Troubleshooting
Rocky Mountain Douglas Fir problems — and how to fix them
Rocky Mountain Douglas Fir (Pseudotsuga menziesii var. glauca) is generally forgiving once you match its basics, but a few issues come up again and again. Here is what each one looks like, why it happens, and the fix.
Douglas Fir woolly aphid (Adelges cooleyi)
White woolly masses appear on new shoots in spring, causing needle distortion and premature drop. Infestations are usually more aesthetic than fatal on established trees. Apply horticultural oil or systemic insecticide in early spring at bud swell.
Swiss needle cast (Phaeocryptopus gaeumannii)
A fungal disease causing yellowing and premature loss of second-year needles, leaving only current-year growth. More prevalent in humid conditions. Ensure good air circulation; avoid dense spacing. Copper-based fungicides can help on high-value specimens.
Root rot in wet soils
Var. glauca is particularly intolerant of prolonged soil saturation, which leads to Phytophthora root rot and rapid decline. Always plant on a well-drained site; do not irrigate excessively once established. Raised planting on berms can help in marginal drainage situations.
Prevent rocky mountain douglas fir problems before they start
Most rocky mountain douglas fir issues are care-mismatch, not bad luck. Dial in the basics:
Rocky Mountain Douglas Fir problems — FAQ
Why is my rocky mountain douglas fir douglas fir woolly aphid (adelges cooleyi)?
White woolly masses appear on new shoots in spring, causing needle distortion and premature drop. Infestations are usually more aesthetic than fatal on established trees. Apply horticultural oil or systemic insecticide in early spring at bud swell.
Why is my rocky mountain douglas fir swiss needle cast (phaeocryptopus gaeumannii)?
A fungal disease causing yellowing and premature loss of second-year needles, leaving only current-year growth. More prevalent in humid conditions. Ensure good air circulation; avoid dense spacing. Copper-based fungicides can help on high-value specimens.
Why is my rocky mountain douglas fir root rot in wet soils?
Var. glauca is particularly intolerant of prolonged soil saturation, which leads to Phytophthora root rot and rapid decline. Always plant on a well-drained site; do not irrigate excessively once established. Raised planting on berms can help in marginal drainage situations.