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Pollen, allergy season in full swing in Las Vegas

Updated April 12, 2021 - 12:17 pm

Pollen alert. That’s what we see on our advisories in the early spring regarding pollen from mulberry, ash and junipers. The alert continues through the pollen season as we go from mulberry to pine to olive.

Pollen season might last until May. Right now we’re in mulberry season. Some pollen like mulberry is light enough and can be pushed by the wind and carried by pollinators like honeybees. This type of pollen causes hay fever while others are considered sticky or heavy, too large to travel long distances in the air and not considered allergenic.

“Hay fever” was a misnomer from the start. It was an old historic association with the cutting of hay in the spring and not paired with the flowering of plants that cause the release of pollen.

Back then pollination by plants was not studied much. Tree and shrub pollen was not considered responsible for hay fever.

Most allergenic pollen comes from uncontrolled, wild grasses growing where rain was available. These allergies were caused by pollen floating in the air but the idea of “pollen fever” never caught on.

Until recently people with severe symptoms were told to move to the desert Southwest where pollen fever was never considered much of a problem. Maybe that was the case back then, but they are wrong now.

As people moved to the desert Southwest and started planting more and more desert trees like acacia, mesquite and palo verde, pollen fever developed into more of a problem.

Typically, trees and shrubs with large showy flowers like oleander do not contribute much to the problem. This pollen is heavy or sticky and did not travel in the air far from the flower, and the plants released pollen too large to cause allergy problems. Most of the problem pollen comes from nonshowy flowers commonly found on olive, mulberry, pines, ash trees, mesquite and the like.

Now we have planting restrictions in population-dense Clark County that prohibit the planting of male mulberry trees or olive trees that produce lots of fruit. So, is it illegal to plant mulberry trees in Clark County? Yes and no. It is illegal to plant male mulberry trees but not female trees.

How about olive trees? Yes and no, but for a different reason. Olive trees always have both male and female parts in the same flower so we focus on the so-called fruitless mulberries and hope that these trees reduce hay fever in large communities.

Mulberries, similar to ash trees, are bought as either male trees or female trees. As I have always said, plant sex is much more interesting than animal sex because of its diversity.

Q: Does pineapple guava need a pollinator plant to produce fruit? The edible flowers bloom in May and have the wonderful taste of cotton candy. Should I get my pineapple guava tree a boyfriend?

A: First, let’s talk terms. A pollinator is an insect that helps plants produce more fruit by transferring pollen from one plant to another. Examples of pollinators are honeybees.

A pollenizer is the plant that supplies this pollen to another plant to help it produce more fruit. So, I think you are asking for a pollenizer plant for pineapple guava.

Now let’s talk pineapple guava. If the flowers are pollinated properly by a pineapple guava that is not exactly the same as the mother plant (pollenizer), the flowers will produce fruit.

Some plants may be even self-fruitful to a degree. The amount of fruit produced depends on the number of flowers it produces and its closeness to a pollenizer plant.

To make sure to get fruit from the flowers, give the plant a “boyfriend” (or girlfriend). The reason for this is because of its genetics. In technical terms, the flowers can be nonreceptive to pollination by the same or similar plant (variety or cultivar) depending upon genetics.

So to make sure you get fruit, plant two different varieties of pineapple guava in close proximity, otherwise it might be a trickle of fruit at best. The flowers of pineapple guava are edible, and the taste is not affected by a pollenizer.

Pineapple guava performs well in desert landscapes. It can handle our heat and can handle our cold. It can even handle a lot of the rock mulch used in many landscapes. But it is a normal water user (mesic) and not xeric like many of our native desert plants.

Pineapple guava is a good choice for our desert climate in landscapes, but it is not a true desert plant so it grows better with a little bit of organics like compost mixed in the soil at planting time.

Q: I have a star jasmine that is very woody growing behind some front greenery. I heard you mention once not to prune flowering wood. What does that mean?

A: It means pay attention to the time of season you’re pruning and its relationship to when the plant produces flowers. We are talking about plants valued for their flowers, not fruit trees.

Fruit trees are pruned at a different time because we value their fruit. The flowers are not as important to us in fruit trees.

The best time to prune any plant valued for its showy flowers is as soon as possible after it finishes flowering. Enjoy the flowers, and then prune.

If it’s in the spring, prune it for flowering after it finishes in late spring. If it flowers all during the growing season, then wait until fall or winter when it stops flowering. Avoid all dramatic or heavy pruning of nearly all plants during the summer heat.

Plants need time to produce flowers. Some plants also need the right time of year. If plants produce flowers all season long when growing, then wait to prune them until they finished their show by mid-fall or early winter.

Star jasmine typically produces flowers after a flush of spring growth, not right away in the spring. The time of flowering for this plant is more similar to oleander or Texas sage.

As long as they are old enough, they start flowering as soon as there is some growth. This tells you they need a little bit of growth to produce flowers. Plants that grow like this we say “flowers on current season wood.”

Can you see why oleander, Texas sage or star jasmine shouldn’t be pruned during the summer? Instead, they are best pruned during the winter when spring and summer growth provides the new stem growth needed for producing flowers. If these plants are pruned just before or after they start their new growth in the spring, it causes their flowering to pause until there is some new growth.

Q: I don’t like what I’m reading about Imidacloprid (frequently found in borer control insecticides) and wanted to know if there is a better insecticide that won’t harm bees.

A: You are talking about one ingredient (lower-case letter on the label) found in several different products rather than an actual product name (capitalized) found on the label. The actual or product name might be Merit for commercial applicators and Bayer Tree and Shrub Spray for homeowners but also many others.

Insecticides containing imidcloprid have been banned for use in some countries. That particular ingredient is still approved for use in the United States but has faced a lot of environmental problems and may be eliminated in the future. It has been implicated in the death of some pollinators like honeybees when visiting the pollen contained in open flowers.

The label information for these types of products gives you a clue about how to apply it properly. If you must use this product, apply it to plants after they have finished flowering. This helps prevent the transmission of this ingredient to pollinators.

At this particular time, I don’t know of any insecticide available or permitted for use that has the same potential for controlling borers as imidacloprid. Its major advantage is also, potentially, its disadvantage; it is a long-lived systemic insecticide that can potentially be harbored in flowers, fruit produced and plant parts for up to 12 months. The only other option I know that can work is the digging of these problem insects from infested trees as they are seen.

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.

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