Nutrient deficiency may be source of yellowing leaves

: I have 30-year-old citrus trees in my backyard on flooder/bubblers and I can’t tell if they are overwatered or underwatered? They were planted before I bought the house three years ago. The tangerines are green and splitting. The Meyer lemon seems healthy with new leaves on the lower branches but the leaves are turning yellow and I am not sure if it is due to fall.

A: The pictures that you sent me of the newest leaves yellowing but with green veins is a good indicator of chlorosis. Chlorosis is due to a nutrient deficiency. In our valley, it is usually iron. This lack of iron can be caused by alkaline soils, overwatering or the plant itself.

One of your pictures shows that the soil is not being mulched with wood or organic mulch. Organic mulches decompose and add important materials back to the soil. Leaving the soil bare robs the plant of this opportunity to have organic amendments, which make iron available to the plant.

Overwatering also can contribute to the problem. Underwatering does not usually contribute to this problem. The rootstock, or the roots that were grafted or budded onto the desirable top, also can contribute to this problem.

There are several different citrus that are used for rootstocks and some of them are better for our soils than others. Rough lemon, mandarin and sour orange are the best rootstocks for preventing iron chlorosis. The worst rootstocks are trifoliate orange and citrumelo. Some nurseries will inform you what rootstock was used.

The best advice I have for you is to use iron sprays or foliage-applied iron solutions to help green up the leaves. You probably will need to apply iron to the leaves several times during a period of a few weeks to get significant greening.

I also recommend that you use some liquid detergent with this solution and mix the solution with either distilled or reverse osmosis water. Our water has a fairly high alkalinity and hardness that can interfere with iron sprays. Always use up all of the iron spray and do not store it as it will lose its potency in a very short period of time when tap water is used.

Fertilizer applications to fruit trees are normally done just before flowering and after harvest. When you make your next fertilizer application also include an application of iron fertilizer that contains the chelate EDDHA. Follow the label’s directions.

Splitting in citrus can be an indication that the fruit is mature and ready to harvest or it can mean irregular irrigations. Apply a surface wood mulch around your trees to help regulate soil moisture and prevent splitting. Citrus does not have to turn color to be mature. When it splits, sample it to see if it is sweet, or mature. If it is not, it may very well be your irrigation practices.

Q: You write that trees should only be staked for the first two years after they have been planted. Is there any reason that they should not be staked after that time? We have one in particular (a California pepper) that has grown like a weed. Our lot is occasionally hit by what we have been told are called “devil winds.” In any case, the tree, now about 20 feet high, has twice blown to the ground. Both times we have been able to get it back up and stake it but we are very concerned about taking the stakes away.

A: As you know, there are no trees growing in the wild that are staked. Staking is done temporarily to a tree until its root system is capable of securing a foothold into the soil, making it self-supporting. If stakes are left on a plant for too long, the plant becomes conditioned to rely on those stakes and may never be capable of supporting itself.

When stakes are put into the ground, they are meant to immobilize the roots long enough so that they can grow into the soil surrounding the hole that was dug and amended. To do this, water has to be available to the roots several feet from the trunk.

Native trees growing in wet climates have root systems that spread anywhere from two to three times their height. Landscapes installed in climates where there is sufficient rainfall may have trees with roots approaching the same dimensions. In desert climates, native trees have root systems that are significantly larger than this.

The other part of this dilemma is delivering enough water at each irrigation so that the tree’s roots will grow deeply enough to support it. To obtain enough support, the majority of tree roots need to occupy about the top 18-24 inches of soil. Frequent, shallow irrigations encourage shallow rooting and possible uprooting during high winds.

Obviously, it is impractical to put irrigation systems for trees at these distances from their trunks. A bare minimum for irrigating these trees would be half the distance to their drip lines. The drip line is the line created on the ground by the edge of a plant’s canopy.

If you want your tree to stand on its own, then you will need to expand your irrigation system so that the soil under the canopy is irrigated deeply. When you irrigate, you need to irrigate deeply, allowing the top of the soil to dry between irrigations so that tree’s roots will grow deeply as well. Shallow irrigations do not allow for the proper mix of air and water that encourages deeper rooting.

Q: You encourage mulching in many of your columns. I have used mulch, but with the first big wind the mulch goes everywhere. Any suggestions or alternatives?

A: Wood mulch that has been chipped from trees and has pieces of variable sizes in it doesn’t blow. Wood mulches with pieces that are small and similar in size as well as larger sized wood mulches of a similar size blow easily in relatively light winds. Wood mulch that we use at the University of Nevada Cooperative Extension Master Gardener Orchard in North Las Vegas has withstood winds of at least 70 miles per hour. Decorative mulches look nice but they blow away easily.

The mulch available from our orchard will withstand high winds. It is free for the taking. For directions, call the master gardener help line at 257-5555. If you need quantities larger than 10 cubic yards, please e-mail me at morrisr@unce.unr.edu.

Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.