COLUMN: GARDENING: Roses need right position to flourish

Question: Can you tell me why my roses do not open fully and look like they have rust on them? Is it because of over watering? Also, the new growth on the tips of the leaves becomes dry and brittle.

Answer: Hopefully, your roses were good quality roses when you purchased them and they are varieties or cultivars recommended for this area.

Second, let's assume you have planted them in a location where they are protected from the hot, late afternoon sun and not in front of a wall or building with lots of reflected heat and light.

Let's make sure the soil was amended with organic matter at the time of planting and mulched to control weeds, conserve water, keep the soil cool, and add organic matter back to the soil. Let's make sure they are not planted in the center of a desert landscape with rock mulch.

Let's next assume that you are using a good quality rose fertilizer at appropriate times of the year and are controlling pests when they occur, and that you are pruning them when they need to be pruned which is typically at the first sign of new growth in the spring.

Last, let's assume you are watering when needed by the plants and not on a daily or twice a day schedule and disregarding when the plant needs water.

With that behind us, I am wondering if the flowers are not opening at any time of the year or if they do not open during the heat of the summer. You mention you use the term roses and I wonder if this is happening on all different types of roses or if it is only on one particular cultivar.

If one type of rose never performs well at any time of the year, I would guess that this one is a cultivar not well suited to our Las Vegas climate. If this problem is occurring across all types of roses, I would guess it may be an exposure problem such as a hot, west or south facing exposure, perhaps a soil problem such as high salts, compaction, or the presence of construction debris, or a management problem such as the wrong watering schedule, a fertilizing plan more appropriate, a pest problem, lack of mulching or too much mulch.

Insects, such as thrips or whiteflies, might cause a problem related to this type of damage. Inspect the leaves and flowers closely for insect pests or damage from insects. If there are pests or you suspect pests, use a rose systemic insecticide to get these critters under control. If it were an insect problem, you would expect it to be a problem across different roses with some cultivars damaged more than other types. If they only show this problem during the heat of the summer, this is most likely a heat-related problem and a relatively normal occurrence for roses. You can extend the time of bloom and reduce midsummer problems by providing some shade during or perhaps frequent misting during the hottest times of the day.

There is a disease on roses called rust but it is relatively easy to identify and control. The reason this disease is called rust is that the disease erupts on the leaf and stem surfaces with pustules that resemble the reddish rust like you might find on a rusty piece of metal. When this disease is active, just like rust on metals, the reddish spores easily rub off on your fingers.

This disease is not particularly common in our climate. It is a disease attacking undernourished roses and common during overcast, wet weather at cooler times of the year.

There are, however, cultivars of roses more susceptible to rust diseases than others. You might have one of these cultivars if you discover that rust disease is a problem. Fungicides are available to help control rust diseases but usually clearing weather, warmer temperatures and appropriate fertilizer applications helps clear up the problem.

My guess is that either you have a cultivar not suited to our area, or your rose is having midsummer, heat-related problems. Check into a better cultivar or move it to a more protected location in the landscape.

Q: I have a row of 2-year-old, 2-foot tall Japanese boxwoods with dark green leaves and always in direct sunlight. They have amended soil, vertical mulched, 1 gph emitters now watering three times per week for 15 minutes per day, and MiracleGro fertilizer about every third week. Watering beyond 15 minutes seems to produce runoff rather absorption.

They do not appear to thrive, as do those on the west and partially shaded side of the property. There is little new growth but there is leaf drop. I see no evidence of bugs on the leaves or branches. The leaf's coloration seems to go from "normal" green to brown quickly and then the leaf drops. The defoliated branches eventually become brittle.

A: Japanese boxwoods make great little trimmed hedges. They take a formal pruning with a hedge shears very nicely. They can add a formal appearance to a landscape. In Las Vegas they are near the upper limits of their summer temperature tolerance so you can use them but with some caution during times of summer stress. They perform better in colder climates.

The term vertical mulching refers to a landscape practice used to improve drainage and aeration in the soils surrounding plant roots. This is normally done when soil drains slowly or plants are growing in low spots that accumulate water or show physical signs of poor drainage.

Drainage holes are dug vertically in the soil to a depth well below the plant's roots. These holes can be dug with a deep soil probe, soil auger, electric drill with an extra long bit, posthole digger for large plants or a water jet or water auger.

When the holes are small in diameter, they are just left open. If the holes are large enough to cause a safety hazard then they must be filled with something like pea gravel or very coarse sand, but not reject sand. These holes act in a manner similar to a French drain.

Let's do the math. From your explanation, I assume they each have one emitter. This means that right now they receive one quart of water at each irrigation and three fourths of a gallon of water per week (one gallon per hour for 15 minutes is one quart).

Just for the sake of visualizing the situation let's picture this two-foot tall boxwood growing in a container. A one-gallon container is too small for a plant this size and a five too big. So let's picture it growing in a three-gallon container. As an example, when we water a plant in a container we want to apply enough water so that water runs out of the bottom.

This provides for leaching of salts and drawing fresh air into the soil to replenish the oxygen needed by plant roots. All this occurs provided there is drainage.

If one half to one fourth of the soil in the container is occupied by empty space, which can hold either water or air, then about three fourths of a gallon to one and one half gallon of water is needed at each irrigation. Applying less than this runs the risk of accumulating salts, suffocating plant roots or both.

This amount of water would be the right amount of water to apply at each irrigation, provided it is applied evenly over the plant's roots, regardless of the time of year. The time of year dictates how frequently this amount of water would need to be applied.

Even though you have vertically mulched these plants, the math indicates the soil is not draining if all that you can apply is 15 minutes of water at a time. If water is not draining, you run the risk that plant roots will suffocate and that soil salts may accumulate. This is not a good situation for boxwood, which has roots fairly sensitive to poor drainage and salts.

I think your alternatives are too further improve the site to provide for even better drainage so you can apply more water at each application. You other choice is to change over to a plant with better tolerance to poor drainage and more heat tolerance.

Bob Morris is a horticulture specialist with the University of Nevada Cooperative Extension.


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