Monitor water, soil to keep mandevilla healthy
Q: I bought a mandevilla vine last spring, and I have brought it into my southeast-facing apartment, where it is doing well. But some of the leaves seem to develop spots, and then after a day or so they turn yellow and fall off. I can’t see anything on the leaves, but when I rub the top of the leaves my fingers get a little bit sticky. What could the problem be? Also, last winter I kept an alstromeria plant indoors, and it bloomed beautifully all summer. It seems to be doing well this winter as well. As this will be the plant’s third year, should I change the soil? Will any type of potting soil work? I have just fertilized it during the growing season. Also, will the soil surrounding the mandevilla vine need to be changed too, or should I just leave it and fertilize it during growing season? A: Mandevilla leaves typically turn yellow and drop off near the base of the plant. This is quite common with many vines, which lose their older leaves as they grow. However, if your plant is losing leaves in other areas then there is cause for concern. Too much water can cause a problem, especially if it is not draining quickly. Check to make sure the roots are not sitting in soggy soil. If they are, you might want to look at a pot that offers better drainage. When you water, do so in the sink, allowing the water to run out of the pot.
A lack of water can also cause problems. Mandevilla likes to have a moist soil, but never wet. Use your finger to check. Insert the finger up to the first knuckle. If the soil feels dry it is time to water. Or else insert a plain wooden chopstick into the soil, avoiding the roots. Leave it in place for 10 minutes and then examine it. If it has changed colour or has a watermark, the soil is moist. If it has only slightly changed colour you will need to check the soil again in a few days.
The spots you are describing may be a fungal infection called leaf spot. You can treat leaf spot with a garden sulphur spray or a garden fungicide such as Serenade. Mealybugs or whitefly can cause the stickiness that you are describing, but since you examined the leaves closely I think we can rule those out.
As for the alstromeria and mandevilla soil, I would change the soil for both plants. You should also consider moving them into to a bigger pot. They do not require any special potting mix, just a good quality one. One tip I would offer is to moisten the potting mix before using it. Place a layer of the moistened soil in the bottom of the new pot, place the rootball of the plant into the new pot, and then add more moistened potting mix around the sides. This method keeps the transplant shock to a minimum. Gerald Filipski is a member of the Garden Writers Association of America. He is the author of Just Ask Jerry. E-mail your questions to filipskigerald@gmail.com. To read previous columns, go to edmontonjournal.com/filipski