The Many Reasons Plants Turn Yellow

This is one of the most common questions I'm asked. Why is my plant yellow? In Desert Horticulture, the reasons can be many. I'll try to list here as many as I can think of and how to tell them apart.

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.

Yellowing due to a lack of available iron, also called iron chlorosis, can cause severe yellowing in leaves and weaken them so much that they are much more susceptible to heat than they would be if they were healthy. Manganese deficiency will cause the leaves do look very similar to this but it's not as common as iron. Spraying the leaf with a little bit of iron solution and soapy water will tell you if it's iron or not.

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.

This is an iron fertilizer containing the chelate EDDHA. Because of the soil chemistry, other chelates may or may not work in your soil. Using the iron chelate EDDHA works in all types of soils and it's a safe bet when looking for a soil treatment in the spring for correcting yellow leaves due to iron problems.

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

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

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

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.

Purple leaf plum leaves turn pink instead of yellow because they are purple when they are healthy. Purple leaf plum, like all plums, will not tolerate continuously wet soils. The leaves will begin to turn pink (its form of yellow leaves) because roots will begin to suffocate. The tree will not be anchored in the soil but will move easily when it's pushed.

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

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.

Desert Horticulture Podcast: Leaffooted Plant Bug, Weeds, Twisted Myrtle

Join me on this episode of Desert Horticulture. We will discuss the Leaffooted Plant Bug (does it deserve to be capitalized?), controlling some weeds like sandbur and puncturevine, planting horsetail, twisted myrtle problems, and ants climbing inside figs. This and more in this episode of Desert Horticulture.

Your Drip Irrigation System Needs Help

Q. I've been using adjustable  drip emitters and replacing them every year because they plug from the hard water. They sputter air or just don’t emit water when they start a watering cycle.  When I find one like that, I open up the watering rate until a steady flow is achieved, but it doesn’t seem to last.

A. Sounds like several possible problems going on with your drip system. Let’s start with the easy ones first. Make sure your drip irrigation system has a filter installed to prevent drip emitters from plugging and a pressure regulator to prevent high water pressure from damaging the emitters.

A pressure regulator (black and white on the left) and a screen filter canister (right) together inside an irrigation box. Arrows on them indicate the direction water should flow when installing them. 

            A removable flush cap must be on each of the irrigation lines so that the system can be “flushed” on a regular basis. This helps to prevent emitters from plugging. I will talk more about this in a minute.

Some flush caps unscrew to release water. 

            The most popular filter for residential use is the inexpensive screen filter. It is oftentimes a black canister installed close to the irrigation valve used for drip irrigation. It is normally installed immediately downstream of the valve in the same irrigation box.

            The screen inside the filter should be removed regularly and gently cleaned or replaced if damaged. Clean the filter two or three times a year when using city water, more often if you’re using well water. The dirtier the water, the more often the filter needs to be cleaned.

Pressure regulators come in different static pressures and there are adjustable ones if you aren't sure of the pressure needed or it varies.

            Pressure regulators prevent the water pressure from rising above the limits allowed for the drip emitter to operate normally. They don’t increase water pressure if it is too low. Match the pressure regulator to the pressure range recommended for correct operation of the emitters. It’s more complicated with a large drip irrigation system. In cases like these, use adjustable pressure regulators.

Flushing can involve the use of valves, screw caps or be as simple as this figure 8.

            Flush caps or valves must terminate all drip irrigation lines that carry water. It is important to “flush” these lines by opening the irrigation line when the system is operating and let the water run until it comes out clean. Open each flush cap sequentially for each drip irrigation line the same time the filter is cleaned. Flush the line if repairs are done on the system that allows dirt to enter that might plug emitters. The number one reason for failure of a drip system is not flushing drip irrigation lines and cleaning the filters regularly and after repairs.

Air release valve manufactured by Jain Irrigation. It is inserted directly into polyethylene drip tubing to exhaust air in the drip lines. Water forces it shut.

            

Air release valves like this one from Netafim are also made to be screwed into PVC irrigation pipe as well.

Air release valves are an add-on to the drip system if trapped air is a problem or takes a long time for the drip emitters to start dripping water. Air release valves are inexpensive and exhaust trapped air in the system quickly before the irrigation lines become fully pressurized with water. They are usually installed at the end of a drip irrigation line or in combination with a flush cap or flush valve.

Adjustable drip emitters are convenient because you can open or close the emitter to deliver more or less water. But they are not precise and are not pressure compensated.

            Adjustable drip emitters are not “pressure compensated”. This means they deliver water at variable amounts depending on the water pressure and their elevation relative to other emitters. One reason I don’t like adjustable emitters is because of their lack of precision; when one adjustable emitter is opened to allow a greater flow of water, less water flows from the other emitters on the same line.

Messy Palm Trees are Females...Sometimes.

Flowers emerging from the spathe on a Canary Island date palm.

Q. We have several Mexican fan palms, plus a few identified as hybrid California and Mexican fan palms, in our backyard. They are sending out flower stalks and make a huge mess for anybody who has a pool. One of my neighbors said that only female palm trees produce these messy stalks. True?

Dates (fruit) produced by a female date palm.

A. There are hundreds of different kinds of palm trees and palms are variable whether they are male, female or produce both types of flowers on the same tree! For instance, both the Mexican and California fan palms have male and female flowers on the same tree. But the date palms, both the true date and the Canary Island date palm, are either all male trees or all female trees but they never have male and female flowers on the same tree.

Suckers from date palm. They can be removed and planted when they are older and their chances of survival are better. The suckers will be the same sex as the parent tree.

            So, if you have the California or Mexican fan palms then all of them are “messy”.  But date palms are different. Male date palms are less messy around pools than female date palms. Look for fruit on older female date palms if you aren’t sure whether they are male or female (they must be about six feet tall before females start producing fruit).

            It is nearly impossible to “sex” a tree before it starts producing flowers or fruit if grown from seed. Palms propagated from suckers will be the same sex as the parent tree. California and Mexican fan palms are started from seed. Date palms may be started from seed or suckers.

Take Fig Cuttings at the Right Time of Year

Q. I tried to start fig trees by cuttings taken from my big, sturdy tree and putting them in water or soil. Not much luck. How can I start new fig trees?

A. Figs are easy to start from cuttings provided you take them at the right time of year, the cuttings are the correct age and they are placed in a soil or medium that drains water and free from disease. They can also be started by air layering or rooting suckers at the base of the tree trunk. Placing cuttings in jars of water or using garden soil can lead to drainage and disease problems.

Suckers growing from the base of the trunk can be covered with soil, kept moist and they will grow roots. These rooted suckers can be potted and put in indirect light until fully rooted and then planted.

Air layering or marcottage can be used to form roots coming from damage done to stems on many different types of trees. Just do it at the right time.

            Cuttings taken from new growth in the spring are 6 to 8 inches long and range in diameter from pencil sized to as big as your thumb. With a sharp, sanitized pruning shears remove a length of new growth. Cut this new growth into pieces just below a node on the bottom and just above a node on the top. Include at least four nodes on each cutting and remove any leaves present on the cutting.

Cuttings don't have to be from large stems.

            Push these cuttings into perlite or sanitized potting soil in a clean container that drains. Stick these cuttings upright, a couple of inches apart, so that two of the nodes are beneath the soil or media and two are above. Use a rooting powder for a larger number of roots that are more vigorous.

Wine grape cuttings stuck in an amended soil and rooted.

            Place this container out of the direct sun but where there is plenty of indirect sunlight and keep it moist. Rooting occurs first followed by leaf development. Leaf development is a sign rooting has occurred.

Grape cutting forming roots at its nodes.

            Figs can also be started from the suckers at the base of the tree or by air layering. The base of the tree is mounded with soil around the suckers and kept moist. Damaging the sucker before mounding encourages root development. Traditional air layering methods can be used but keep them in the shade of the canopy so it is not damaged by direct sunlight.

Main and Briba Figs Come from Different Locations on the Same Branch

Q. You said the main crop of figs is the second crop of figs. But on my Black Mission fig, the first crop has large fruit, and the second crop, although much more plentiful, has fruit that is about 1/3 the size. Both are delicious.

Fruit size has more to do with the distribution of "food" for fruit growth and the number of fruit it is divided into.

A. The size of the fruit has more to do with the total number of fruit growing on the tree; the fewer fruit on the tree, the larger each individual fruit becomes. The term “main crop” has more to do with where on the tree the fruit originates.

Picture showing both Briba and Main crop figs coming on at the same time but at two different locations on the same stem.

            Fruit from figs comes first from stems that grew last year as well as a second crop on this year’s stems. “Main crop” figs are only produced on stems that grew this year. The early or first figs, called the “Briba crop”, come from stems that grew the previous year.

            Some people claim that “Main crop” figs taste better than the Briba crop figs. I have never tasted any difference between the two. But generally smaller figs have more flavor than the larger figs and there is a difference in taste between different varieties of figs.