I was reminded to remind you all that the reason for not using oil on blue spruce is because the oilremoves the wax that makes the needles blue. New growth will be blue again but the sprayed needles will stay green. This is also true of other blue species like Concolor fir and blue junipers.
As I look through some articles, I am also reminded that there are several terms that can be confused –
Horticultural oils is the overall term. Dormant oils are used when the trees are dormant (makes sense) and may be less refined. ‘Summer’ oils are more refined (92% or higher unsulfonated residues – listed with active ingredients). For example, I just looked up Suffoil X – which has a minimum of 92% unsulfonated residues. You can use some oils for either dormant or summer use. Check your Tree and Shrub Guidelines!
*** This is not the same as the % active ingredient!
Other species can also be damaged by horticultural oils – check labels and do a test application if you are new to using them.
An article from Michigan State on using dormant oils on Christmas trees
And more from Connecticut
We are always looking for efficacy data – this is from Jill Sidebottom in North Carolina. (I’m translating acronyms so I hope I am correct – Jill’s text in italics below, my comments not).
A 2% solution of oil applied in mid-March (remember – this is for North Carolina) will do a good job of controlling the following pests:
- All stages of HRM (hemlock rust mite- I haven’t heard of this in NYS) — actually oil works as well as expensive miticides when controlling rust mites
- BTA (balsam twig aphid) eggs
- SSM (spruce spider mite) eggs
- BWA (balsam woolly adelgid) crawlers, nymphs, and adults though the adults are covered with white wool and are therefore harder to wet.
- Cinara aphids
Control of the following pests is not as good. A 2% solution of oil will only suppress the following:
- BWA eggs – however, there shouldn’t be any BWA eggs present in mid-March
- BTA nymphs or adults. Therefore be sure to treat before the twig aphid eggs start to hatch
- EHS (elongate hemlock scale) — this is really too early in the year to get good EHS control. You can treat for this pest in the summer with oil, but problems with burning foliage increase in hot weather.
Be sure to keep scouting through the spring to make sure treatments have worked.
Thanks to readers with sharp eyes who let me know when I need to update my updates!