Can someone please explain why my Rhodo & relatives are always chlorotic? I have them planted in various beds under different light & soil conditions. It seems like once a month I have to give them an iron foliar spray despite the regular soil amendments that I make. At least twice a growing season I mix Ironite and greensand into the top few inches of soil. Many folks that I know never treat their shrubs and they are gloriously dark green.
There can be many explanations for this but I would start with a soil test if you have never had one or if it has been more than 5 years since the last one. Alkaline soil ties up certain minerals that plants need and this causes iron chlorosis once the plant cannot absorb the iron that it needs. Calcifuge plants originated from places where the soil is acidic. If you are familiar with the pH Scale, rhododendrons and azaleas do best in soils whose pH ranges from 4.5 to 6 but will tolerate less acidity up to a point. At a reading of 7, the soil is considered neutral and at higher numerical readings, the soil is considered alkaline. This pH can vary widely in a close geographical area. About 15 or so miles from my house, the type of soil changes and is even more alkaline. In the opposite direction, it is more sandy so go figure.
When planting rhododendrons in alkaline soil, compost can help counteract the tendency to have iron chlorosis attacks but it will be less of a problem if you grow them in raised beds filled with acidic soil. That is because of a tendency of the soil to return back to its normal pH some time after you have ammended it. I have lightly alkaline soil and to counteract it, I always add some garden sulphur, iron sulphate or an iron chelated liquid amendment in Spring. I water, apply about 1 tablespoon (under the mulch) of sulphur per plant and water again. Sometime in the Fall, I have a few plants (hydrangeas) that will require another application. In more alkaline places, this needs to be done twice or three times a year. Certain things can also make this a more common problem too. For example, cement foundations -especially new ones- tend to leach lime which makes the soil alkaline and rhodies planted next to the house then get iron chlorosis. Rhodies will tolerate this problem but you may discover some rhodies that tolerate this more than others too. Places where it rains a lot tend to suffer from this as well.
A soil test will give you an idea of where you stand, ph-wise, now. Then purchase a soil pH kit that shows you the soil pH in numerical increments so you can amend the soil, see what effect it has and then continue applying these amendments forever. Remember that the soil pH will go back to normal if you stop amending the soil.
City water can also be counteracting all the amendments. The water we drink can be very alkaline so each time you water the plants, you would be undoing the effect of these amendments. To prevent this, some people using water from rain barrels. So test your water to see how alkaline it is. Some local city water departments have information on the Internet that indicates the pH of the local water supply but I do not know in PA.
This message was edited Oct 3, 2009 5:24 PM
Thanks, Luis. I haven't tested the soil in a while. I do use compost and mushroom soil as a topper on my beds every fall. I will add some sulphur. Several of my Hydrangea are also chlorotic and yes, we have VERY wet springs here. I also feed these with Hollytone and occasionally with Osmocote for acid lovers.
It's odd, I don't have this problem with the Camellias, Skimmias or Mahonia. The Laurels, Rhodos, Azaleas, Hydrangeas and some Daphnes are repeatedly chlorotic.
Some plants of the same species, rhododendrons for example, may tolerate alkalinity more than others. I too have azaleas, camellias and hydrangeas and here some comments about those. The problem occurs most often for me with hydrangeas, specifically some unnamed ones that the previous owner planted by the front door facing north. Other hydrangeas that I planted have not shown as much sensitivity. But when any one hydrangea shows signs of chlorosis, I amend the soil for all of them at the same time. I had chlorosis once with camellias; once within the last four years. The affected camellias are located quite close, 1-2 ft way from one set of hydrangeas. I cannot remember if the azaleas had this problem before. One bunch is close to the hydrangeas and the other bunch is away from all others. I do not fertilize them; just provide 3-4" of mulch and some soil pH amendments in Spring (garden sulphur).
One of the things that I've intended to try this year is drenching all of the shrubs with a Mycorrhiza bath. I have the product and have been using it all summer, but have not gotten to all of the shrubs. It is supposed to help the plants take up and use the available nutrients. I will try to get around to it this week.
I too have noticed that it is only certain Hydrangeas and Azaleas that have this problem. But all of my Laurels, whether near the house or out in the beds have it.
This property was neglected for 25 years before I moved here 4 years ago. My fertilization and soil amendments are an attempt to bring the soil back to life. It took almost 2 years before I started seeing worms.
My niece just saw some worms a couple of days ago and she started screaming SNAKE! SNAKE! It was -oh- a barely 6-inch long worm but try explaining it that when she is running away yelling. Oh well......
I wondered as I typed my last post if some of those dry and hard pan or sandy soils in Texas actually have many worms!!!