I have a small jasmine that was likely the cutting of a branch on another plant. When I got it, it was far taller than I’d like with foliage only on the top. I’ve had it a couple years and reduced the height by half. Though it flowered nicely this past summer with delicate white flowers I’d hesitate to ever call it robust, and now that it’s spring when the plant should flourish, the leaves are loosing their luster. They’re a pale green, almost yellow.
One of the ways jasmine likes to tell you it’s unhappy is with yellowing leaves. If a jasmine has unhealthy leaves it’s likely pests or environmental factors.
First give your tree a careful inspection. Black scale bugs feed on jasmine. Scale bugs are good at disguise looking mostly like black stationary bumps on the trunk, branches, or leaves of the tree without a visible head, legs, or wings. They suck nutrients from the plant and in so doing can cause yellowing leaves. It’s also possible to have had a minor scale problem, cured by insects eating the pests and still have a problem. Black scale bugs leave behind a sticky substance that can provide a home for mold and also yellow leaves. Try first washing the plant off with forceful water spray, followed by treatment with horticultural oil. You might also look for spider mites, which are tiny spiders (not insects) that may look like a bunch of red or white dots on plant leaves. Mealy bugs are another possibility. They’re also small and similar to scale as small oval shaped bugs, which anchor themselves on your tree, sucking nutrients from the plant. But instead of black and smooth bumps these are fuzzy gray/white bugs. You can see them as individual bugs alone or after infestation you can also see their eggs, which make the leaves look almost covered in a fuzzy powder. These can be removed with a toothpick, by hand. Scale and mealy bugs will spread, infesting other trees. For this reason it’s best to treat the infested tree as soon as possible and separate it from other un-infected plants.
If you’ve looked carefully and the tree is pest and disease free, environmental factors are to blame. Jasmine plants like full sun and well drained soil. Like any bonsai watering is a top question – both too much and too little water can yellow jasmine leaves as the roots are either starved or drowned
Jasmine is also prone to chlorosis. That is a fading or yellowing of the leaves that may leave the stem and veins of the leaf a different color than the space between and around them. It’s caused by mineral deficiency. Make sure the tree is in a pot that drains well with holes to allow excess water to pass through, preventing wet feet or the plant drowning by roots sitting in water. Similarly the soil must be well aerated, allowing water to pass through. Soil that is too compact can restrict the roots and prevent them from reaching necessary nutrients and can hold in water too long, risking root rot. In this manner either too much or not enough water can be the culprit. Repotting or fertilization may be necessary. Another possibility is pH imbalance Jasmine prefers acidic to alkaline soil (optimally 4.9 – 8.3), which may require correction.
First examine the tree for pests. If your tree is free of pests and disease check to see that the pot has holes in it, and the soil isn’t too compact or hard. If these steps have all been corrected, try testing a soil test.