High temperatures can affect pollination, fruit set

Q: My friends are giving me grief because I can’t grow zucchini or other squashes. I get female flowers and I have male flowers too. We seem to have insects around enough to pollinate other plants. But zucchini fruit withers at about large grape size. What can I do to become one of those zucchini and squash growers who have so much they can’t even give it all away?

A: It is possible that they are not open at the same time. Having more than one plant should solve that problem. During high temperatures, pollination and fruit set can be a problem because of sterile pollen.

If the plant has a very tight canopy and bees have a hard time getting to the flowers then this could prevent good fruiting as well. I will sometimes remove leaves on very compact zucchini plants to open it up a bit.

It is also possible that you have a variety that is not a good performer in our climate. Not all varieties will perform well in the desert.

You may have to act like a bee and do some hand pollination. This requires a soft paint brush or a Q-tip and transferring the pollen from the male flower to the female flower. This is a pretty good video on hand-pollinating zucchini: https://www.youtube.com/watch?v=FCVIu82eXGY.

Q: I have two African sumac trees and the past few weeks many of the leaves are dropping a lot each day and they are dry or yellow in color. I am wondering if it is not getting enough water or if with the heat that may be its problem.

A: First of all, African sumac is a messy tree. It can drop a lot of litter on the ground. The female trees can drop a lot of seed. Sparrows and mockingbirds love the fruit and will spread seed everywhere with seed sprouts given to your neighbors.

If the tree is in rock landscape and on drip irrigation it might be a lack of water. Some people give trees a small amount of water daily. You do not want to water daily but every two to three days and deeply in this heat at the most.

If you do not think you are watering too often then I would run a hose out to it and let it get an extra 20 to 30 gallons under the canopy once a week. See if that helps over the next few weeks.

It is possible aphids could be feeding on the leaves. Check them to see if leaves are sticky or shiny with honeydew from aphids. If so, spray with soap and water a few times, a few days apart.

The soil will dry out faster if it is covered in rock rather than wood mulch. Bare soil dries out the fastest. This tree performs better in desert landscapes if it has irrigated plants under its canopy.

Q: I have been working compost into my soil and mulching the soil surface trying to improve it. Much of my compost looks kind of mulch-like, but I use it quickly so that I have space to compost my current leaves and grass. Does incompletely composted material draw fertilizer components away from plants?

A: The finer the compost, the faster it will decompose in the garden. I don’t like compost that looks “mulchy” because it decomposes slowly and it can present a problem when preparing soil for vegetables.

Compost for vegetable gardens should be screened, be dark brown or black, smell good and earthy. Composts that have not yet finished should be kept longer before applying it. Composts that are “mulch” can be good for preparing soil for the planting of trees and shrubs.

If compost is not completely broken down it can draw nitrogen out of the soil. Usually in regularly fertilized garden beds I do not worry about that much but I lightly fertilize vegetables once a month to keep them in production.

Q: I have a gardenia that is approximately 4 years old. It blooms yearly and was doing great. All of a sudden, literally overnight, the leaves turned yellow and started falling off. What happened? I water it daily. It is on the patio outside where it has always been and in the shade.

A: I think your gardenia developed root rot. This can be a common problem with gardenia. The symptoms of root rot are yellowing leaves and leaf drop. Frequently the plant dies a relatively slow death unless it’s in the heat, then it would be rapid.

Container plants need to be repotted regularly. When we grow in containers, the soil organic matter (this is the component of the soil that helps keep soil loose and gives good air exchange to the roots) begins disappearing at a steady rate. In a couple of years it will be in critical short supply.

As this soil organic matter disappears, the open spaces in the soils that help with drainage and air exchange diminishes. At the very beginning, a good container soil may contain as much as 50 percent of its volume as pore space.

In about three years, this pore space may drop to only 20 or 15 percent. Basically the soil collapses, losing its pore space. This might be OK for a palm growing in a container but gardenias need soils with lots of pore spaces.

The soil becomes more dense, water drains through it more slowly, the soil stays wet longer, salts begin to accumulate and the roots begin to suffocate.

Soil diseases attack suffocating roots, roots begin to die, leaves begin to yellow and drop from plant.

Soils in containers need to be renewed every two to three years depending on the type of plant. Since gardenia is very susceptible to rots and grows much better in aerated soils, I would repot it no longer than every other year.

I will give you some directions on how to repot it on my blog.

Q: Last year we purchased a potted dwarf Meyer lemon tree. In the fall we had about two dozen delicious lemons. We brought the plant inside for the winter and placed it outside in the early spring. It flowered and produced several lemons. However, they grew no larger than lima beans before falling off.

The tree reflowered and again produced numerous lemons. These grew slightly larger than the last growth but they also fell off the tree.

Now we have no fruit and no flowers but plenty of new leaf growth. What should be done so we can get fruit from the tree, if not this year than certainly next season?

A: This sounds like a watering issue, either too often or not often enough or both. Both types of watering will cause fruit drop.

When you water, make sure water comes out the bottom of the container to keep salts flushed from the soil. About 20 percent of your applied water needs to drain out the bottom.

If you don’t, you will start to see leaf scorching, leaf yellowing and fruit drop. Water the entire surface of the soil in the container, not just the spot where the tree is planted if this is a large container.

Surface mulch on top of the soil will help. Apply a layer of wood mulch to the top of the soil in the container about one inch deep. This will help keep the soil from drying out quickly and give the soil in the container wide fluctuations in soil water content.

Wet, dry, wet, dry cycles may cause fruit drop. Use an inexpensive soil moisture meter, such as one used for houseplants, to help you judge when to water again. I will push a container with my foot or lift a side of it to judge how wet the soil might be.

Don’t forget to repot the lemon every three years to keep the soil “fresh.”

Getting a handle on when to water will be a key to keeping fruit on the tree.

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.