Q: I purchased a few waxleaf privet plants in February. They were planted in my yard using drip irrigation with one drip emitter per plant for about one month now. I am starting to see some black markings and yellowing on the leaves. I also think there may be some root rot, but I am unsure.
A: I don’t like the idea of only one drip emitter on any plant unless it is very small. Two emitters give you backup in case one plugs. Also, two emitters give you better water distribution around larger plants.
I think you are seeing the results of a harsh climate, harsh desert soils and some rising spring temperatures. It’s probably going to get worse if you don’t make some changes.This is what I would suggest doing.
Waxleaf privet is also called Japanese privet. As this name suggests, it is native to Japan and Korea and that part of the world. It can handle our climate and soils, but it needs a little bit of help.
This help comes in the form of soil improvement at the time of planting, watering it appropriately and using the right surface mulch. It will start to decline in about five years when surrounded by rock surface mulch.
Install two drip emitters per plant rather than just one and put them on either side of the plant: 6 inches when they are first planted and then move to 12 inches away after a couple of years.
Root rot comes from watering too often not by giving it too much water. Whatever you do, don’t water these plants every day. Even when they are first planted, supplement their irrigation by watering daily using a hose for the first few days then let the drip emitters take over.
These plants can handle poor soils but not unimproved desert soils. Hopefully, you added some amendments to the soil at the time of planting.
If these plants are surrounded by rock, get rid of it. Use wood chips or some organic mulch on the surface that can decompose. This decomposition will continue to add organics back to the soil which is what they will prefer.
If these plants are surrounded by an organic surface mulch that rots or decomposes with time, then all they will need it is a fertilizer applied once in the spring.
From the pictures you sent, they don’t appear to have root rot but, still, never water trees and shrubs every day. Always give them one or two days without water between irrigations. This gives the soil a chance to dry out before the next irrigation.
Root rot starts because of poor drainage; soils are full of water, and root rot diseases love continuously wet soil. It will never happen because of giving the plant too much water, only because water is applied too often.
The last issue you need to address as they get older is proper pruning. Most landscapers don’t know how to prune them properly. If pruned with hedge shears, then new growth will get discolored because it grows too close together and shades itself.
Q: What’s the best way to get rid of those ugly green worms with a spine on their rump? I found them now on my grapevines and tomatoes.
A: Thanks for the alert. The best way is hand picking them, but they can be hard to see since they are green. On a calm day, I will stand back and watch the plants for a few minutes. The leaves and stems will move where these larger critters are feeding. Find them where you see this movement and remove them by hand.
Both Bt and spinosad will control if you must spray. The spinosad is a little rougher on honeybees than the BT product, so I usually stay away from spraying plants with flowers when I use that product.
I like Spinosad because it’s one of the few “natural” products that control western flower thrips that are so damaging to nectarine fruit and roses. It can also control leafhoppers on grapes if sprayed when they are young, as well as grape leaf skeletonizer. Except for the honeybee problem, it is more versatile than Bt products.
Q: Can you recommend a treatment for powdery mildew on roses? Is it necessary? The plants look very healthy.
A: Powdery mildew treatments are almost never needed in our desert climate if the roses are growing in full sunlight for at least six hours every day. If you see powdery mildew in the spring because of rain or overcast weather, and if the roses are in enough sunlight each day, it will disappear on its own.
Some varieties of roses are more susceptible to powdery mildew than others. But in our desert climate, as long as the leaves stay dry, they are planted in plenty of sunlight, and they are pruned properly, it shouldn’t make much difference which variety you grow. Keep in mind, however, there are varieties of roses that perform better in our hot desert climates than others.
Never water roses with an overhead sprinkler like those used for lawns. Overhead, sprinkler irrigation wets the leaves and the splashing water causes powdery mildew to spread. If you have no choice in the matter, never water with sprinklers at night. Irrigate them early in the morning, say about 4 or 5 a.m.
When watering with drip irrigation, it makes little difference when watering is done. That being said, it’s always best to irrigate plants so they enter the heat of the day with water around their roots.
If roses are pruned correctly, there will be enough dry air between the leaves and wind movement to eliminate powdery mildew problems.
Q: I converted my grass landscape to desert plants. A bottlebrush plant was planted with mulch about 10 days ago. Its leaves appear very dry. What is going on? Could this be bad stock and should I have it replaced?
A: If the plant looked good at the nursery when you bought it and then it looked bad after planting it, then it was probably planted incorrectly. Mistakes I see in planting include planting them too deeply, not watering them in thoroughly and surrounding new plants with too much surface mulch, whether that’s chips or rock.
Make sure the plant was not planted too deeply or hasn’t sunk after planting. The surface soil in the container should be the same height as the surface soil in its new environment.
A major reason I tell people not to dig planting holes deeply is that the soil settles when plants are watered in. When holes are dug deeply, these plants can sink. If the soil or surface mulch is kept continuously wet, then rotting of the stems and roots can occur; the plant will first yellow and then die.
If you must dig the hole deeper because of drainage problems, then fill the hole to the proper depth and settle the soil at the bottom of the hole before planting. In most of our soils, the hole needs to be dug only as deep as the container. It’s more important to dig the hole wide and amend that soil.
Plants purchased in 5-gallon containers should have two drip emitters, one on each side of the plant. Install three drip emitters on 15-gallon plants. Water these plants with a hose two or three times after planting and before turning the irrigation over to the drip emitters.
It’s OK to have the drip emitters 6 inches from the plants when they are first put in the ground. But these drip emitters should be moved further from the plant to about 12 inches the following year or as they get larger.
Never water woody plants daily. This includes immediately after they’ve been planted. If they need to be watered the first few times after planting, do it with a hose. Always give them at least one day between irrigations so the roots can dry out and breathe.
When surrounding these plants with a surface mulch, whether it’s chips or rock, keep this mulch a few inches away from the stems so they don’t stay wet. The first year after planting is a critical year for plant establishment. It’s tough enough for them to get established in our desert environment. They don’t need to have the extra headache of staying alive because the stems are surrounded by a mulch they’ve never contended with before.
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.