Neem, horticultural oils differ in toxicity to insects

A: Horticultural oil is used for spraying trees and shrubs when leaves are absent during the winter. Horticultural oils, sometimes called dormant oil or even dormant spray, have a very specific viscosity or thickness that makes them effective in killing pests. These oils are petroleum-based and refined so they don’t damage plants if applied correctly.

When spraying trees, fruit trees and shrubs make sure the entire plant is covered with horticultural oil from top to bottom Neem oil is used primarily during the growing season when leaves and the offending insects are present.

Neem oil is a bit trendy right now. And it is plant-based rather than petroleum-based which for some is important. One major difference between neem oil and horticultural oil is the toxicity of neem oil toward some insects. Horticultural oil is not toxic on its own. It relies on suffocating insects for control.

Remember, just like many organic pest control chemicals, neem oil is nondiscriminatory in what it kills. It cannot recognize a good insect from a bad insect. You direct the spray toward pests you want to control or food that you want to protect. This is also true of horticultural oils, which, unlike neem oil, is frequently applied when leaves and fruit are not present.

Neem oil has been promoted for disease control. A little-known fact is that horticultural oils will control nearly all of the same diseases as neem oil. Similarly, horticultural oils can be applied when leaves and fruit are present as long as temperatures are below 80 degrees Fahrenheit.

Personally, I have never found neem oil particularly effective in killing pests although I do use it in rotation with soaps and other organic pest controls. I worry a little bit that there is a wide variation in the quality of neem oils being produced and available on the market. Horticultural oils are pretty much all the same. Stay with name brands of neem oil.

Q: Our Bermuda grass lawn is covered with little mounds of dirt. Something is digging in or digging out of the dirt and I suspect it can’t be good. Grubs? What do I do?

A: These mounds appear in lawns after a long rain or if the soil has been flooded. Those mounds of dirt are excavations from earthworms. Earthworms move to the soil surface when they can’t get enough air, leaving behind little piles of dirt. We also see it in soils that are compacted for the same reason.

There are many earthworms in our soils, particularly where plants are growing. They provide a lot of benefit to the soil and recycle nutrients from decaying plants and very small animals like insects. They also create air and water channels in the soil.

Earthworms do not like to be exposed, but they will come to the surface if they can’t get the air that they need. The only way they do that is by digging these channels to the open air.

In some parts of the country, earthworm populations are so dense they create problems when they surface, leaving behind the piles of dirt you’re seeing. In cases like these, people are more likely to ask how to get rid of earthworms.

In your case, don’t worry about it. It’s a good sign. These mounds will eventually disappear. If the problem persists and it does bother you, aerate the lawn with a commercial core aerifier and dethatch it.

Q: I am new to this climate and want to plant two apple trees in my yard: one for eating and one for cider. Do you have any experience growing Dabinett apple trees or other apples indicated for USDA zones 4-6 in this climate? I ask because I want apples that produce the right kind of fruit.

A: All apple tree varieties will grow here. Many will even produce fruit here. But there are only a few that will produce high-quality fruit here. There is a concept in wine production called “terroir.”

Picking the right terroir is the reason why some wine grapes are better suited to specific agroclimatic regions than others. For instance, in Las Vegas we focus on the warm climate reds and a few warm whites, with cool season wine grapes not producing the right balance of acids and sugars. Of these warm climate reds, it is still too hot for the best quality.

The same concept, terroir, applies to fruit quality and apples. First off, our hot desert location is not the best for apples to begin with. It is better suited to apricots, peaches, plums and their relatives. However, it is best suited to pomegranates, figs and dates.

There are only a handful of apples I would recommend for this climate regarding flavor. Some produce here better than others, and still others produce better fruit than most. I would recommend finding an apple that grows well here and produces quality fruit that you like.

Otherwise, what you are doing is experimental and chances are most will not produce the fruit quality you would like. There is a possibility some might but chances might be slim.

Regarding Dabinett, an English variety, you are taking a risk. Don’t get me wrong. I love this kind of risk but be aware of the downside in trying. Be aware that, particularly with apples, it may take three to four years before you get decent fruit.

Q: My dwarf oleanders have quit flowering. Should I cut the flower pods off now or never? If yes, how far down the stem should I cut?

A: You can do just about anything to oleanders without problems. They are a tough plant.

Oleanders are planted for aesthetics. Cut the seed pods off if you want or leave them still attached. It will make no difference to the plant.

Some of the dwarf oleanders are tender to cold winter temperatures and may or may not freeze back to the ground depending on the winter and your location. Wait to prune them until mid-February.

Every three to four years, cut three or four of the largest stems in diameter to the ground. This will cause them to renew the plant from the base. Trimming with hedge shears is never needed and should be avoided.

Q: I recently saw an article about cactus in which the author discussed how to plant a cactus in a new pot. He advised to always plant the cactus “with its front facing the sun.” How do you tell which is the front of a cactus? Darned if I can tell when looking at mine.

A: I have no idea what the front of a cactus is. I could make some wisecracks about it but I think the author is telling people to orient the cactus with the same side toward the sun to reduce problems from sunburn.

Like many plants, cactuses grow differently on sides facing extremes in climate. The top, south and west sides are conditioned for high light intensities. They have a skin that is thicker with more protection provided by the plant in a reaction to high light intensities.

The north and east sides are less protected. If the north side of a cactus is oriented toward the south, this side may sunburn as a result.

Q: My canary palm is getting hit with sprinklers to the point that it’s eating into the trunk. This is damaging to the palm, isn’t it? I talked to my gardener about it but he didn’t seem to think it was much of an issue.

A: Yes, water continuously hitting the trunk can be damaging to the trunk and possibly lead to disease. Eventually, the tree may need to be replaced if the damage is severe.

I understand why your gardener responded like he did since it is common, but, yes, the water should be diverted from the trunk or the tree irrigated in some other manner.

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