Knockout rose tree, in large planter, has leaves turning yellow. Help!

Question

I water regularly Nd pot has good drainage. I don’t think I’m overwatering, as it’s been hot, and it is in a pot. What else causes yellow leaves? Any treatment ideas? It is looking worse, by the week…

Answer 1

Oh, a picture is worth a thousand words! I’ll start by assuming this is not blackspot fungal disease. If you aren’t familiar with blackspot, read up on it as it is a common cause for black spots followed by yellowing leaves. Make sure the top 1-2 inches of soil are dry before watering and then water enough so a bit of water comes out the drainage holes. You want the soil moist, not wet. The other issue that comes to mind is nutrient deficiency, particularly low nitrogen. Roses are heavy feeders and need a regular schedule of fertilization. Follow instructions on the fertilizer container. If you routinely water so lots of it runs out the drainage holes, you are washing out soil nitrogen. Phosphorus and potash move through soil much, much slower than nitrogen. Lastly, did you use good potting soil? If you planted using yard soil or bagged topsoil, that’s a problem and you should repot using bagged “potting soil”. These are specially formulated for growing in containers. I’m including a link to an article that discusses many rose diseases. https://extension.illinois.edu/roses/disease.cfm