Common Gardening Questions for Prescott, Arizona

How much should I water?
This is probably the most common question we are asked and the most difficult to answer.  How much to water depends on many factors, including the type of soil, weather, and the type of plant.  Please see the following link for watering information from the Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona: http://cals.arizona.edu/pubs/water/az1298

Which plants require the least amount of water?

We define Water-Wise Plants as those requiring moderate or less than moderate water, once they are established.

We use Sunset Western Garden Book as our primary source to determine watering guidelines.

Those plants that need Little Water are the most drought tolerant plants.

Those plants that need Moderate Water require less than regular moisture - which may mean a soaking every 2 to 3 weeks. In exposed, windy sites or in extreme heat they may need watered more often.

Please note that Water-wise does not necessarily mean drought-tolerant! If the plant requires moderate water, it will probably suffer if not given some supplemental irrigation during drought periods.

Click here to see a list of our Water-wise Plants. This list does not address other needs – for instance, some of these plants may require a rich, loamy soil and would not fare well in a lean, infertile soil.  Please ask our staff about any special needs for the plants you are interested in.

What can I spray so my plants won’t get bugs? 
There is no preventative spray to keep bugs off of plants.  Pesticides are only effective when a bug is present. 

What is dormant oil and when do I use it?
Dormant oil is a refined petroleum oil and is intended for use when a plant is dormant (during late fall and winter). Dormant oil kills over-wintering insects on trees and shrubs.  It is best to apply dormant oil when the temperature will stay above freezing for 24 hours.
Dormant oil is effective against: 

  • Aphids that curl leaves in spring

  • Caterpillars that winter as eggs on the plant (leafrollers, tent caterpillars)

  • Mites that winter on the plant (e.g., conifer-infesting species)

  • Scale Insects (e.g., pine needle scale, striped pine scale, Kermes scale, cottony maple scale)

Although dormant oil can be applied after bud break there is a risk of harming young leaves. There are new oils on the market, referred to as horticultural oils or summer oils, which can be used more safely during the growing season.

How do I prevent peach leaf curl from re-occuring?
If peach leaf curl has been a problem in the past, then you can spray your peach trees with lime sulfur during the dormant period or you can use a fruit tree spray during the growing season.  Peach leaf curl appears as a thickening, curling and puckering of leaves. Fruits become swollen and deformed. 

It is summer – why are the leaves on my tree brown?
The most likely culprit is leaf scorch.  Leaf scorch is most common when the weather is hot, dry and windy. Plants often need extra water during these times to ensure that they have enough moisture to combat the rate of evaporation.

Leaf scorch can also be caused by a lack of oxygen in soil that is too wet.  This interferes with water uptake. 

Check your soil before watering – insert a probe several inches (at least 6”) into the soil and let it sit for several minutes.  If the soil on the probe is dry then it’s OK to water, it the soil is moist then wait a day or two to water. Remember – infrequent, deep watering is best.

What is the difference between an annual and a perennial?
An annual completes its entire life cycle from seed to flower to seed within a single growing season. All roots, stems and leaves of the plant die annually. Only the dormant seed bridges the gap between one generation and the next. Petunias and marigolds are well-known annuals.

A perennial lives for many seasons. The upper portion (leaves and stems) dies back during the winter but then regrows from the same root system the following year. 

Because nothing is ever simple in the plant world there are also:
Half-hardy annuals that are perennial in warm winter regions but are treated as annuals in cold winter regions. They can survive some frost and may even survive a mild winter. Osteospernum and Snapdragons are examples of half-hardy annuals.
Frost tender perennials would also be treated as annuals in cold winter regions because they will be killed by a heavy frost.

What zone are we in?
The most commonly used zones in North America are the USDA Plant Hardiness Zones which divide the United States and southern Canada into 11 areas based on a 10 degree Fahrenheit difference in the average annual minimum temperature.

Depending on the elevation at which you live in the Prescott area, your USDA Plant Hardiness zone would be either: Zone 6a: -10/-5F minimum temperatures or Zone 6b: -5/0F minimum temperatures . Lower elevations can be a Zone 7: 0/10F minimum temperatures.

With this said, the USDA Plant Hardiness Zones have not been updated since 1990. The National Arbor Day Foundation has developed an updated Plant Hardiness Zone Map based on the most recent 15 years' data available from the National Oceanic and Atmospheric Administration's 5,000 National Climatic Data Center cooperative stations across the United States. This new map is not an official government document and its release has caused some controversy. Using this map, Prescott is in Zone 7: 0/10F minimum temperatures.

Although this is confusing, if you’re a conservative gardener then stick with plants that will over-winter in USDA Zone 6. If you’re a risky gardener then give a few USDA Zone 7 plants a try.

Western gardeners often use Sunset's Western Climate Zone system that divides the Western U.S. into 24 climate zones. This system takes into account winter cold and summer heat, humidity, elevation and terrain, latitude, and varying degrees of continental and marine influence on local climate.

The Prescott area lies within Sunset Western Climate Zone 3A in the 2007 edition while older editions put Prescott in Zone 2. Zone 3A has minimum winter temperatures averaging between 15 to 25F. The frost-free season growing season runs betweens 150 to 186 days.

Why are the leaves of my plants yellow?
Yellowing leaves could be the symptom of either nitrogen or iron deficiency.  If the entire leaf is yellow, the problem could be nitrogen, however, in severe cases of iron deficiency, the entire leaf could be yellow, too.

To determine whether your plant is iron or nitrogen deficient look at the location of the yellowing leaves. Iron deficiency shows on the new growth toward the tips. The veins of the leaf stay green while the leaf tissue turns yellow.  Nitrogen occurs from the bottom of the plant on the old growth.

Iron deficiency is fairly common in highly alkaline soils as the high pH of the soil ties up the iron in the soil, making it unavailable to plants. High soil temperature and high light intensity can also increase problems with iron deficiency.  

Adding liquid iron is a quick-fix and should only be used in severe cases. Otherwise, chelated iron is longer lasting.  Iron products should be applied at the drip line of the tree. Avoid contact with sidewalks, patios etc… as staining may occur.

If my tree or shrub is unhealthy, should I fertilize it?
If a plant is showing signs of stress such as:

  • Light green or yellow leaves

  • Leaves with dead spots

  • Leaves smaller than normal

  • Fewer leaves and/or flowers than normal

  • Short annual twig growth

  • Dying back of branches at the tips

  • Wilting of foliage

Then it is best to determine the cause of the stress rather than applying fertilizer and hoping for the best.  Stress can be caused by many factors, including but not limited to: inadequate soil aeration, too much or too little water; adverse climatic conditions (excessive heat, drying winds); improper planting; high soil pH; pests and disease.

After you have resolved the underlying problem and taken measures to correct it, then a light application of fertilizer may help the plant recover from stress.

When is the best time to plant?
Transplanting is less stressful for plants during the cooler months of spring and fall or during the monsoon season if we are getting rain.  If you plant during the hot, dry summer months then you must be very diligent about watering to help your plant survive. 


What is the white dust-like covering on the leaves of my plants?
From mid-summer to fall you might see a dusty white coating on the leaves of some plants – this is powdery mildew. The fungi that cause powdery mildew are quite common in semi-arid areas. The spores of powdery mildew do not require standing water for germination, instead high humidity (like during monsoon season), shady locations and crowded plantings with poor air circulation will all promote spore germination.

Powdery mildew occurs on numerous plants but here are some of the most common: roses, lilacs, squash, cucumbers, melons and gourds.

Some ways to reduce the occurrence of powdery mildew:

  1. Avoid overcrowding your plants

  2. Avoid overhead watering, especially in the evenings

If the fungus is already present:

  1. Avoid late-season applications of nitrogen fertilizer as the fungi enjoy new, succulent growth

  2. Remove and destroy infected plant material.  Do not compost infected plant material.

  3. Prune overcrowded plants to allow for better air circulation.

  4. If cultural controls are not sufficient, you can apply sulfur or neem oil

Are there any plants the deer, rabbits or javelina won’t eat?
Although there are plants, often referred to as “deer resistant, rabbit resistant and javelina resistant”, that these critters do not prefer, there are none that are 100% full- proof. They may resort to these plants if there is not enough natural forage.

Ask for lists of these plants when you visit the nursery. 

Grasshoppers!  How do I get rid of them?
Here are some products you can use to control grasshoppers. All products are most effective on nymphs rather than adults so effective control must include the breeding areas.  Grasshoppers typically breed in dry, undisturbed areas, such as pastures, empty lots and roadsides.  Eggs generally hatch in late June through July. You can identify breeding areas by searching your property during the hatching season to detect areas with high concentrations of nymphs.

  1. Nolo Bait: a biological insecticide that suppresses grasshoppers and Mormom crickets. Nolo Bait contains naturally occurring spores of a protozoan called Nosema locustae. The spores are applied to wheat bran which acts as bait. When grasshoppers consume the bran they become infected with the spores. Young grasshoppers will die from the infection while older grasshoppers will become lethargic and will reduce their feeding. Infected grasshoppers will be cannibalized by healthy grasshoppers who will then become infected themselves. Nolo Bait can be used around humans, pets, birds, wildlife, and beneficial insects; it will not contaminate waterways. It can take at least a year to effect populations.

  2. Safer Brand Insect Killing Soap with seaweed extract: certified by OMRI (Organic Materials Review Institute).

  3. Diatomaceous Earth: a lethal dust of razor-sharp shards of silica (feels and looks like talcum powder to humans). It scratches and absorbs an insect’s wax coating so the insect dies of dehydration. Wear a mask when working with this product. 

  4. Hi-Yield Permethrin Concentrate: contains 35% permethrin. Can be used in lawn areas.

  5. Ferti-lome Eliminate:  a granular insecticide that contains Carbaryl. Can be used around ornamentals and vegetables

Should I add sand to my clay soil?
Sand is not effective in loosening clay soil. In fact, mixing sand with clay soil will turn that soil into a rock-hard substance.  It is better to use organic matter, such as compost or well-composted bark.  Adding these products will not only help aerate clay soil but they also improve the nutrient quality of the soil.

Do I need to water drought-tolerant plants when I first plant them?
Yes.  All plants need water to help their roots become established. It is best to give them moderate to regular water during their first season. During their second season they can be slowly weaned to little to moderate water.  Native plants can be weaned to a deep monthly soaking once they are well-established (except if they are a native riparian plant that grows along waterways in which case they require regular water to mimic their  native habitat).

Do I need to water over the winter?
Yes.  Plants are dormant during the winter but because our soil does not freeze the roots still require water.  Water trees and shrubs deeply at least once a month but no more than twice a month. 

Should I use pruning sealer when I prune trees?
Trees heal better without an application of pruning sealer. In most cases, applying the sealer causes faster decay of the tree’s heartwood because the sealer holds moisture near the new wound – creating the perfect environment for fungal decay organisms. Make a clean cut just outside the branch collar and leave the wound open to air to heal. 

Are there plants that are more fire-resistant than others?
Yes, some plants have certain characteristics that make them less likely to burn in a wildfire. These characteristics are: 1) Leaves are moist and supple; 2) The plant generally produces little dead wood and tends not to accumulate dry, dead material within the plant, and 3) Sap is water-like and does not have a strong odor.  Characteristics of highly flammable plants include: 1) Plant contains fine, dry or dead material within the plant such as twigs, needles, and leaves; 2) Leaves, twigs and stems contain volatile waxes, terpenes, or oils; 3) Leaves are aromatic; 4) Sap is gummy, resinous and has a strong odor, and 5) May have loose or papery bark.  (This information is adapted from “Fire-Resistant Plants for Oregon Home Landscapes” by Oregon State University Extension Service). 

Why aren't the buds of my roses and fruit trees opening?

The culprit is probably thrips.  Thrips colonize  buds, furled leaves and other enclosed plant parts. The symptom most commonly noticed is buds that never open and that often have blackened tissue on their edges. 

To check for thrips tap a possibly infected branch against a piece of white paper.  If thrips are present you'll see their very tiny bodies on the paper.  

Infected parts of the plant should be pruned off and, if necessary, the plant can be sprayed with insecticide.  Neem oil based insectides are very effective against thrips. Before sparying, keep in mind that most ornamental plants can tolerate a thrip infestation and new unaffected leaves will emerge in warm summer weather. However, thrips can prevent fruit from setting on fruit trees so it is best to monitor for thrips early  and take action as soon as possible.