I would avoid Tipu - I have one and it develops a wide and broad canopy... mine has only been in the ground a few years and it's already 25 ft tall, and has a canopy that's almost 20ft wide as well... check out some of the acacia trees, salicina and saligna... both go more upright vs wide... one is greener than the other, both stay green all year round, low liter, low water, and very very fast growth. Arizona ash can be good when young, will drop leaves in a frost, but eventually they get huge... I have the same scenario as you with postage stamp lot and living just outside the heat island here in Maricopa, outside of Phoenix. Good luck with your tree hunting!

Might want to check into the Chinese Elm. I was told by a landscaper they do quite well here once established. They are also low litter. He said to drive a pipe in about 3 feet that can be capped to keep trash out or to mow. Water through the pipe to get them deep watered. They have a few where I work and they look like very nice trees, grass will still grow under them and they put off a nice shade.

Gemfire

chinese elm sheds leaves in winter, its lot of litter in winter. All other seasons they look lush green, fast growing provides lot of shade/provacy. They can get huge, but you can keep it trimmed to fit the yard. Sisso is tall, lighter green leaves, they grow upright and grows fast. they shed some leaves in winter when temps get to 26-28 deg lows.

Neoconman,

For great information on allergies and asthma, check out the University of Arizona web site at the link below. Michael J. Schumacher has compiled extensive information on this topic and has lists of plants that are low or non-allergenic.

A general rule of thumb when it comes to plants is that if they have large showy flowers, they have heavy pollen which is less likely to affect people. These plants rely on insects and other pollinators to move the pollen from flower to flower.

Plants with small, inconspicuous flowers (ash, mulberry, Bermuda grass) produce 'light' pollen and rely on the wind to move it. These are the worst allergy offenders.

You can always take a sample of a plant you like to a local nursery or to the U of A Cooperative Extension office. The main location is in Phoenix (44th St. and Broadway) but the Master Gardeners have satellite offices in the far east, north and west parts of the Valley.

I hope this helps.

Here is a link that might be useful: Allergy and Asthma in the Southwest