Ask Mr. Smarty Plants

We realize you are having too much heat and intermittent sandstorms in Phoenix, but Tecoma stans (Yellow bells) is a desert plant, native to Arizona. It would have been better to have planted them in the cooler part of the year, like from November to January. Woody plants are very susceptible to transplant shock

Previously answered question on Tecoma stans (Yellow bells).  Too much water, from Florida.

Please read this article from the University of Illinois on Chlorosis, which often is the result of nitrogen deficiency in the plant.

So, we have established three possible reasons for the failure of your yellow bells to thrive and all are linked to the time and method of planting. If your plant was not planted to allow for good drainage, such as adding compost or other organic material to the soil, the roots are drowning. The loss of green in the leaves is lack of chlorophyll, again a result of too much water on those roots.

From another previous answer:

"Chlorosis is a yellowing of leaf tissue due to a lack of chlorophyll. Possible causes of chlorosis include poor drainage, damaged roots, compacted roots, high alkalinity, and nutrient deficiencies in the plant. Nutrient deficiencies may occur because there is an insufficient amount in the soil or because the nutrients are unavailable due to a high pH (alkaline soil). Or the nutrients may not be absorbed due to injured roots or poor root growth."

Our recommendations:

1. Since you can't undo planting a woody plant in June, let a lesson be learned: Don't do it again.

2.  If you didn't plant with organic material, put a good quality shredded bark mulch over the roots. This will shelter the roots from heat and, as it decomposes, it will sink down into the soil and help amend it.

3.  Since your soil is almost certainly alkaline, take the planting provisos above to heart for all planting. Drainage, especially for desert plants, is vital for accessibility of moisture and nutrients. Compost, compost, compost.

4. Cut out all that water, probably a thorough watering once a week is sufficient. And no fertilizer. Most native plants do not need fertilizer and this plant particularly dislikes it.