Mandevilla vines are vibrant bloomers and popular in summer gardens. It’s also best to grow these vines in containers so that you can overwinter them indoors.
They won’t bloom indoors but they will give you a lush trailing foliage plant with plenty of greenery to brighten any indoor space.
Unless, that is, you see the leaves on your mandevilla turning yellow, in which case, take it as an early sign that your plant needs some extra care and attention.
3 Common Causes for Mandevilla Leaves Yellowing
1 – Water-logged Soil
As mandevilla plants are typically grown in containers, the soil is going to need to be changed periodically. If you’ve had your plant in the same soil for a year or two, chances are, the soil’s past its prime.
The soil you use needs to be well-draining to prevent the soil from water-logging, which will lead to root rot. Yellowing leaves on any plant is a sign of the roots are being distressed.
The main concern with water-logged soil is that it starves the roots of oxygen. To ensure the soil continues to offer your plant good drainage, renew it every two years at most and if it begins struggling after a year, try repotting it in fresh soil.
A good potting mix to use should include plenty of organic matter, peat moss, and you can add some sand to improve aeration. Adding a layer of pebbles or gravel will help to increase both humidity levels and boost drainage too.
2 – Lacking one or several key nutrients
Another soil related problem that yellowing mandevilla leaves can be a sign of is that the soil is lacking in key nutrients. When leaves are yellowing on any plant, the specific problem you need to address is chlorosis, which sets in when one of four nutrients are lacking.
The four main nutrients required for chlorophyll production are:
Iron deficiencies are more common with mandevilla plants than the other nutrients. If this is the case, the younger leaves near the base of a mandevilla vine will yellow first.
The soil pH for mandevilla ought to be under 7. Any higher than a 7.0 and the plant will struggle to absorb iron from the soil. The most recommended type of soil amendment for increasing soil pH is Canadian Sphagnum Peat (pH 3.0 to 4.5).
Every other type of Sphagnum Peat is pH neutral so unless you’re using Canadian Sphagnum Peat (or another acidic soil amendment), it’s unlikely to increase your soil’s acidity.
Adding just 1 to 2 inches of Canadian Sphagnum Peat to your topsoil will acidify it enough to allow it to absorb more iron and other essential nutrients.
3 – Insufficient Air Circulation
Air circulation is often only considered when growing plants indoors. It’s just as important with garden plants too because without sufficient air flow, water will evaporate slower, and it’ll prevent condensation building up on the leaf surface, which can lend a hand at preventing fungal infections like powdery mildew.
Maintaining a healthy growing soil helps prevent soil compaction, thus ensuring there is sufficient air flow to the roots of the plant. Up top, the plant needs to be pruned to ensure oxygen can freely flow to every part of the plant.
Mandevilla plants should be pruned periodically and drastically at that. Especially if you’re over-wintering your mandevilla indoors. The more vigorous you prune it, the better.
You can cut mandevilla plants back to just 12” and over-winter it anywhere with temperatures above 50oF (10oC). Below 50oF can kill it, which is why these should be potted and over-wintered indoors.
Before you reach for fertilizers or making soil amendments, check for pest problems first, because insects can suck enough sap from plants to cause the leaves to yellow, mimicking what would otherwise be a nutrient issue.
3 Pest Problems that Cause Yellowing Leaves on Mandevilla Plants
1 – Scales and Mealybugs
Both of these pests are crawlers and entomologists regard them the same because the life cycles are so similar. So are their feeding habits.
Both mealybugs and hard and soft-scale insects suck the sap from plant leaves and stems. They don’t quite suck them dry (unless they’re on the plant for years) but what they mostly do is infuriate gardeners by masking what looks to be a fertilization problem.
The yellowing of leaves can be caused by nutrients being drained by scale insects and mealybugs rather than the soil amendments you could be making if you don’t inspect for these pests.
What do they look like?
Bumps! That’s it. They’re only a fraction of an inch in size and the scale insects are a tan brown color, or in the case of mealybugs, they have a fuzzy white cotton-like appearance.
2 – Whiteflies
Whiteflies are a nuisance pest to control because you can’t see them until there’s hundreds. They are most active when 1) temperatures are higher, and 2) when there’s a reduction in natural predators outdoors such as the green lacewing and ladybugs, which help control the population of whiteflies in the garden.
Bring them indoors and they can quickly multiply when there’s fewer to no natural predators to control them.
Whiteflies are only partially white when they’re in their adult stage. The eggs they lay on the underside of plant leaves behave the same way as scale insects.
They crawl and suck the sap from the leaves and stems and they excrete the same honeydew that can lead to black sooty mold developing on leaves.
The best course of action to control whiteflies is to use a repeated application of neem oil as that’s safe to use indoors and outdoors and it won’t have a negative impact on the population of beneficial insects.
Insecticides and pesticides can effect all types of insects – beneficial or not.
See #2 of 10 Foolproof Ways to Get Rid of Bugs on Indoor Plants information for instructions on using neem oil applications to get rid of whiteflies on mandevilla vines.
For the best protection, use a repeated application 2 to 3 times at the end of the season before you bring your plant indoors to over-winter the mandevilla plant.
3 – Spider Mites
Spider mites are among the most prolific of breeders that require inspection and instant treatment. These insects reproduce fastest when temperatures rise above 85oF (30oC).
A female spider mite will lay on average 100 to 200 eggs during the short lifespan of just 30-days. That’s thousands of sap-sucking parasites being laid by each female adult spider mite!
The math is 1 adult x 100 (min) eggs, x 30 days and that’s 3,000 eggs at least per adult and they feed at every stage of their life cycle. These could very well be the death of your plant and they can cross over to nearby plants too.
Your biggest problem with these is the fact that you can’t see them without inspecting for them.
Back in 2015, the Michigan State University issued a warning to gardeners to be inspecting mandevilla plants for spider mites when they noticed a growing trend over a few years of Mandevilla and Bougainvillea plants being grown in southern states and shipped to other regions with spider mites on the underside of the leaves.
Bottom line, if you don’t periodically inspect for this pest, chances are, if you do get an infestation, it’ll be more than a pesky few.
Worse still is they just look like tiny black dots. You need to look closer to see if those tiny black dots have legs.
A good way to inspect plants for spider mites is to use a piece of plain white paper or card, tap it against the plants leaves and if you do see black dots drop, either look closer, preferably with a magnifying glass to see if they have legs, or leave them on the paper or card to see if they move.
Often, they can sit still and you won’t notice they are insects until there’s a colony of mites infesting your plant.
Check for any pest problems every year before you bring your mandevilla indoors for the winter, and every time you see more than a few leaves on a mandevilla turning yellow. In particular, yellowing leaves near the base of the plant where the leaves are younger and should be healthier.