Pruning should be done to maintain plant health (remove dead, damaged or diseased portions, cross branching, etc.), to highlight the “natural” shape of the plant, to train a young plant, and to eliminate hazards. Excessive or heavy pruning causes significant stress to trees and shrubs. The best practices are to prune the least amount necessary and prune for legitimate reasons. How much to prune depends on the size, species, age, as well as your intentions. Two good principles to remember–a tree or shrub can recover from several small pruning wounds faster than from a single large wound and never remove more than 25% of the canopy in a year. For more information register for a Garden class offered on pruning that will teach you the proper pruning techniques for trees and shrubs or visit http://www.treesaregood.org for information on proper pruning of young and mature trees.
If necessary, native and desert-adapted spring-flowering shrubs can be pruned after flowering has diminished. However, prune before the upcoming hot, summer months.
Lightly prune your Mesquites (Prosopis spp.) and Palo Verdes (Parkinsonia spp.) to remove dead and crossing branches.
Pruning newly planted trees and shrubs is not recommended and in fact, can be detrimental. However, at planting time prune broke or torn and diseased branches. Save other pruning activities for the second or third year. For more information on developing a healthy tree visit http://www.treesaregood.org
Prune Prickly-pears (Opuntia spp.) or Chollas (Cylindropuntia) after flowering. If pruning your prickly-pears or chollas to maintain size, for propagation or to remove a damaged or diseased stem, prune at the joint or segment. Use a sharp, clean pruning tool and spray tool periodically with a 70% alcohol solution to prevent infection. If the pruned stem is to be used for propagation, allow the cutting to dry out for a week before planting.
Continue to remove spent stalks of aloes, agaves, and other succulents.
Spring-blooming perennials such as Penstemons (Penstemon spp.), Brittlebush (Encelia farinosa), Globemallow (Sphaeralcea ambigua), Fleabane (Erigeron divergens), and Golden Columbine (Aquilegia chrysantha) should be producing seed. Allow them to produce seed before pruning spent flowering stems. The seed is a valuable food source for many animals.
Deadhead herbaceous perennials to encourage continued flowering including Desert Marigold (Baileya multiradiata), Chocolate Flower (Berlandiera lyrata), Red Sage (Salvia coccinea), Mealy-cup Sage (Salvia farinacea), Gaura (Gaura lindheimeri), Tickseeds (Coreopsis spp.), Angelita-daisy (Tetraneuris acaulis), and Gaillardia (Gaillardia pulchella).
Most native and desert-adapted plants in the landscape do not generally require fertilizer as they are adapted to our soil conditions. In most cases, fertilizers are generally applied to prevent deficiencies. If fertilizers are needed, one application for the year is usually sufficient. If you did not fertilize in March or in April, go ahead and fertilize your landscape plants if necessary. However, it is recommended to fertilize your landscape plants the early part of May. For more information on fertilizing and plant deficiencies go to the University of Arizona Cooperative Extension Publications: http://ag.arizona.edu/pubs/garden/az1020.pdf and /sites/dbg.dd/files/nutrient_deficiencies.az1106.pdf0_s.pdf
Periodically fertilizing may be needed for plants in containers as nutrients in the soil will have diminished over time. Always follow directions on the label.
Continue to fertilize your warm-season annuals and herbaceous and woody perennials in containers if necessary.
Cacti and warm-season succulents in containers should be fertilized at least once during the month. Do not fertilize any winter-growing succulents such as Live Forever (Dudleya saxosa ssp. collomiae), Succulent Geraniums (Pelargonium spp.), Ice plants (Malephora spp.,Drosanthemum spp., Cephalophyllum spp.), Living Stones (Lithops spp.) and crassulaceous plants (Kalanchoe spp., Cotyledon spp., Echeveriaspp.) as they are now undergoing their summer dormant period.
Continue to fertilize your vegetable and herb garden as needed.
A noticeable, fine web may be present on your Palo Verde trees (Parkinsonia spp.) and even from time to time on the Whitethorn Acacia (Vachellia constricta syn. Acacia constricta). This “webbing” is produced by the Palo Verde webworms often called Palo Verde webbers. The webworm is a small caterpillar that feeds on the leaves and occasionally the bark of the small stems. The Palo Verde tree and Whitethorn Acacia are resilient to webworm infestations so control methods are unnecessary. The caterpillars and adult moths are an important food source for many lizards and birds.
If you notice a tattered appearance on your landscape plants such as Evening Primroses (Oenothera spp.), Sacred Datura (Datura wrightii) and Gaura (Gaura lindheimeri) it may be the flea beetle in action. A different species of flea beetle may also harm your vegetables including tomatoes and eggplants. The flea beetle larvae and adults can be destructive and they can be difficult to control. To find out more information on flea beetles go to the following link: http://ag.arizona.edu/maricopa/garden/html/t-tips/bugs/flea-btl.htm
Agave snout weevils become active during the warm months and infestation may not be apparent until it is too late. For detailed information on the life cycle, symptoms, prevention, and treatment go to our Agave Snout Weevil Desert Gardening Guide.
While sitting underneath your desert trees, you may notice a light “rain” falling. This is the smoke tree sharpshooter insect expelling sap as it draws from a variety of plants including Palo Verdes (Parkinsonia spp.), Beebrush (Aloysia gratissima), Bird of Paradise species (Caesalpinia spp.) and Hackberries (Celtis spp.). These sharpshooter insects have gone virtually unnoticed until they threatened oleanders, shrubs widely used in low-desert landscapes for screening hedges. They gained notoriety because they are able to transmit a deadly bacterial disease called oleander leaf scorch. Our native plants do not appear to be adversely affected by the smoke tree sharpshooter so no control is necessary. To find out more about oleander leaf scorch check out the University of Arizona Extension Plant Pathology at http://cals.arizona.edu/PLP/plpext/diseases/trees/oleander/oleleaf.htm
As the weather warms, whiteflies may be present on your landscape, vegetable and herb plants. These tiny, white insects have sucking mouthparts that cause leaves to yellow, wilt and drop prematurely. The immature nymph stage does more harm to the plant than the adult. Whiteflies can be difficult to control. Allow natural predators such as spiders, ladybugs, lacewings and even hummingbirds to control the population. Yellow sticky traps can also be used to control the adult population.
Noticeable leaf damage may be seen on the Texas Mountain-laurel (Calia secundiflora syn. Sophora secundiflora) during the warm months. The damage is caused by the sophora pyralid caterpillars feeding on the tender new growth. These ravenous caterpillars are approximately an inch long with orange bodies and interesting black spots with white hairs.
Psyllids can still be active during the month of May, but activity will decrease as the temperatures climb. Psyllids are sap feeders and many are plant specific or feed on a closely related group of plants. High populations of psyllids can cause distortion and die back of new growth, and in some cases defoliation. To keep populations under control do not overwater or over fertilize your plants as this causes excessive growth. Yellow sticky traps can also be used to control the adult population.
Cochineal scale, the cottony, white substance on your Prickly-pears (Opuntia spp.) and Chollas (Cylindropunita spp.) may be active now. Remove by using a fast stream of water or spray insecticidal soap.
Fine webbing between leaves or stippling on leaves may indicate the presence of spider mites. These plant mites cause damage by sucking contents from the leaves and are difficult to detect due to their small size. Plants that are water stressed may become susceptible to infestation. Dusty conditions can also lead to spider mite outbreaks. Make sure your plants are well-watered and wash off accumulated dust on plants to manage spider mite problems. You can also remove by using a fast spray of water or by spraying insecticidal soap to control populations. There are many biological controls that feed on spider mites including lacewings, predatory mites, lady bugs and big-eyed bugs. Using insecticides is not recommended as insecticides do not help manage the population, but can actually cause the population to intensify because insecticides used will often kill their natural enemies. Some insecticides can even accelerate mite reproduction.
If you notice a rank odor and black ooze dripping down the saguaro stem(s), the plant may have developed an infection as a result of an injury or frost damage. The infection is caused by the Erwinia bacteria, a common bacterium found in the environment.
For more information on diseases or problems of landscape plants go to the University of Arizona Cooperative Extension publication/sites/dbg.dd/files/pldiseases_urban-1124.pdf or the University of California UC IPM Online athttp://www.ipm.ucdavis.edu/PMG/menu.homegarden.html.