Rhododendron Yellowing Redux


Posted by Horticulture Guy - Peter Punzi | Posted in Northwest U.S. Gardeners | Posted on 08-06-2012

Q. I have 2 rhodies that both have leaves that are looking chlorotic with leaves that are yellowing between the veins. I have fertilized like I should, I have bark mulch on them and they still dont have the dark green they should. I did do a soil test with a PH tester and it says its too alkaline but I have no idea what I can do to get the soil more acidic. We have been having above normal rain and have been running 11 degrees below normal temps with temps only getting into the low 50’s.

Can you offer any suggestions on why the rhodies are doing this, is it because the temps are below normal with little sun so far this year? I have read that coffee grounds can be good at increasing soil acidity but how much and how often? Also Im confused on peat moss, some say it doesnt help, others say it does. We ony have access to store bought peat moss, would it help mixing it with compost and applying that?

Thanks in advance for any help. Sandra in Juneau, Alaska

A.  This is a common issue called “interveinal chlorosis” in horticulture parlance (the spell checker doesn’t even recognize those words!). You are on the right track as it is a deficiency of iron and/or manganese (most often iron) and low pH can make these nutrients (called micronutrients as they are needed in smaller quantities than Nitrogen, Phosphorus and Potassium (Potash)).  As I have mentioned in prior posts coffee grounds are great for organic matter but don’t alter the pH of the soil enough to make a difference (the actual coffee you drink would do much more).  So you have two options. The first short term solution is to apply a special type of fertilizer that protects nutrients from the effect of high soil pH (through a process of chelating the minerals) like Miracle Grow’s MirAcid.  This does not change the soil pH but rather delivers the necessary nutrients in a protected formulation.  The long term solution is to drop the pH of the soil using a soil acidifier – which is usually made up of elemental Sulfur.  Follow directions and bring your pH down to pH 5.5.  That should free up iron and manganese if they are present in the soil of course (they usually are since the plant requirements are so low).

Excessive Rainfall can leach out nutrients like iron and low soil temperatures can slow down microbial activity which is essential to the breakdown or organic nutrients.  But I would still get you pH down and then see if this solves your problem.  I would start with the miracid or similar product that has chelated micronutrients to get them greened up quickly and also lower the pH.

Live long & Garden!