I just got my favorite gardening magazine and a reader asked the question, “The leaves of my roses are yellowing but the veins are green. What causes this?” The given answer is, “This is called chlorosis and is caused by a deficiency of iron”. The answer might be true, or it might not be true. It is certainly an incorrect statement. Find out the truth about chlorosis.
Chlorosis – What Is It?
According to all the dictionaries I could find including ones dedicated to botany, chlorosis is a yellowing of leaves due to a lower than normal amount of chlorophyll.
It is incorrect to define chlorosis as a yellowing of the leaf but not the veins. When the veins stay green the proper term is interveinal chlorosis. This might seem like a trivial matter but the difference can be important if you are using this as a diagnostic tool.
Chlorosis – Is it an Iron Deficiency?
No! An iron deficiency can cause interveinal chlorosis but it does not cause chlorosis.
What causes Interveinal Chlorosis?
A lack of iron in the soil can cause interveinal chlorosis but so will a number of other soil issues. Just because you have a plant with inverveinal chlorosis does not mean you have an iron deficiency. Each of the following conditions can produce the same symptoms.
- manganese deficiency
- a high soil pH
- zinc deficiency
- herbicide damage
- wet soil conditions
- compacted soil
- trunk-girdling roots
- plant competition
- high organic content in soil
- high salts
- high levels of phosphorus, copper, zinc or manganese
Some plants are more sensitive to iron deficiency at higher pH than others. Many acid loving plants have trouble getting enough iron from the soil at higher pH and therefore show interveinal chlorosis while their neighbors don’t.
It should be obvious from this list that adding iron to the soil may not solve the problem. But what if iron is deficient–will adding more iron solve the problem? Not necessarily. If your soil has a high pH, or high levels of some of the other nutrients mentioned above, then the iron you add will be bound up in the soil so tightly that your plants can’t use it. In these situations, adding iron will not help.
Will a Foliar Spray Work?
Spraying an iron solution on the leaves may make them green up in the short term. But since you have not fixed the underlying problem, interveinal chlorosis will return in a few weeks and any new leaves will immediately show interveinal chlorosis.
The second problem with foliar sprays is that there is a small difference between concentrations that work and ones that damage the leaves. It is just too difficult for home owners to apply the right amount.
Chlorosis – What Causes it?
The only thing that causes chlorosis is a deficiency of nitrogen.
The Cure for Chlorosis and Interveinal Chlorosis
If the symptoms really are chlorosis, try adding nitrogen to the soil. It should solve the problem.
If the problem is interveinal chlorosis, then the solution is more complex. If all plants seem to be affected, get a soil sample tested and follow the recommendations from the lab. It is unlikely you will solve the problem on your own, unless you are watering too much, or adding far too much organic matter.
If only a few plants are affected, do some research on the plants and see if they like acidic soil. If they do, you probably can’t grow these plants. Don’t fight mother nature–grow something that does well in your soil.
If you are like me, you won’t take this advice and you’ll try to grow the plant anyway. You can try adding iron sulfate around the plant. This will add iron, in case you do have a deficiency. It will also add sulfur which might help lower your soil pH. You can also try just agricultural sulfur which will lower the pH. When the pH goes down, plants have an easier time getting at the existing iron. In a future post I will have a closer look at acidifying soil–but it is more complex than most references suggest. It really is much better to just grow a different kind of plant.
1) Photo Source: Scot Nelson