Hello. I would appreciate some advice. I live in Hamilton, Ontario. My Saucer Magnolia tree has scale and I’ve been reading lots online and had an arborist out for a look. He suggests two applications of insecticide, then dormant oil spray in the fall. That seems to make sense to me based on what I’ve read. However he’s also recommended a deep root fertilizer to support the tree because it’s currently under stress. While I understand that in theory, I’ve been reading that fertilizing actually increases the scale problem. I’m thinking of doing everything except the deep root treatment. Do you agree that I should not do the fertilization? Also, if you do advise the deep root treatment – how late in the year can I do it before winter for it to help. I greatly appreciate your time and expertise! My tree has sentimental value and I want it to survive.
Thank you for contacting Toronto Master Gardeners with your question.
Magnolia scale is a nasty problem to deal with. Even the treatment programs administered by professional arborists can take a couple of years to eradicate these pests. Still, magnolias are some of the most beautiful spring-blooming trees we have in Southern Ontario, and an older specimen is well worth treating and preserving.
As you note, the programme suggested by the arborist you consulted follows the standard treatment plan with insecticide applications and dormant oil. Fertilization is not part of the generally recommended plan, but may be called for in your particular situation. The resources I consulted (perhaps the same ones you found in your online research) recommend against a nitrogen-heavy fertilizer because it promotes new green growth which is exactly what the scale insects prefer. In general, we do not recommend using nitrogen-heavy fertilizers at this time of year because new green growth will not have time to harden off before winter sets in and will be vulnerable to die-back. However, you have not specified the fertilizer components recommended by the arborist. A “deep root” fertilizer may emphasize the phosphorus or potassium components rather than nitrogen. Perhaps the best idea is to consult the arborist again to obtain more detail on the proposed fertilizer before deciding on the best course of action.
Regardless of the decision you make regarding fertilization, do ensure that you water your tree regularly and deeply throughout the fall so that it is as strong as possible going into winter. Best of luck in restoring your saucer magnolia to vigorous health!
You may have already seen this useful article regarding magnolia scale:
If you have other gardening questions, since you are based in Hamilton, you may also like to try using the online service provided by the Halton Region Master Gardeners. Here is a link to their website:
https://haltonmastergardeners.wordpress.com