Native trees worth planting for shade in a Phoenix yard

Shade is the only thing in a desert landscape that gets cheaper over time.

Everything else a homeowner does about the heat costs money every month: a bigger cooling unit, a second shade screen, more water. A tree costs money once and then does the same work year after year, at a scale no piece of equipment can match. Which is why the choice of which tree is worth more thought than the rest of the yard put together.

But shade trees in Phoenix are asked to do something unusual. They have to survive the thing they are being planted to relieve. A tree that dies in its third summer has shaded nothing.

What a tree has to survive to get there

The Phoenix year puts three tests in front of a young tree.

The first is heat, and it is long. From 31 May to 18 September the normal afternoon high is at or above 100 °F, a run of 111 days. Inside that, 18 June to 22 August stays at or above 105 °F, with July’s normal high at 106.5 °F and the normal low at 84.5 °F. There is no cool night in the middle of that to let a plant recover; the tree spends nine weeks with no part of the day below the mid-eighties.

The second is water, or rather the shortage of it. The annual rainfall normal is 7.22 inches, and the three months from April to June, the ramp into summer, deliver 0.22, 0.13 and 0.02 inches respectively. A tree planted in March without an irrigation line to it is being asked to establish itself through the driest quarter of the year.

The third test is the one newcomers miss, because it is the opposite of the first two. Some of these trees are at the edge of their range in the cold. The station record has 511 nights at or below 32 °F behind it and a coldest January night of 17.1 °F, and that number is why the hardiness column in the table below is not a formality.

The seven trees, compared

These are the desert trees AMWUA lists for Arizona landscapes, with its own figures for each. Every number in the table is from the association’s plant profiles.

Read the growth-rate column before the size column, because the two together decide what the tree is like to live with. A velvet mesquite at 25 by 25 ft is a large tree by any standard, and it gets there at a moderate to fast rate. An ironwood of exactly the same mature size is slow, which means a decade of waiting and a tree that will outlive the mortgage. An ironwood is also, in AMWUA’s wording, a keystone Sonoran Desert species (the sort of plant that other desert species depend on) which is a reason to plant one that has nothing to do with shade.

The hardiness column is where the list separates. Velvet mesquite and desert willow are rated to 0 °F, which is beyond anything the station record has produced. Palo brea at 25 °F is the marginal one: a 25 °F night has happened in November and March in the record, and a hard January would test it. If your yard is on the cold side of the Valley, that is the number to weigh before buying.

Texas ebony is the outlier on origin: it is not a Sonoran Desert plant at all, as the AMWUA profile states, coming from southern Texas and Mexico. It is on the Arizona list for the qualities the table shows: very low water use, a moderate 20 by 15 ft, and evergreen foliage. AMWUA also notes one practical caution worth repeating, that the bark can burn in strong sun if the tree is heavily pruned. Pruning a desert tree hard in this climate exposes wood that has spent its life in shade.

Foothills palo verde is the small one, 15 by 15 ft, and the one to reach for if the space is a front-yard strip rather than a yard. AMWUA describes its mature form as shrub-like, with sharp branch tips rather than thorns. It is also the slowest of the three palo verdes, so the smallest tree is not automatically the quickest small tree.

How far the shade actually reaches

Here is the figure that changes how people plant, and it is in the chart above.

At solar noon in June the sun at Phoenix is almost 80 degrees above the horizon. A shadow cast by a sun that high is shorter than the object casting it: the multiple is 0.18. For a 25 ft tree, that is about 5 ft of shade at midday in the hottest month of the year.

The same tree in December, with the noon sun at about 33 degrees, throws a shadow 1.5 times its height, roughly 38 ft from a 25 ft canopy. The tree that barely shades its own trunk in July drapes the whole yard in December.

This is not an argument against planting. It is an argument for not measuring a tree by its noon shadow in summer. The shade a desert tree delivers in the hot months is not under it; it is beside it, in the morning and the late afternoon, when the sun is low enough for a shadow to stretch. On 15 July the sun at 4 pm is 43.3 degrees high and due west, so a 25 ft tree is laying down about 27 ft of shade straight to the east of itself: across the yard, at the hour the west wall of the house has begun to heat. By 6 pm, at 18.5 degrees and west-northwest, the shadow is over 70 ft long.

That is the practical case for the western side of the house, and the Sun Path & Shade Finder will give you the bearing and altitude for any date so you can check it against your own walls. The companion piece on reading the sun across a yard works through the arithmetic in more detail.

Where to put it, and what the table cannot tell you

Two rules come out of the numbers, and both are about the afternoon rather than midday. Plant to the west of anything you want cooled, because the western afternoon sun is the load that a shade tree is best at intercepting. And plant for the tree’s mature spread, not its nursery size: a 30 ft palo verde planted 6 ft from a block wall will be in the wall’s business within a decade, and the arborist’s answer to that is usually to remove the tree rather than the wall.

Beyond that, the table has limits and it is worth saying what they are. Water-use ratings are landscape ratings for an established plant in the ground, not figures for a sapling in its first two summers, which needs more water than the mature tree ever will. Growth rate is a relative rating, not a measurement in feet per year, and a fast tree in a low-water yard grows slower than the same tree on a lawn. The table says nothing about litter, pollen, thorn size or how a species behaves in wind, all of which matter more to some households than shade does.

For the ones that are not on this list, the Desert Plant Picker filters a wider set of desert trees, shrubs and perennials on sun, water need, mature size and pet safety. It is the right place to start if the seven above do not fit the space you have.