Needled evergreens, and in particular Colorado Blue Spruce, can have sensitive new growth severely damaged and even killed from phenoxy-type herbicides (2-4D etc.) applied to lawns containing these conifers during the new growth period. Do not apply these herbicides around conifers from budbreak (usually occurs in mid-May) to about July 4th. By this date, new growth “hardens off” and becomes very resistant to injury.
When applied during the susceptible “free growth” phase, phenoxy herbicides can be absorbed through shallow roots and foliage exposed to herbicide over spray or volatilization of vapors during warm days.
Some twisting or distortion of new growth may be observed, but more commonly the needles turn purple and drop from the tree.
Watering and fertilization are remedial strategies for damaged trees, but the fertilization should be delayed a year because damage can be exasperated in some situations.
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Harold Hoover, ISA Board Certified Master Arborist