Leaves turning yellow a sign of complex problem

Why are the leaves on my plants all turning yellow? While this is a frequent question every year, it seems to be more common this year. While I am not certain of the exact reason plants are showing more problems, the problems are definitely affecting more plants.

The word chlorosis means lack of chlorophyll, which means that the plants do not have their normal green color. Without the important green color, plants cannot manufacture their needed food.

There are many kinds of chlorosis. Lack of nitrogen, zinc, manganese, sulfur or other elements prevents leaves from making chlorophyll, so they have a chlorotic symptom. However, the nemesis for most of our plants in Utah is iron chlorosis. It is the most troublesome micronutrient problem in Utah.

Plants with iron chlorosis have distinctive symptoms. The leaves turn light green, yellow or even white if the problem is severe. Typically, the leaf veins remain dark green.

Our soils in Utah make correcting chlorosis problems difficult. These desert soils are mostly formed from weathered limestone that are very high in calcium carbonate. There is not a lack of iron in our soil, but the iron that is in the soil is unavailable to the plants because it is tied up with the limestone.

High lime content also makes our soils alkaline. Most soils in Utah have a pH between 7.2 and 8.3. The high pH makes it more difficult to correct iron chlorosis problems.

Controlling chlorosis is difficult. The easiest way is to choose iron-efficient plants. Plants that prefer acidic soils are highly susceptible to iron chlorosis. Azaleas, rhododendrons and blueberries thrive in acidic soils, but struggle and die here.

Trees that are highly susceptible to iron chlorosis include silver maple, red maple, sugar maple and Amur maple. Add to that list many birches, as well as pin oak, dawn redwood and sweetgum. Bald cypress, crabapple, white pine, cottonwood, aspen, Bradford pear, mountain ash, horse chestnut and many other species are also susceptible to iron chlorosis if they are grown in high pH or heavy clay soils.

Many shrubs also develop iron chlorosis. These include boxwood, cotoneaster, flowering dogwood, hydrangea, privet, pyracantha and spirea.

Roses also often show severe iron chlorosis. Specific cultivars vary in their susceptibility to the problem, so check local rose gardens to see what kinds are not as susceptible.

Unfortunately, raspberries, strawberries, peaches and Concord grapes are all prone to show iron chlorosis. In many cases, it is difficult to get these to grow well.

If you have established plants or must plant susceptible kinds, look for other solutions. Excessive moisture or soil salinity aggravate iron chlorosis. Too much phosphorus, copper, manganese or zinc can tie up the iron and make it unavailable.

Plants also have a hard time absorbing iron when the soil is wet and cold or too dry. Keep the soil evenly moist but never waterlogged. Improving soil drainage is one way to reduce chlorosis.

In some cases, it is necessary to add iron to the soil. Before you start burying old car parts and nails, remember that the iron must be available to the plants. That usually means adding chelated iron.

The product that works best with a soil pH of 7.4 to 8.4 contains the chelating agent EDDHA, (sodium ferric ethylenediamine di-(o-hydroxyphenyl acetate). Compounds that contain EDDHA include Sequestrene Sprint, Miller's Ferriplus Iron and Grow More EDDHA.

Sequestrene 330 is a less expensive iron chelate. It works best when the soil pH is below 7.4. Chelating compounds also come from tree bark or other organic sources or synthetic organic molecules. These are sold under several brand names and formulated as dry or liquid products. Baicor LC, based in Logan, makes products to help correct chlorosis problems.