Lack of Iron
Plants must have iron. If they don't, their newest leaves turn yellow. It's not totally correct to say it's a lack of iron. There is plenty of iron in the soil, it's just not available to the plant. The plant can't use it because of some chemistry problems involving the soil. The pH is too high or its alkaline. This type of soil chemistry turns the iron in the soil into something that the plant can't use.
|Iron chlorosis when it's mild is on the new leaves and the veins of the leaves stay darker green than the blade. If this is early in the season, the future leaves may turn bright yellow with no green veins. Here it is on Apple.|
Because of iron is going to be a big problem that season, the yellowing starts out early in the season and on progressively gets worse and worse. The first thing you see are leaves with a light green color with darker veins. As the season progresses the green veins may disappear and the leaf is just plain yellow. As it gets hot, many plants can't handle the heat anymore because they are not as healthy and the edges begin to scorch, turn brown.
Some plants have leaves that react differently when iron is a problem. But the yellowing is always on the newest growth.
|Iron chlorosis looks different on bottlebrush but the yellow leaves are the newest leaves at the tips of the branches.|
Control yellow leaves due to iron. If you know you have a plant that has this tendency every year, apply an iron chelate spread beneath it with lots of water to get it out of the sunlight and into the soil before new growth begins in the spring.Several chelates are available but I usually push people toward the chelate abbreviated as EDDHA. This is because this chelate works in all types of soil whereas other chelates may or may not.
The second method of applying iron is to the leaves as a liquid spray. This is done later in the season when the leaves are out and applying an iron chelate to the soil may no longer work effectively. Watch my video on applying iron to the soil or apply an iron spray to the leaves. I don't have a video on doing the ion spray yet but hope to do it soon and post it on my channel.
When making the iron spray concoction I would use distilled water, the iron fertilizer mixed in the water with about a tablespoon of liquid detergent mixed with it afterwards. Spray the leaves until this mixture begins dripping from the leaves.This spray concoction may need to be applied three or four times a few days apart to get it dark green. Another option you have is to do the soil application and the foliar spray at the same time. We have done that in the Ahern Orchard and had good luck with it in late spring or May.
Salts in the soil can also cause leaves to yellow but this is often times accompanied by damage to the edges of the leaves. The veins can appear green just like it was described above but the edges of the leaves are the telltale sign that salt in the soil may be the problem.
|This is yellowing of the leaves of a pepper plant because the of salts in the compost mixed in the soil and the heat of the day. Edges of the leaves scorching was the telltale sign it was salt damage.|
Fixing salt damage. There isn't anything that you can do to the existing leaves that show the yellowing and browning on the edges when salt is causing the damage. But the new growth will show improvement just threw better irrigation management. Flush the soil with lots of water to remove the high level of salts. The soil must drain the water for this to work. If the soil doesn't drain, then drainage is the problem combined with the salts. To fix that, you have to make the soil drain. A different topic.
Intense Sunlight and Yellowing
Some plants don't belong in strong sunlight.Know which plants can handle intense desert sunlight and which ones can't. Just because a blog or a YouTube video says you can plant it in full sun doesn't mean you can plant it in full sun in the desert. It depends on the plant. This is why Desert Horticulture can be so different from "normal" horticulture.
|This cycad or Sago Palm has yellow leaves because it's in full sun near a hot wall and surrounded by hot rock mulch. This plant will perform much better in a cooler location, woodchip mulch with some filtered light.|
|Carolina cherry laurel is another plant that can show yellowing, similar to the cycad, if it's planted in intense sunlight and surrounded by rock mulch.|
Know the plants going into your landscape and plant them in locations where they won't struggle. Just because some book or blog post tells you he can grow in full sun, that might be true in North Carolina but not in the Mojave Desert. That's why it's called Carolina cherry laurel.
Sago Palm is native to southern Japan. Do you know any desert in southern Japan? Be careful of plants with the name "Japanese" in it such as Japanese blueberry. It's also not a good one to plant in full sun surrounded by rock for the same reason.
Nitrogen is a plant nutrient supplied by many different types of fertilizers, animal manure and compost needed in large amounts by plants. It helps plants develop dark green leaves and lots of stem growth. In fact, the amount of nitrogen that plants receive is directly related to how green the leaves are and how much growth it produces.
A lack of nitrogen doesn't happen very often but mixing woodchips or sawdust in a soil or other things that decompose and are brown can cause a temporary lack of nitrogen in plants. The soil microorganisms responsible for tearing down these wood fibers require lots of nitrogen. If wood chips or sawdust is mixed in the soil these microorganisms will steal nitrogen away from plants so they can break down wood.
Just the opposite of iron, a temporary shortage of nitrogen in the soil will also cause yellow leaves but these will be the oldest leaves, not the youngest.
|The yellow leaves of this corn plant are the oldest leaves at the bottom, typical of yellow leaves due to a nitrogen deficiency.|
Wet soils, either from watering too often or poor drainage, can cause yellow leaves in plants because roots begin to drown or suffocate. Suffocating roots as plants are trying to get bigger causes the plants to have fewer roots and decreases their chance to take up iron and manganese from the soil. This is sometimes called "water induced iron chlorosis".
Plant roots must have a supply of water as well as a supply of air. Plants are different in this regard. Lawns and palm trees can survive in soils that have lower amounts of air in them. Other plants like rosemary, Italian Cypress, bottle trees, and many of our desert plants need soils that drain easily and allow air to enter the soil. There are differences among plants and which can handle soils that have water in them and which ones can't.
Solving the wet soil problem. Amend the soil at the time of planting so that air can enter the soil as well as water. This helps water to drain. Know which plants can tolerate rock applied to the surface of the soil around them and which ones are better off with woodchips. Rock eventually causes the soil to stop draining. Woodchips help to keep the soil open. For instance, never put rock around roses, Photinia, mock orange, southern magnolia and other plants that come from places that have better soil than ours.
Put plants that have a similar need for wet soils and water on the same irrigation valve. Put desert plants together on another valve. Put non-desert plants together on one valve but don't mix them together with desert plants on the same valve.
Cold temperatures will sometimes cause "bronzing" or yellowing of plant leaves. We see this a lot in Palm trees, Sago Palm, evergreens like citrus and even
|Sometimes cold temperatures can cause leaves to yellow.|
|Cold weather can cause bronzing are yellowing of Mesquite leaves. If it gets really cold, it can cause the leaves to die and drop even though people tell you it's Evergreen.|
What can be done? Nothing. Wait for the new growth and new leaves to replace the damaged leaves from winter temperatures.