Both Nitrogen and Iron Fertilizers Contribute to Dark Green Plant Color

Q. Thanks to your previous advice I used an iron chelate, applied it to the soil in March, and got my plants to develop a dark green color rather than a yellowish green color.

Shrubs that the reader was speaking about. Not sure but they do look like desert natives.

A. There are many things that develop a yellowish color in plants besides needing an iron fertilizer or chelate. The plant you questioned appears to be native desert plant, a Texas sage. That is odd needing an iron fertilizer for native desert plants. They are usually accustomed to our soils and don’t need iron. Watering or a nitrogen fertilizer may be the issue.

Not sure but I do recommend iron chelates that contain EDDHA as the chelating agent. It works in all the different alkalinities of desert soils. EDTA and DTPA iron chelates don't.

Regardless, the two fertilizers that can create dark green leaves are nitrogen and iron. If that plant is native to the Western US, then yellowing leaves is more likely issues involving either nitrogen or watering too often. Nitrogen causes stem growth as well as dark green leaf color. Adding only an iron fertilizer or chelate causes the new growth to become green but does not stimulate new growth that much. When iron is involved, the yellowing occurs on newer growth. Yellowing due to a need for nitrogen occurs all over the plant. Also, the yellowing of leaves due to iron may be a yellow leaf color while the veins of the leaves stay a darker green.

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Winter yellowing I call bronzing of a shrub due to very cold temperatures for Las Vegas.

Two types of “overwatering” can occur; watering too often or giving the plant too much water all at once. It is easier for the plant to resist “overwatering” from the second kind than the first kind. It is easy to water desert native plants too often when placed on the same irrigation line as non-desert plants.

Mesquite leaf yellowing and leaf drop during winter cold temperatures.

Another reason for yellowing of plant leaves are cold temperatures. This type of yellowing is more of a “bronzish yellow” leaf color and happens during cold weather. Cold weather damage to mesquite leaves (yellowing or bronzing) is a common occurrence during cold weather just before the leaves may fall from the tree if it gets cold enough.

Pine Trees Don't Need as Much Fertilizer as Fruit Trees

 

Pine trees in the desert do not need as much fertilizer as fruit trees. We don't expect as much from them as we do fruit trees.

Q. I have a few Aleppo pine trees that I fertilize and water regularly growing in Logandale. I am wondering how much fertilizer to give them each year and how much to water them. I am wondering if I can get them off of the irrigation due to a shallow water table here and apply less fertilizer.

Pine trees access water from shallow aquifers if they are within a few feet of the surface of the soil. One way to see if that is the case is to look for salt pushed to the soil surface by shallow water.

A. In my experience trees like Aleppo pine need fertilizer applied AT THE MOST once a year and perhaps less often. In the case of pine trees mostly nitrogen and potassium because we don’t need them to flower or fruit. That requires fertilizer higher in phosphorus, the middle number. Whenever the trees are “improved” (hybridized or improved for some reason) they need to be fertilized more often and need more care. For example, most fruit trees require one full or a split application of fertilizer twice a year.

This is Burgundy plum growing in Las Vegas, NV. Fruit trees will need more fertilizer if we want large fruit to be produced. Are expectations aren't as high for pine trees so they can get by without applying as much fertilizer.

My guess is that your pine trees, at the most, will need fertilizer applied once a year in the spring. Improved trees like fruit trees need about one pound of a nitrogen fertilizer in the spring (or the fertilizer divided in half and applied twice) for each 1000 square feet under their canopy. Aleppo pine trees require it less often or apply less total amount each time you fertilize.

New growth of older pine trees should be at least 8 inches or more to give adequate growth for a full canopy.

Look at the results of irrigations and applications of fertilizer. The fertilizer is needed every year or every other year. A tree of that size should put on about 8 inches of new growth every year to keep it full. That takes primarily nitrogen similar to a lawn fertilizer (21-7-14). Don’t skimp on nitrogen and potassium in the fertilizer for pine trees. Because they are “all green” and no flowers or fruit you can apply less of the middle number (the amount of phosphorus).

As far as applying irrigation less often or eliminating them, you need shallow groundwater to about 5 to 7 feet deep. At the same time force its roots to grow deeper in the soil so it can discover the water. Pine trees have taproots or large roots that can grow deep if given the chance. Groundwater can go down in the summer months and up to normal in the winter. What I am telling you is that you might need to water occasionally during the summer months.

This is bubbler and basin irrigation on pine trees. The basin doesn't have to be deep but it must be flat and capture the water from the bubbler so it stays put long enough and penetrates the soil deeply.

When you irrigate, water the trees deep when you do, water them less often and watch the tops. Force the tree roots “to go after” deeper water and see how much fewer extra irrigations they need and still maintain 8 inches of new growth during the early summer months.

Winter Cold Damage and Freezes

Oleander after a strong freeze.

This is the time of year, the second week of December, we normally experience the start of freezing temperatures. The likelihood of freezing temperatures increase through the winter. Sometimes freezing temperatures occur in November, as it did a few years ago, but that's rare and not “normal”. What I mean by “freezing temperatures” is plant damage that occurs anytime the air temperature drops below 32 degrees Fahrenheit (F). Those familiar with Celsius or centigrade, may realize this temperature is the same as 0 degrees on the Celsius scale.

Cold or freezing damage.

Refrigerators Don't Freeze

As a reference point, the temperature inside most refrigerators is set to around 40 degrees F, or 8 to 10 degrees above freezing; too cold for most tropical fruit and plants but not too cold for temperate fruit like and plants apples and peaches. Chilling injury (plant damage that occurs because air temperatures are too cold for the plant but not yet freezing) is one reason many ripe tropical fruits, like tomatoes and (more obviously) bananas, should not be exposed to the 40-degree F temperatures of a refrigerator. 

If winter temperatures aren't cold enough to kill, the plant will regrow from buds.

Chilling Injury

All parts of tropical plants such as tomatoes and bougainvillea, experience “chilling injury” when temperatures drop a few degrees above freezing and may extend to 50+ degrees F. Chilling injury (as opposed to freeze or frost damage) occurs at different temperatures and depends on the plant.

Tomato plants might freeze at 32F but the fruit is damaged below 55F.

Chilling injury damage to tropical and subtropical plants include small stem and leaf discoloration, leaf roll, poor growth, and susceptibility to some diseases like root or collar rot. Symptoms of chilling injury include a change in color such as yellowing or bronzing of leaves that ultimately result in leaf scorch or drop, the slowing or halting of growth, leaf drop, water-soaked patches in soft and semi-hard tissues, susceptibility to diseases, and wilting. Chilling injury is due to cooler or cold weather (above freezing) temperatures to tropical plants growing outside of, or close to, the fringes of their normal range. As a side note, I noticed leaf and stem discoloration (closer in color to leaf “bronzing”) in mesquite, palms, citrus and a wide range of plants growing at different temperature ranges.

Chilling Injury and Vegetables

Vegetables can exhibit chilling injury and freezing damage as well. So-called “warm season vegetables” such as tomatoes, peppers and eggplant can show chilling injury anytime the air temperature drops into the damaging temperature range I mentioned earlier. Cool season vegetables, on the other hand, may sail through the same temperatures, or lower, or require a crop cover when temperatures are below freezing. Vegetable varieties may differ in their chilling injury by a few degrees. The ‘Dragon’s Tongue’ variety of bush bean is more susceptible to collar rot (chilling injury) when grown in garden soil a few degrees cooler than other bush bean varieties.

Nopales freeze damage during the winter of 2007-2008

Temperate Flowers and Fruit Can Freeze

Temperate fruit like apples and pomegranates, unlike tropical fruit like tomatoes and bananas, are not damaged at refrigerator temperatures (around 40 degrees) because fruit from these trees can handle these lower temperatures. The ideal storage temperature for these types of fruit is somewhere close to freezing (0 degrees F) and combined with high humidity. A high humidity slows water loss and helps delay some fruit from shriveling.

Refrigerator Freezers and Freezing

The freezer part of our refrigerator is set to around 32 degrees F, or about 10 degrees below the “refrigerator temperature.” Our nighttime winter temperature frequently drops to a “refrigerator temperature” range at night during the late fall, winter and early spring and occasionally into the “freezer temperature” range during the early morning hours of December, January and occasionally early February. When nighttime temperatures reach the “freezer range” is when we often times see plant damage or experience fruit loss, but we may not know it yet.

Open Flowers can Freeze

Open flowers of any fruit (citrus, peach, apple and others) can’t handle temperatures below freezing (32 degrees F) even though most plants or trees might show no damage at all! When flowers are simply buds and not yet open, there is a small amount of freeze protection provided to the developing flower. This freeze protection starts disappearing as the flower buds mature into open flowers. As the flower begins opening, and the frost-sensitive ovary is surrounded by the freezing night air, is when we experience damage or fruit loss. Fruit loss due to a frozen flower ovary can happen in a few seconds. This is why sprinklers, ultimately resulting in applied water turning to ice on the flowers, are used in orchards to prevent freeze damage to flowers (ovary). The act of water freezing releases a small amount of heat that protect flower ovaries from death.

Early winter freeze damage to persimmons.

Was the Flower Frozen?

If you are curious if the ovary of a flower from your fruit tree was damaged during a freeze, pull the flower apart a few days after a suspected freeze and inspect the ovary for death. Ovaries that eventually turn into fruit will be robust and green. Dead flowers drop from the tree early or have a dull, water-soaked appearance if they are still attached. Just because the flower you inspected was “dead” doesn’t mean there will be no fruit produced at all. It takes about two or three weeks for all the flowers to open in spring flowering plants. Several consecutive light freezes in a row (or only one hard freeze) are needed to totally wipe out a crop of fruit from a mature tree.

when a fruit tree flowers it is hold your breath that no freezes happen during this time. When fruit tree flowers are open is a dangerous time for freezing temperatures.

Freezing Death Differs

 There is a temperature difference between the freezing death of open flowers and the freezing damage or death of the plant or tree. For citrus this difference can range from the same temperature as flower death (32 degrees F; limes and true lemons) to lower temperatures (mid 20 degrees F; Myer lemon, grapefruit, and kumquat). Much of this depends on how many minutes or hours a killing freeze lasts and at what temperature.

Temperature, and duration of that temperature, are what is critical. With open flowers, freezing temperatures are needed for just a minute. With plant or tree death, the same temperature is needed for a longer time period. With some fruit trees (such as citrus), the temperature difference between flower and tree death can be small (0 to 7 degrees). In other fruit trees (such as apples, pear, peach and apricot) it can be much greater (20 degrees or more).

Weather Apps

What to do about freezing temperatures and fruit or plant loss? Track and use weather station apps during times when freezing temperatures might occur. Buy a recording thermometer for your backyard and hang it in your important trees. If you are into growing vegetables, use a soil thermometer to judge when to precisely plant.

Weather stations like this one can play a critical role in helping to predict if freezing temperatures are a possibility.

What to Do?

Move fruit trees to a new location. If your fruit trees are less than three to four years old, now is the time of year to move them. South and West-facing sides of walls and buildings are tough to grow plants because of the heat they reflect. Some citrus are in landscape “wind tunnels” that remove flowers during strong winds. North and east sides of buildings and walls as well as backyards have a gentler environment. In cooler, gentler areas of the landscape expect flowering to be two to three weeks later. Group plants together so they can share water, fertilizer, and protect each other. If they can't be grown together, consider constructing wind barriers.

Try kumquats which are a cold hardy, subtropical citrus trees. The small fruit can be freshly harvested and eaten from the tree but still gives you that citrus “kick”. Kumquat flowers several times during the summer instead of just once and yet is small enough to grow in a container. Flowering more than once during a single growing season increases your chances of having a successful crop if you have late or multiple freezes.

Wrapping a fruit tree correctly might give you a few degrees of winter protection from cold.

Grow fruit trees that flower continuously. Pomegranate, fig and jujube are temperate cold and heat tolerant fruit trees that flower continuously through the growing season.

Fruit Trees Benefit from Neighboring Plants

Always group plants and fruit trees together. Improve the soil at the time of planting and every couple of years. The water, soil improvement and shade that they provide and share is a big benefit to them when grown in the desert.

Pines in Logandale Need Less Fertilizer But the Same Amount of Water

In my experience trees like Aleppo pine need fertilizer applied AT THE MOST once a year and perhaps less often. 

Non-Flowering and Non-Fruiting Trees Need Mostly Nitrogen and Potassium

In the case of pine trees mostly nitrogen and potassium because we don’t need them to flower or fruit. That requires fertilizer higher in phosphorus, the middle number. Whenever the trees are “improved” (hybridized or improved for some reason) they need to be fertilized more often and need more care. For example, most fruit trees require one full or a split application of fertilizer twice a year.

Force Them After this Water

My guess is that your pine trees, at the most, will need fertilizer applied once a year in the spring. Improved trees like fruit trees need about one pound of a nitrogen fertilizer in the spring (or the fertilizer divided in half and applied twice) for each 1000 square feet under their canopy. Aleppo pine trees require it less often or apply less total amount each time you fertilize.

Look at the results of irrigations and applications of fertilizer. The fertilizer is needed every year or every other year. A tree of that size should put on about 8 inches of new growth every year to keep it full. That takes primarily nitrogen similar to a lawn fertilizer (21-7-14). Don’t skimp on nitrogen and potassium in the fertilizer for pine trees. Because they are “all green” and no flowers or fruit you can apply less of the middle number (the amount of phosphorus).

As far as applying irrigation less often or eliminating them, you need shallow groundwater to about 5 to 7 feet deep. At the same time force its roots to grow deeper in the soil so it can discover the water. Pine trees have taproots or large roots that can grow deep if given the chance. Groundwater can go down in the summer months and up to normal in the winter. What I am telling you is that you might need to water occasionally during the summer months.

When you irrigate, water the trees deep when you do, water them less often and watch the tops. Force the tree roots “to go after” deeper water and see how much fewer extra irrigations they need and still maintain 8 inches of new growth during the early summer months.