Evergreens normally do shed previous years’ needles on a regular basis. Often there can be a “heavy” needle drop on Front Range landscape pines, spruces and firs. The most common causes of excessive needle drop are too-wet and too-dry soils.
Most conifers prefer loose, well-drained soils with plenty of oxygen. When grown in predominantly clay soils, excessive irrigation easily displaces whatever oxygen is available to roots. Too wet soils cause the death of many small roots and root hairs upsetting the shoot-to-root ratio. An affected pine may shed needles that it can no longer support.
Many parts of the Front Range can receive significantly less than average rainfall. Even soils receiving irrigation to maintain turf grass may not have recharged deep moisture. Too dry soils can cause reduced conifer vigor and excessive needle drop.
Landscape managers should water around drip lines during the winter months, using soil needles or sprinklers to recharge soil moisture. Monitor winter weather and snow cover, and water affected conifers during winter dry spells.
For more information, see the following Planttalk Colorado™ video(s).