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Healthy plants have better tolerance to heat

Updated August 21, 2023 - 1:26 pm

Q: What do I do with shrubs and plants that get heat or sun damage? A rep at a local nursery says to remove the stones from around the trunk and be sure it is getting enough water. Also, their “tonic” might help. We have done all this. Will these plants survive this stress, or should we do anything else?

A: Improve the health of the tree or shrub. Healthy plants have better tolerances to heat and cold. For most plants, it’s not the heat or the cold but it’s the intense desert sun that causes damage. Sometimes moving plants to a new spot in the landscape helps them avoid direct sunlight.

Be sure to water your plants so that they have at least one day without water. This means never water daily unless you have no other option.

The only plants that require watering daily are lawns, vegetables and annual flowers. Otherwise, plants that require daily watering will control the irrigation of others. Plants with deeper roots than this should get watered every other day at the most, even if the top of the soil looks dry.

Plants originally coming from a desert are watered less often than those that don’t come from deserts. This is because desert plants have methods of surviving a longer time between irrigations.

Fertilizer applications are usually done during cooler months, not the heat of the summer. Fertilizer bags stress the importance of applying a total amount of nitrogen. Slow-release fertilizers last longer after an application than quick-release fertilizers. Printing on the bag and their more expensive cost will tell you if it’s a slow release or not.

It takes longer to correct but soil improvement might be necessary for many plants. Rock surface mulch doesn’t add organics back to the soil. Some plants prefer growing in soils with organics and may require it in the long run. Adding organics under the rock mulch may take months to get results.

Q: I’ve had several agave quadricolor plants in pots for four to five years now. Potting medium is about ⅓ each of vermiculite, perlite and coarse sand. I occasionally fertilize with a granular fertilizer each season. They don’t grow too much, maybe one flush of leaves per season but it always looks healthy. They get a half day of early sun (two to three hours), and I water every two weeks or so in spring and summer.

But this hot July, I fried some of the bottom leaves and started to fade the others. I moved the plants into a shaded part of the yard a couple of weeks ago and the plants seems to be happy and on their way to recovery. Could I repot the plants lower by a couple of inches to hide the exposed trunks? It’s my thought that the new growth will spread back out and cover the top of the pots.

A: The quadicolor agave is a small (2 to 4 feet tall by 3 to 4 feet wide) but beautiful agave with dark green leaves edged with white. Like all agaves, it is susceptible to spring and summer agave weevil damage. Check and make sure there are no agave weevils active at the base of the leaves or in the trunk. The trunk and fleshy leaves near the trunk will be mushy by early to midsummer. If you have an agave of any sort, it is better to treat them in the spring than to be sorry you didn’t later in the year.

Agave weevils are the No. 1 problem with agaves in this climate — that and watering them too often. The plant should be solid in the soil, and you should not see any adult black weevils about ⅜ inch long near that plant. If they are present, then drench a systemic insecticide near the plant. I would do it now if you see some. The imidacloprid (systemic insecticide ingredient) works best in the middle of spring if it is watered into the soil and the roots take it up.

Even though this agave has a different species name I still consider it a smaller version of Agave americana and has some of the same traits (such as attraction of agave weevil). It is native to the desert Southwest, so it is xeric. See if it produces pups (small plant on rhizomes close to the mother plant) as it gets older. That’s how agave regenerates itself as it gets older.

I would Not change the planting depth but leave it alone. Let it produce a pup and grow a new plant from that. I would not chance that it will root from the stem at that age or that species of agave. I am not sure what will happen except to produce pups as it gets older.

If you water it about every three weeks (good drainage) and fertilize it, it should produce pups by next spring. It’s a good plant. Sometimes Home Depot and Lowes nurseries have some good finds.

Water to a depth of at least 12 to 18 inches when you water.

I would also put them in larger containers. They need extra soil while the container is in direct sunlight. That soil mix is light and would heat up fast. Not the same when they are planted in the ground. Roots of plants are oftentimes less tolerant of hot and cold.

Q: Can you tell me what’s happening with our mastic tree? The tree was planted about five years ago and had been doing well until the last few months. We stopped applying anything when the leaves started changing color.

A: I am thinking the color change in the leaves is from new growth. I think the new growth is filling a hole made when the plant became shrubby again.

Mastic is a Mediterranean tree that grows slowly and naturally to a bush shape that is about 20 tall by 20 feet wide. It requires, at the least, annual pruning when it is young to shape it into a single or multitrunked tree. I think the pruning will lessen with age.

It is best used as a background shrubby tree or shrub and not a smaller version of the larger Chinese pistache. It is not as pretty. It lacks fall color, it is more shrubby and smaller in the pistache family but does have red ornamental nuts when they are young and before they ripen around August.

If this plant were mine and I wanted a small tree instead of being shrubby, as it tends to be, I would start to prune it in the winter or late fall months. Decide whether you want it as a single or multitrunk tree and make the appropriate cuts.

I would expose the trunk or trunks of this tree up to my knees by starting at the bottom of the tree. I would eliminate any growth below my knees and keep any upright growth. If I saw any suckers at the base, I would eliminate them.

While it’s young, I would eliminate any growth growing downward or horizontal. I would concentrate most of my pruning efforts on keeping any upright growth to make it look like a tree and give it some height.

Water it like you would an olive tree. It is mesic in its water use which means you will water more frequently than true desert trees and shrubs.

Shearing this tree with hedge trimmer is a mistake. If you want a tree, then prune it as a tree. If you want it as shrub, then prune it as a shrub. It will attract the leaf-footed plant bug during summer and winter. Fertilize this tree once a year, or maybe twice a year lightly, with a standard landscape fertilizer when it is cooler such as 16-16-16.

Q: I just read your advice column in the RJ this morning about iron shortages in sago palms. We have a similar problem; however, all our sagos are planted in the ground. We have seven of them in our front and backyards for over 15 years and we never saw them yellow like this before. Some are doing better than others.

We have cut down on watering due to the new water restrictions by the Las Vegas Water District but have heard that yellowing is usually caused by overwatering, which is not our case. Just wondering if the extreme heat this year is the cause. Noticed that our neighbor’s sagos are also yellowing. What do you recommend?

A: Let me be clear that I am not positive it’s iron, but suspicious, because that is usually the nutrient in short supply. Iron, manganese and zinc can all cause yellowing of plants growing in our landscapes in the Western U.S., but iron is the most common.

Many nutrients in short supply might cause yellowing including magnesium, which was asked about. The reasons I doubted it was due to a shortage of magnesium was because of its sufficiency in most natural soils, the yellowing didn’t fit the magnesium shortage description, and it would not cause any harm if over-supplied to the soil. If applied to the soil and it solved the problem, it would “kill two birds with one stone.”

There were other suggestions (compost amendments and water) so follow these suggestions first as well as chancing it with a specialty fertilizer.

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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