborist with an air gun. It’s friendlier than a shovel or backhoe.
• Soil compaction. I’ve seen people park vehicles under shade trees to keep them cool in the summer. This added weight compacts the soil and deprives the roots of water and oxygen.
Often one or more of the abiotic conditions cited above stresses a plant and the decline makes it a good target for insects or disease organisms. When the plant dies, the owner blames it on the biotic cause rather than the abiotic problems that made the plant so attractive to pests or pathogens.
Our Plant Health Care (PHC) professionals can diagnose abiotic as well as biotic causes of plant decline. The prognosis will probably be more positive if you have your plants inspected as soon as you notice signs of stress, rather than taking a wait and see attitude. Left to the plants natural defenses, it’ll get worse before it gets better.