So here are some tips on what to do in May from the University of Arizona College of Agriculture Maricopa County Extension - I omitted the part on turf which is of no interest to me. There's also a lot of other helpful informaion and FAQ's on that website. Another site of interest on what to do in the garden here in the low desert is John Chapman's site.
To Do List . . .
Black eyed Peas, Melons (Cantaloupe, Muskmelon), Okra, Sunflowers
Jerusalem Artichokes, Sweet Potatoes
Place shade cloth over tomatoes.
Continue fertilizing established roses, liquid fertilizers can be added at 2 week intervals, follow the directions on the container.
Fruit and Nut Trees
Plant Citrus Trees - Young two to five year old trees transplant most successfully. Larger, older trees are more costly, harder to transplant without injury (to yourself and the tree), and suffer more from transplant shock. It will generally be three years after transplant before fruit production and that is the same whether you plant a 2 year old tree or a 10 year old tree. Go small!
Pick early-maturing deciduous fruit varieties, which are particularly prone to bird damage, before full maturity. Ripened at room temperature to lessen the bird peck loss.
Cover fruit trees to protect from birds
Give special attention to watering deciduous fruit trees, provide adequate soil moisture for fruit sizing in the late April and May period.
Apply nitrogen and zinc to pecan trees to produce normal size leaf growth and to enhance kernel development. Pecans also need more water than most other shade trees.
Increase water application as the weather warms.
Tree water use, desert types being the exception, increases rapidly during this period of leafing out and gradually higher air temperatures.
Apply mulch to the ground around heat sensitive plants keep the roots cooler and prevent evaporation. Be sure to keep the mulch several inches away from the trunk to prevent pest and disease problems.
Apply chelated iron to bottle brush, pyracantha, silk oak, and other plants with iron deficiency symptoms.
Prune palms when flower spathes show or delay pruning until after the palm has finished flowering to prevent infestation of Palm Flower caterpillars. If palms are pruned in the spring, leave the top five rows of peels so the caterpillars have a place to hide.
Don't List . . .
Do not prune citrus except to remove dead or damaged wood and branches obstructing pathways, views, or structures.
Do not use pre-emergent herbicides in an area in which you intend to plant seeds.