Time running out to apply iron fertilizers to soil

As I mentioned last week, we are nearing the end of any soil applications of iron fertilizers that work. You can still do it now but don’t expect glorious results like you would see when you apply it during the spring. These iron fertilizers include Kerex, EDDHA iron chelates and any other iron fertilizer applied to the soil.

Once early spring through early summer has passed, the only thing that cures leaf yellowing is the leaf sprays applied directly to yellowing leaves. That’s a poor substitute for soil applications of iron, best done earlier in the season, but sometimes the yellowing is so severe it must be done when it’s seen.

Just remember the alkalinity of our tap water is very important to a spray’s effectiveness, so either adjust the alkalinity of tap water with a cup of vinegar for each gallon or use distilled water for your iron spray.

Regarding applications of fertilizer, do the trees really need it? It’s best to apply plant growth fertilizers in the spring, but applying it now, if desperate, is still better than nothing. Applying fertilizers when trees don’t need them is a waste of fertilizer, a waste of money and unhealthy for the trees.

Look in several locations at the color of the leaves and the amount of new growth. If the leaves are very dark green and new growth is exploding, your trees don’t need a fertilizer application. Save the fertilizer application for next February or perhaps the November coming up.

Fireblight disease popped up in May on Asian pears and their hybrids, many European pears like Bartlett, some apples, quince, ornamental pear, pyracantha and cotoneaster; it was not seen on citrus, just the rose family. Where did it come from? Anybody’s guess but it could be your neighbor or at least from the neighborhood.

The most effective treatment is pruning it out. When removing any systemic disease, you have to make deep cuts to get all of it. Cut or prune out 8 to 12 inches below where you see it. Sanitize your pruning blades with alcohol.

Q: We have a 10-year-old live oak tree that was established when we moved into our home last year. I noticed that it sheds leaves seemingly all year long, which didn’t seem right to me. It is happening very fast now, and I’m really worried.

A: Let’s get something out of the way quickly. That’s irrigation. I know it doesn’t seem logical, but irrigation can — and notice I said can — be the source of a lot of problems in the desert.

Live oak from Texas is not a desert tree, but for a large tree — 40 to 50 feet tall — it can use much less water than some other trees around the same height, such as mulberry. It will survive in a lawn, and it will survive in rock landscapes if given enough water.

Whereas mulberry might require between 5 and 6 feet of water under its canopy each year, Texas live oak can still look good with 3 to 4 feet of water applied to the same area. Watering too often, not watering often enough, giving the tree not enough water all at once, and not distributing the water over a wide enough area under the tree can cause leaf drop.

When water is applied to Texas live oak, it should be applied to at least half the area under the canopy and to a depth of 2½ feet. It is watered again when the upper 6 to 12 inches start drying.

The usual spring problem on live oak is aphids. Aphids become a lesser problem when it gets hot.

Trees with leaves infested with aphids are easy to spot because aphids leave a sticky residue on the leaf surface and can cause leaf drop when they feed on plant sap. The sticky residue glistens in the sunlight. An application to the soil in the spring of a systemic insecticide like imidacloprid or a rose systemic insecticide will take care of the aphids and stop the excessive leaf drop.

Q: You suggested buying a waterproof, battery-operated recording maximum/minimum thermometer for about $15. I’m having trouble locating what you’re talking about.

A: I suggested buying a recording maximum/minimum thermometer and placing it in your gardening area so that you could track temperatures during the late summer, fall, winter and spring. This thermometer, combined with a weather app on your phone, makes a powerful and inexpensive combination that predicts the weather and shows the weather that already passed. Use this information to determine tree fruit losses and the best time to prepare the soil for planting cool- and warm-season vegetables.

Placing it as close as possible to the gardening area helps you decide the low temperatures you received during the night. Watching these low temperatures tells you how cold your microclimate got in your backyard so you can accurately compare yours with other microclimates.

Look for manufacturers with well-known names in the weather or instrument categories such as Taylor and AcuRite. Make sure the thermometer records minimum and maximum temperatures over a time period such as 10 days.

Q: I have a large pine tree in a natural area losing limbs from the bottom up. Its needles are browning and dropping. I don’t water the tree much at all, but I have a water channel flowing 3 feet away on one side of the tree. The channel is mostly filled with moving water and is about 1 foot wide.

A: It’s probably either a water or light issue. Not enough light because trees are planted too close together will also cause lower limbs to die and drop. Sometimes light is not considered. Pine trees planted 4 to 6 feet apart are too close together, and lower limbs will die as they age.

My guess is that it’s water-related — not enough. An easy way to find out is to put a sprinkler on the end of a hose and give the tree one to two hours of water once every three weeks to a month. I would use a nonoscillating sprinkler and adjust it to about 8 feet wide. Use a 4-foot-long piece of rebar to adjust how many minutes to water.

There are two ways that I use to tell if a pine tree isn’t getting enough water: the amount of new growth and its canopy density. On well-watered, fertilized smaller pine trees (about 20 feet tall), I like to see about 18 inches of new growth and a solid canopy density. On older well-watered pine trees (40 to 60 feet tall), the growth will be a lesser amount, perhaps 8 to 12 inches, but the trees still have a dense canopy.

Pine trees with drought problems usually grow 2 to 4 inches in height each year. This small amount of growth translates into a very open canopy. Because pine trees can have other problems such as borers and woodpeckers (sapsuckers), I inspect the trunk for this kind of damage as well. A tree trunk with extensive damage by borers or woodpeckers may also show signs of drought. Pine trees with only a few inches of new growth each year and an open canopy usually suggest drought.

Q: I have six bottle trees in my backyard for a privacy hedge with my neighbor. One of these trees, possibly two, is not growing as strongly as the others. These trees were planted from 36-inch boxes by a local nursery three or four years ago. I installed deep watering tubes for these trees a couple of years ago.

A: Deep watering tubes are plastic tubes inserted in the soil that force irrigation water to wet the soil deep. When I measured soil moisture in the top 6 to 12 inches when using these tubes, the soil was dry. The soil was sopping wet below this.

The tubes did exactly as promised; they kept the upper surface of the soil dry by bypassing it. But the soil was very wet starting 12 to 18 inches deep.

I prefer the old-fashioned method of irrigating and plant water extraction from the soil following the 40:30:20:10 rule. Water trees and large shrubs deeply but infrequently.

The roots of any plants are lazy. They will extract 40 percent of the applied water in the top fourth of the soil, 30 percent in the next quarter, 20 percent in the next quarter and 10 percent in the last quarter.

As we wait longer between irrigations, the tree or plant must use water from deeper in the soil. As they use this deeper water and shallow water becomes less available, plants and tree roots develop deeper in the soil.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.