Two applications of olive spray work better than one

Q: We have seven large olive trees on our property. In the past, we used two different local tree services to spray our trees to minimize the amount of olive production. One company told us we needed two applications in early spring, and the other said we only needed one. Both came at a very high cost, and the results left much to be desired. Every day we had to sweep up hundreds of olives. Do you have any suggestions as to what might work and the timing of when the trees should be sprayed?

A: The chemicals used to prevent the formation of olive fruit work if the timing is right. The timing depends on when the flowers open. If the right chemical is used, applied at the right concentration and applied correctly, it should prevent 98 percent of the fruits from forming. Two sprays are more effective than one spray.

Nearly all the chemicals now being used need to be applied when flowers are open to get satisfactory results. If flowers are open, pollen is released, so applying these chemicals has little to do with preventing allergy problems. Its purpose is to prevent fruit from forming.

Here is why two applications work better than one. First, flowers must be open when the spraying is done. This is because the spray must enter flowers to be effective. It is not effective if it lands on the outside of the flower when the flower is closed.

Secondly, flowers do not open all at the same time. Flowers exposed to full sun open first. These are primarily on the south and west sides of the tree. Flowers on the north and east sides open later. Flowers that are not in full sun, scattered throughout the inside of the tree, also open later.

For all the flowers to open may take up to 10 days. The length of time needed for all the flowers to open depends on the weather. If it is cold, all the flowers take longer to open. If it is hot, all the flowers finish opening much sooner.

For best control of the fruit, two sprays should be applied: the first when 20 percent of the flowers have opened and the second when 80 percent of the flowers have opened. One spray prevents fruit from forming, but two sprays prevent more of the fruit from forming. Never expect 100 percent control.

Applicators have a very limited amount of time to get all their customers’ trees sprayed. If this spray window is missed, it results in very poor control of the fruit.

I should mention there is one chemical that prevents flowers and fruit from forming. This chemical is normally applied from February until March, before flowering. It is hard to get, but there may be a few applicators still using it.

Q: I was told I need chelated iron for my roses. So, per instructions, I added the granulated type today. How often do I do this? It’s not mentioned on the label.

A: Chelated iron applied to the soil only needs to be done once every year. The time of year to do this is now, during January and February. This applies not just to roses, but all landscape plants, including fruit trees. Chelated iron applied to the leaves as a liquid spray may need three or four applications a few days to a week apart to be effective. Chelated iron sprays are applied after leaves have emerged but avoided during the heat of the summer.

Once plants have begun growing in earnest, the single application of iron to the soil won’t do much. At this point, multiple liquid applications to the leaves are the only way to correct iron deficiency, yellowing or chlorosis.

The best iron chelate applied to the soil contains EDDHA in the active ingredients. When applying chelated iron to the leaves, use distilled water when making the spray. Avoid using tap water.

Include 1 teaspoon of liquid detergent per quart in the liquid spray mix to help the iron move inside the leaves. Add this liquid detergent at the very end, so you don’t get a bunch of bubbles.

Q: I have a high brick wall in my backyard that has white crystals, or “whiskers,” forming on it. This is because of the overwatering by my neighbors on the other side of the wall. I asked my neighbors if they would decrease their watering, but that hasn’t happened. I have used vinegar and brushed it off with a stiff brush. I also tried brushing it with a dry brush. Neither one worked. I bought some muriatic acid, but I am afraid to try it, because I have plants and trees right by the wall. I would appreciate any advice you can give me to solve this problem.

A: You are correct. This white powder forms because of the water coming through the wall from your neighbor. When the wall dries, white salts are left behind on the surface of the wall that were previously dissolved in the irrigation water. The only long-term solution is to reduce the amount of water applied on the other side of your wall. Eventually, irrigation water coming through the wall and containing salts will undermine the strength of the wall.

I am afraid this is a common problem with no easy solution if your neighbor is not willing to help. In the meantime, use phosphoric acid, rather than muriatic acid, to remove white “whiskers” from the wall. Phosphoric acid is safer to use, and the phosphorus contained in the acid is a plant nutrient. Muriatic acid contains chlorine, also a plant nutrient, but it is needed in very small quantities by plants compared to phosphorus. Chlorine can be toxic to plants if too much is available.

Ideally, your neighbor’s landscape should have a dry zone at least 3 feet wide next to this wall. This dry zone does not have to be void of plants. It can contain water-conserving plants irrigated with drip emitters.

An even better solution is to convert this landscape to a desert landscape using a mini-oasis landscape design. Mini-oasis landscape designs feature high-water-use areas of the landscape close to the house, located where people like to congregate. The other areas are designed and landscaped to use less water by having fewer and smaller desert adapted plants.

Q: We have Mondell pines that are 5½ years old that are still staked. I’m afraid these bindings will strangle the trees soon. But we also have a lot of wind that could blow the trees over. Can I safely remove the braces now?

A: Yes, remove the stakes. Tree stakes or, as you call them, bindings should be removed after the first growing season. If the planting holes are prepared correctly, these trees should be firmly anchored in the soil after one growing season.

I understand your concern about the trees possibly being blown over by wind. As trees get taller with a full canopy, strong winds can uproot them easily if they are not firmly anchored in the soil.

Solutions to this problem are twofold. First, prune these trees so that wind can flow more easily through the canopy. A method I don’t like, but used quite frequently, is to “rat tail” the limbs. This technique removes all side branches along the limbs so that only a cluster remains at the ends.

It’s true, it allows more wind through the canopy but unfortunately results in limbs that become weak and break easily during strong winds. Don’t use this method.

A better method is to selectively remove entire branches from the trunk. This method also allows wind to move through the canopy but without weakening and eventually breaking the remaining limbs.

Secondly, increase the area irrigated under the tree as the tree gets larger and water deeply. This increases the size and depth of the root system, thus improving its anchorage in the soil.

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

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