I've grown catnip for years in a 6' x 15' raised bed. At the beginning of this season, I added equal parts of composted manure and topsoil to raise the level of the bed which had settled around 3-4". The plants almost immediately started to turn red, both the stems and leaves. I tested the pH, and it was 7.0. So after about 2 weeks, I pulled all the plants and tilled the soil really well. Tested the pH again and it was 7.1. Replanted the plants and added a few new ones. The new plants started to turn red, slightly, but not too bad. The other plants struggled to recover. All in all the plants had stunted growth, and did not do well. My questions are what is causing the plants to turn red, and what can I do to the bed before next spring to rectify the problem? Any advice will be appreciated. I have a lot of little felines counting on me.
Soil Additions Caused Catnip to Turn Red
Certified GKH Gardening Expert
Yes, this is probably an issue due to excessive nutrient levels or other byproducts in the manure. Adding even composted manure in a very high amount could potentially burn plants. It is also possible that the manure you purchased was not 100% composted yet.
To fix the problem, you could get a soil test to check what is really going on with the nutrient levels. You could try digging up the bed and mixing in the top portion of the soil with the deeper soil to dilute it, and/or adding extra soil on top. Another idea is growing a "heavy feeder", a plant that appreciates high nutrient levels, in a late summer/fall garden to soak up some of the extra nutrients.
See page 3 of this article for a list of heavy feeders: