Question
I just purchased two pink mandeville plants. They flowered beatifully, but the leafs have developed  brown blight/spots and many of the leaves have turned yellow.
Some have started to fall off.
I have them indoors, near a slider door that allows indirect light.


How should I treat this?

Jeff

Answer
Hi Jeff,

I don't think your Mandevilla sanderi (Dipladenia) has a blight. I suspect lack of light and, possibly, improper watering may be the cause of the leaf discoloration.

Your Mandevilla needs direct sun indoors and should be right in front of your sunniest window.  Keep the soil evenly moist in summer and allow the soil to dry a bit in the winter.

Keep it tightly potted. Repot in the spring but only if extremely potbound.

Best temperature range is 60 to 70 degrees so keep it cool in summer and warm in winter. In winter it benefits from increased humidity. Fertilize in summer only.

Flowers develop on the previous year抯 growth, so it is best to do any pruning immediately after it finishes flowering. Regular pruning inhibits flowering, but it keeps the plant full and compact. Unpruned, it will grow very long; a trellis or wire hoop will help support and contain the long stems. The white sap is a skin and eye irritant. Propagate by using 4 inch cuttings that are left overnight to harden off. Watch carefully for spider mites and mealybugs.

Please let me know if any of this is unclear or if you have any additional questions.

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Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC