Laidback Gardener Tip of the Day

Bye-Bye Blues! Beware of horticultural oils and insecticidal soap

Warning: horticultural oils (dormant oil, summer oil, etc.) and soaps (insecticidal soap, dishwashing soap used as a pesticide, etc.) will melt away the bloom that covers the leaves of many conifers, hostas and other plants: that is, the waxy white powder that gives them their blue coloration.

Blueberries on plant
You may see your blueberries go from blue to deep purple when sprayed with horticultural oils or soaps!Photo: MAKY.OREL, commons.wikimedia.org.

These products have the same effect on the bloom that covers many fruits (blueberries, grapes, plums, etc.). So if you spray your blue spruce or your blue hosta, their leaves will turn green! And you will see your blueberries go from blue to deep purple! The effect is not harmful, but it can be disappointing.

'Blue Arrow' Hosta
Blue hostas like this ‘Blue Arrow’ may also lose their blueish hue. Photo: James Steakley, commons.wikimedia.org.

Fortunately, the effect is temporary. No, the discolored leaves and fruit will never regain their original color, but the new ones that grow back the following year will be as blue as they ever were.

So, you can spray these plants with water, but avoid fats (like soap) and oils.

Translated from French by Mathieu Hodgson

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