The Q&A Archives: Yellowing leaves on roses

Question: What can I do about my roses? The leaves are turning yellow and falling off. There are some whiteflies and spider mites around now. However, when I spray for these pests it seems to kill the plant. Also, when I spray the leaves with water, the roses get mildew. What can I do?

Answer: Roses grow fairly well in the Phoenix area in general and miniature roses are even easier to grow than your average full size rose. They take the desert heat and sun fairly well and cold weather is not a problem as all types of roses can take much colder temperatures than occur in the lower desert. In fact, roses never go fully dormant here. Roses need regular water, especially in the summer.

While roses prefer full sun to produce flowers, afternoon summer sun will make flowers short lived, so extreme microclimates like Western exposure with reflected sun should be avoided. Roses are fully hardy in the Phoenix area.

Basin or flood irrigation is recommended because it helps keep the salt in our salty water from accumulating around the roots. Furthermore, deep watering will encourage the plant to develop deeper roots, making the plant tougher when the weather gets hot and dry. Plants that like lots of water are happy on the same schedule as your grass. This schedule equals a watering frequency of every other day in the hottest part of summer and every one to two weeks in the coldest part of winter. Other times of the year fall between these two extremes. Definitely do not water any plant (except maybe lilly pads) every day no matter how hot it is. Watering every day is a waste of water and will make most plants sick.

The duration of the watering each time can vary depending on what type of plants you have and how much sun they get. For example, grass does well with 15 minute sprinkler waterings in shadier spots and 25 to 30 minute waterings in sunnier ones. For this reason, water can be saved by having different zones for the sunniest and shadiest parts of your lawn.

In addition to controlling watering frequency and duration, one can also control the rate at which water is applied. For example, sprinklers have screws for adjusting flow and throw, and for plants other than grass bubblers offer precise control of flow rate.

Entire books have been written on the intricacies of fertilizing roses. Fortunately, healthy miniature roses with beautiful flowers can be grown with very little care. A one inch layer of compost applied in the spring and fall can go a long ways in making a rose happy. Nutrients will be supplied as the compost decomposes and it will also help to keep the soil around the rose more moist and cool.

Chemical fertilizers and organic fertilizers can be used as well. It is best to use a bloom producing variety which are generally low in Nitrogen and high in potassium and phosphorus. Keep in mind that Phoenix soils are high in salts already and chemical fertilizers contain salts. Therefore, it is very easy to burn plants with chemicals, so go very light on them. It might be worthwhile to try the pure compost approach first before trying anything stronger, then graduate to organic fertilizers if needed. Organic fertilizers generally provide nutrients by degrading over time and are therefore less likely to damage the plant than chemical fertilizers.

Aphids are the main pest of roses. Fortunately, the aphid season is very short in the lower desert, lasting from the beginning of March to the middle of April. Aphids can usually be ignored on well established plants, because they aren't around long enough to do significant damage. On newly planted roses, they can be either periodically sprayed off with the hose or killed using chemical means. Chemical sprays that include bifenthrin appear to work the best because they continue to kill for over two weeks. One spraying in the spring, when aphids appear, is usually all that is needed. Pesticides should be used minimally if at all.

Best wishes with your roses!

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