I want to grow trees in rocky desert soil in Phoenix, AZ.

Hello Permies,

I hope to get some feedback on soil building, watering, and sequencing trees in the Arizona desert. I will be building a small house and growing some shade on a South Phoenix, 1-acre urban lot later this year. I am starting with rocky "moon dust" soil and 7" of rain per year. I will soon have city water and would like to make sunken planter cells that are fed by curb cuts when it rains, similar to what Brad Lancaster has promoted in Tucson.

The lot came with a few Ironwood trees and one Palo Verde that are surviving on natural rainfall with a bit of help from compost and wood chips.  Most of the existing trees will get to remain with the plans. There are some chaparral and small cacti that seem to be happy on the land as well. I suspect they may thrive with a little additional water.

The lot has about 7 feet of elevation drop from the SE corner to the NW corner. Eventually, the perimeters and spaces between structures will be filled with trees and plants. When I start the project, I will need to do some earthwork to level and grade building pads and parking areas. My thought was to excavate 3-4 feet deep in areas that will eventually be fruit and shade trees to supply some of the fill material for the lot plan. I thought I could bury longer rows of 3-4" perforated tubing 3-4 feet apart with elbows that allow for watering from above grade to saturate the excavated areas. The fill material could be a mixture of wood chips, compost, topsoil, manure, or possibly volcanic rock as a bed under and above the tubing. My thinking was that if I could water several feet below ground, I could directly feed water into a tree root zone and lose less to evaporation. If similar water-need trees are planted along a tube, they could share a similar ground moisture/watering schedule. If the plan was implemented during earthwork, I could stretch the usefulness of street runoff water when we have heavy rains by placing an inlet watering loop in the sunken planter cells near the main road. I could also feed the Aluminum shingle roof capture runoff, shower grey water, and select washing machine discharge to the loops, sinking those water sources into the soil.  If the ground is unable to suck up the water in heavy rains it should just fill the vertical sections of the pipe and flow as water does with gravity, grade and elevation? Eventually, the grading for the property will sink and slow water from the sky, road, roofs, and land, west and north before spilling over a long level gabion wall where the "natural wash" of the lot exits.  My first intent is to build a space to live, create sunken areas to plant some trees, grow some strategic shade, build a small yard with a sunken garden, and observe what is working.

I would like to start 50-100 mesquite and moringa seedlings in the coming weeks. My hope is to keep 10-20 of the genetic winners for transplant in the fall as well as purchase a couple of mulberry trees for strategic shade. When the pioneer trees provide enough shade, I would like to plant date palms, figs, pomegranates, and citrus. Water, time, energy, and success/failure will determine what trees will be planted and culled in the future.

I run a junk removal company in Phoenix, and have a trailer capable of hauling large quantities of topsoil, manure, volcanic rock, or compost. A friend of mine built his own retort furnace to recycle discarded Mesquite into charcoal. I could have a nearly limitless supply of BioChar for amendment use. Looking for advice and feedback on the sunken perforated tubing watering strategy, as well as feedback on various "soil" mixtures to fill the planter cells with after excavation.

I'll bet doing many of the things Mr. Lancaster has done will get you off to a fantastic start. Have you wandered through the Permies forum Greening the Desert? Also, although he is in Austria, Mr. Sepp Holzer has done projects to help with irrigation in areas all over the world, and his wonderful work would probably provide you with a lot of ideas. Best wishes for your projects!

It sounds pretty sound to me. If you are starting mesquite from seed make sure to treat the seeds by scarring and or a vinegar soak. Mesquite normally germinates AFTER it has gone through the digestive system of a cow or a goat. As in cows and goats eat the pods then poop out the seeds which sprout up in the cow pie--goat dropping.

I also live in AZ in similar soil and conditions.

I disconnected my grey water pipes from my septic and rerouted the grey water to water my trees.  Best decision I ever made.  I also recommend rain gutters and a capture system.  Even a small house can capture up to 7,000 gallons a year on 7 inches of rain.  Even if you do not put in barrels, you can direct the water to where it can slow and sink.

If you are planting trees from seed, protect them from underground and above ground intruders.  I lost an 6 months of seedlings in pots on a greenhouse shelf to mice.  I lost 50 12-inch acacia seedlings to quail and antelope squirrel. 1/4 inch metal mesh will keep out mice.  With the drought we have had everything is hungry.  I have even had to protect my agave from jackrabbits!

I would recommend starting with a mix of 15 gallon  and 5 gallon trees to speed up your shade capacity.  Citrus trees do great in Arizona.  As do almond and pecan. Chilean mesquite will grow very quickly, much faster than our native species, and provide shade for underplanting in a year.   I started with a six foot tree last year and it is already as tall as my house.  It gets a lot of grey water.  Check out local nurseries for better quality trees.

I am part of a seed dispersal program in Southern Arizona.  If you are looking to plant a vegetable garden reach out to me.  The seeds are free.