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Friday, October 9, 2015

Cause of Ragged-Looking Japanese Privet Leaves

Q. I have in front of my house 5 big shrubs. Unfortunately I have no idea what they are called. They are getting water over our irrigation system.Today I took a closer look to my shrubs and found all 5 shrubs in really bad condition.
1. The leafs are full of white, at the edge brown dents, but only on the upper side.
2. I've noticed a lot of bees around and inside one of the shrubs.Perhaps cutter bees but I don't have  circled bites on my leafs
3. On one shrub I have a small white net, perhaps a spider net?

Here are my questions: Can you tell me the name of my shrubs and do you have any idea what my problem is?

A. The plant you asked me to identify in the pictures is called wax leaf or Japanese privet. It is not a desert adapted plant so it should be managed the same as other plants which are not desert adapted. It is native to Japan and Korea and so it prefers non-desert soils, medium light intensity, cooler temperatures and rainfall.

It is planted quite a bit in our desert and it always tends to look a little bad because it doesn't like it here much. It prefers climates that are not desert. This is a plant that should not be grown in rock surface mulch but in the wood chip surface mulch instead.

The usual reasons for this plant to decline in appearance is because the soil declines or becomes mineralized because of the loss of organics in the soil over time. The other reason it may tend to look badly is because of poor irrigation management.

If rock surface mulch is surrounding these plants please pull it away from them down to bare soil. I would put about half a cubic foot of compost at the base of each plant and lightly work it into the soil and irrigate thoroughly.

Next, I would check to make sure the irrigation is working correctly and there are enough drip emitters for each plant. Each plant should have at least two drip emitters located about 12 inches from the main trunk.

I would apply about 3 to 4 inches of wood chip mulch, not bark mulch, around all of the plants. Remove by pruning any parts of the plant that are ugly in appearance.

Fertilize the plants once a year in January with an all-purpose tree and shrub fertilizer. The easiest way to apply it is put a handful of fertilizer next to each drip emitter and water it into the soil. Irrigate these plants as you would any other trees and shrubs that are non-desert plants.

Follow-up Q. So, your opinion is that all the white dents (scars) on the leafs are a  fertilizing and irrigation problem? I have no fungus or bugs? And also the cutter bees inside the shrub are not connected to the visual problems ? Should I buy some insecticide?

Followup A. I know it seems improbable that irrigation could cause leaf damage like some of what you are seeing and I agree that is not the only problem. However, my experience with wax leaf privet is that they are out of their climate zone when grown in Las Vegas and growing them here puts a lot of stress on these plants.

The way to decrease the stress on these plants is to give them better growing conditions. This means soil improvement, planting them in the right light and heat exposure in a landscape, using organic surface mulches to continually improve the soil and maintain soil moisture and elimination of rock surface mulches that tend to mineralize the soil.

Once you begin to improve their health and decrease their stress most of the problems that you see will disappear. Then we can start looking for other things that are causing problems to these plants but the major problem you are seeing is due to their environment, either natural or manmade.

We do not have a lot of disease and insect problems on these plants. The diseases that we do have on these plants are from watering too often and poor drainage of water from around the roots. If these same plants were grown in central Japan where they are native you would not see the majority of these problems.

Las Vegas is not central Japan. Create an environment in your home landscape that is closer to the environment where they originate; soils have higher organic matter, light intensity is less, temperatures are cooler, humidity is higher. Try to re-create as much of this environment as you possibly can in Las Vegas; plant on the north and east sides of buildings where they receive shade from late afternoon sun, add organic material back to the soil, avoid rock surface mulches, etc.

When growing plants in the desert that don't belong here you must try to find locations that emulate where they came from as closely as possible. Apply management practices that emulate their original envrionment. Insects and diseases are not the majority of the problems with this plant.

By the way, the attraction of bees to this plant is typically an indicator that one of the sap sucking insects that secrete honeydew is present; aphids, whiteflies, mealybugs or scales. The bee problem could be cleaned up with sprays of soap and water on the undersides of leaves. After the plants are in better health then start working on the bee problem.

1 comment:

  1. The net that looks spider cobweb-ish is likely a fungus. I have seen that in Pahrump on Mondel pines and occasionally on other plants with waxy leaves. I remove the segment of branch where it is growing, bag it, and put it in the cart for trash pickup.