Soil moisture sensors not always accurate

Q. You recently gave me some advice on planting a crepe myrtle tree. I dug the planting hole as deep as the container and five times wider, but I didn’t do a water percolation test to make sure the planting hole drained water. I backfilled the hole with a 50/50 mix of the existing dirt plus a rich Viragrow compost. However, a soil sensor constantly registered a “10” (very wet) even for weeks after a single watering.

A. Be careful when buying a soil moisture sensor. Sometimes they don’t always work.

Before buying it, wet your hand and make sure the sensor on the tip is working. A “wet hand” grasped tightly around the tip should move the needle to the “10” you mentioned or at least close to it. When the sensor is in the dry “open air” the needle should read “0” or close to it. Buy only moisture sensors that are working properly.

Crepe myrtle trees are from the Eastern and colder parts of Asia including Northern and Central China, Korea and Japan. When it was introduced to England during the mid-1800s it didn’t flower profusely because it was not hot enough. This tree, much like the native Carolina laurel, really likes the heat of the south and southeastern parts of the U.S. for flowering.

The reasons it was introduced to the desert are because it likes the heat, is fairly drought-tolerant and tolerates a wide range of soil alkalinity. Crepe myrtle doesn’t like soils without compost, the very intense west and south sides without some afternoon shade and the low humidity of our desert. In our climate, this tree likes northern and eastern exposures, and regularly amended soil.

If the soil is not draining water very well, then mound the soil where you’re planting to a height of 12 inches and 4 feet in diameter. Do not plant in the bottom of a swale unless it likes the soil swampy. The soil mound allows for the roots of the plant to grow by providing water drainage to the lower soil surrounding the tree.

Q. I’m concerned with the appearance of my jasmine vine and rose bushes in my yard. I fed everything 1 cup of Epsom salts two months ago, and fed the rose bushes rose food last month and this month. I watered thoroughly before and after feeding. My yard is on a drip system that waters for 30 minutes three times a week.

A. It’s true some rosarians like to give their roses Epsom salts, but this does not excuse the condition of your plants. Epsom salts provides the soil magnesium in the sulfate form. Typically, in our alkaline soils, compost has enough magnesium to enrich the soil without using Epsom salts. Make sure the soil used in mounding drains water.

I don’t know where your trees are growing but the preferred landscape exposure for both plants is the east and north sides of a home or wall. Jasmine and roses like soil amended with compost. About a half-inch of compost sprinkled on the soil each year is enough to keep these plants healthy and growing at their best.

Just like us, when these plants are healthy, they can handle heat and cold better. When these plants are not healthy, they are not as tolerant of these extremes in temperature. If the soil is covered in rock and not improved, they will struggle a few years after planting.

What to do? If the soil is amended with compost at planting time and the surrounding soil covered in wood chips, usually just fertilizer is all that is needed. The addition of compost sprinkled on the soil surface once a year adds a lot of plant nutrients to the soil that is missing as rock degrades. Just don’t overdo it and keep rock or wood chips at least 6 inches from the trunk of young plants. Both plants will benefit from annual applications of the EDDHA iron chelate in the early spring.

Q. Please see picture of this fan palm that doesn’t look healthy.

A. The picture you sent isn’t a palm although it is called “Sago palm.” This plant is also called a cycad. Let’s agree to call it a “cycad” instead.

These plants originated from China and Japan and are now grown in nurseries all over the world. They can handle a lot of harsh locations and poor soils but look their best when growing in filtered shade and sandy but enriched soils.

If I were to grow them in a landscape in the desert, I would put them on the east or north sides of a home or wall, spacing them about 3 feet apart and 3 feet from walls and buildings. Keeping in mind that they “like” filtered light, I would find a place where sunlight is intermittent. They give a tropical and junglelike look to a landscape and should be used where there are lots of other plants growing as well.

The soil should be enriched and covered with 3-4 inches of wood chips or wood chip mulch and need between 5-10 gallons of water each time they are irrigated. They are “mesic” in their water use and should not be watered with the same valve used for desert plants like cacti, saguaro, Joshua trees and ocotillo.

The absolute worst way to grow this plant is on the west or south sides with reflected heat, grown in unimproved soils covered in rock and watered like a desert plant. If you can put a checkmark by any of these, your cycad is, or will be, in trouble.

Q. A few years ago I was lucky enough to get a Japanese dwarf pomegranate to grow from a 6-inch cutting. It is now over 5 feet tall. What is a properly pruned pomegranate supposed to look like?

A. The Japanese dwarf pomegranate is an ornamental shrub. It is not valued for its fruit. Pruning it is like any other ornamental shrub. Pruning is done after flowering has finished. I am guessing your ornamental pomegranate flowers through most of the year, so it should be pruned in the winter after leaf drop, just like pruning pomegranates that are used for their fruit.

The five- to six-stem rule that we use for pruning fruit-bearing pomegranate is not important if we are using it as an ornamental. This means you can leave as many stems as you want. As the shrub gets taller, remove one-fourth to one-third of the largest stems by pruning them close to the ground. You should be removing three or four of the larger stems every two to three years. Pomegranates grown for their fruit are pruned every year during the winter.

Q. Can I use burlap for shade cloth?

A. Don’t use burlap as a solid piece in place of shade cloth for permanent shade. It is fine for creating permanent shade for people or pets but not for plants. Be careful how much heat it traps if it is located too close to humans and animals. Plants are green and need sunlight for photosynthesis. Usually about 60 percent to 70 percent sunlight. People and pets don’t need as much.

Use shade cloth instead. It comes in different percentages of shade ranging from about 30 percent to 100 percent. Shade cloth for plants ranges from 20 percent to about 50 percent shade. Plants that flower and produce fruits like tomatoes, peppers and eggplant need more light than leafy plants: 20 percent to 40 percent shade. More shade than this interferes with flowering and fruiting.

Plants that only grow leaves such as lettuce, spinach, Swiss chard and greens can handle shade up to about 50 percent. After that, they grow poorly as it’s too dark.

Most burlap I have seen produces much more shade than this and would not be a good choice when growing plants.

You can make 50 percent shade out of wooden 1-by-2s by omitting every other piece of wood. These are called lathe houses. Similarly, 30 percent to 40 percent shade can be done the same way but by eliminating two and leaving every third 1-by-2.

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.