Why Are the Leaves on My Morning Glory Turning Yellow?
Morning glories have long been a staple among climbing vines, providing a green blanket across a trellis and sumptuous color when the sweet flowers open in the morning. But morning glories (Ipomoea spp.; USDA zones 10 to 12) with yellow leaves are in trouble, because that means the plant is stressed.
Morning Glories Need Sunlight
Morning glories must have full sun, which in the garden translates to six hours of sun daily. If the location is too shady, the vines won't be able to photosynthesize properly and their leaves may yellow.
Try pruning any overhanging branches that might be shading your flowers. If that's not possible, you may need to transplant your vine to a different, sunnier location.
...But Not Too Much Sun
While full sun is ideal, morning glories planted directly against southern or western walls or fences might develop sunscald. Leaf scorching usually causes yellowing in the margins or between the veins of the leaves.
Water Plays a Part
Interestingly, sunscorch can be exacerbated by the level of water they are getting, whether it is too little or too much.
If the soil is very dry due to afternoon sun, they may be unable to take up enough water to remain healthy. Conversely, if the soil is not draining well and you are overwatering, their roots may be unable to develop properly.
Monitor the soil around your plant to allow it to dry out sufficiently before irrigation or to make sure it does not dry out too much.
Morning Glories Need Warmth
Morning glories are tender annuals. If it's nearly fall or winter and your morning glories develop yellow leaves, they are simply responding to the colder temps.
Alternatively, if you have planted them too early in the spring and your area experiences a late frost, they may respond by wilting and developing yellow leaves.
Note that some morning glories are perennials, including Ipomoea aquatica and Ipomoea hederacea. These are considered invasive in many states, so avoid planting them.
Morning Glories and Fertilizer
You should avoid fertilizing your morning glory vines, unless you have very poor soil and you've identified specific nutrient problems. Overfertilizing can result in a yellowing of leaves.
However, if you know your soil is nutrient-deficient, your vine may be suffering from a nitrogen deficiency, which would cause a chlorosis of the leaves. It may also not be getting enough potassium or magnesium.
If you suspect that soil nutrients are the culprit, get a soil test, which is available from most nurseries or from your local extension office.
Morning Glories and Fungal Diseases
Some fungal diseases can target morning glories. If your morning glory leaves have yellow spots, they may have rust. Check the undersides of the leaves for yellow-orangish powdery pustules, in which case, rust is likely. Your plant might also develop Fusarium wilt, which can cause yellow leaves, stunting or wilting.
To prevent these kinds of diseases, never water from overhead, which is true for most plants. In addition, remove any infected leaves and dispose of them in the trash rather than composting them. If the entire plant appears infected, dig it up and dispose of it.
I garden in the Pacific North west, previously Hawaii where I had an avocado orchard. I have a Master Gardeners certificate here in Eugene, Oregon.