Turning plumbago flowers a deeper blue

Question

Does anyone know if Plumbago flowers can be made more blue by adding aluminum to the soil or potassium sulfate in the fertilizer as one would for hydrangeas? I recently planted two Cape Plumbago in containers and noticed that the flowers on one are much darker blue than the other. (I am a garden novice in zone 9 and Plumbago seems to thrive here). Any help with boosting the blue color would be appreciated.

Thanks you, Maxx

Answer 1

Plumbago is not known to change its color in response to pH or to aluminum availability. In fact, aluminum can be harmful to plants that are not adapted to high-aluminum soil.

The difference between the two plants you have may be based on small genetic differences, but environmental factors like too-bright sunlight on the pale flowers could also play a role. Making sure the plants are well nourished with compost or fertilizer and a source of micronutrients won't hurt:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/rock-phosphate-fertilizer.htm