|Less than 10% of our 9 month old Vincas have a FEW yellowing or nutrient deficient appearing runners. Most of the plants look vibrant but my wife is pannicking when she sees the yellow. Should she be concerned? I have used triple 15 one time.|
|Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. (Lack of nitrogen is a more common problem than lack of iron.) Soil should be kept moderately moist (but not wet). Finally, transplant shock can contribute to yellowing.
Having said all of that, I wouldn't be concerned about such a minor amount of yellowing, especially in the midst of summer heat. Monitor it and if that's how it stays, I'd say you're doing a good job with your vinca! If it gets worse, consider the above info, one at a time to isolate the issue.