Sick knockout roses
Hello again, Up until two years ago, these were really healthy (see photo). So last fall, I had soil testing done. The results were too acidic and low percentage of humic matter. So I replaced ALL of the soil in that bed with organic matter as Selina advised me to do and added lime. I feed them every six weeks during the growing season (12-4-8 recommended by Selina). I prune them correctly every winter. I had to cut back on watering because they were suffering from chlorosis, which fixed that. This spring, they were doing okay. Now they’re just alive. The knockout roses on the side of the house have always been healthy (knocking on wood). This is really frustrating. Thank you.
Harnett County North Carolina
Hello and thank you for your question!
Regarding the organic material, you only need to incorporate 2-3 inches of it into the topsoil, rather than replacing the entire bed with compost. Additionally, adding lime and fertilizer into new planting media for which you do not currently know the nutrient content or pH could cause problems such as excesses or toxicities, which can lead to plant decline. Additionally, over-amending the soil can lead to plant decline as the roots have not developed in native soil, but in soft, fluffy, nutrient-rich soil, and struggle once they encounter native soil. You can read more about this at the link: https://s3.wp.wsu.edu/uploads/sites/403/2015/03/soil-amendments.pdf
It may be helpful to do another soil test or even a soilless media test to identify the current pH and nutrient content. Soil samples are free until Thanksgiving and soilless media samples, which is usually suited for heavily amended soils, are $5 per box. You can always stop by our office to pick up free soil sampling materials. You can also find the forms online here.
- Soil samples: https://www.ncagr.gov/agronomi/uyrst.htm
- Soilless samples: https://www.ncagr.gov/agronomi/uyrmedia.htm
Additionally, it would be helpful to see a physical sample of the shrub. If you are in the area, you can bring in a sample of the leaves and roots to our office. Just pull up and give us a call at 910.893.7530 and someone can meet you outside to pick up the sample. You can also send detailed photos to Selena McKoy at email@example.com. If you are interested in rose diseases, you can check out this article: https://hgic.clemson.edu/factsheet/rose-diseases/
I look forward to hearing back regarding what steps you would like to take next!
Have there been any other signs or symptoms of disease, such as lesions, black spots, gray fuzz, distorted growth, discolored canes, etc?
If you are not sure, you can always bring a clipping of a cane + leaves and flowers (and roots, if you can get to any) to our extension office for me to take a look at. Our office is currently closed to the public, but samples can be dropped off in the wooden cabinet next to the front doors. You can also email detailed photos to firstname.lastname@example.org if you cannot make it.
It sounds like you have amended the soil correctly and something else may be going on. I look forward to your reply!
I haven’t noticed signs of mold or fungus.
It looks like doing soil testing again and submitting tissue samples or detailed photos will be the next step.