Ask Mr. Smarty Plants

Okay, quick now, do you have drainage holes in that galvanized container? If you don't, the roots of that Tecoma stans (yellow trumpetbush) may be in serious trouble. This plant is a desert plant, used to limestone-based soils, and growing on hillsides, slopes and canyons. If there is any water, the roots will greedily drink it up, but they don't tolerate standing in it. 

From an article "Focus on Plant Problems" from the University of Illinois Extension:

"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."

More information from the Virginia Cooperative Extension Iron Chlorosis Signals Problems indicates that often the trace mineral missing and causing chlorosis is iron. If you planted your Tecoma stans in potting soil, it should be balanced and not be causing the chlorosis problem. If, however, you dug up native soil for the plant, it could be too alkaline even for the alkaline-tolerating Yellow Bells. Iron chelates are organic compounds containing redily absorbed iron, and may provide a temporary solution to the problem. 

Yellow Bells is a good container plant, but it needs to be a big enough container to provide sufficient room for the roots, and it needs to have drainage. 

Tecoma stans

Tecoma stans

Tecoma stans

Tecoma stans