What a great question! The answer to the question relates to soil types, depth, moisture, and soil disturbance. Whitebrush or Aloysia gratissima is found typically in draws or other areas receiving extra runoff and areas that have deep (or in our case deeper soils). Whitebrush is found on PHP West. You just haven't seen it yet because we currently do not have an open trail through the Whitebrush thickets but that will change in September. Areas with "LvA" for Lewisville silty clay are areas with the highest density of Whitebrush in the park. Only LvA and LvB soil types (where most of the Whitebrush is found) have soil profiles down to 62 inches, other soil types at the park are much less or more shallow.
Finally, if all these conditions are met, Whitebrush can be associated with highly disturbed and overgrazed areas. We know the property was heavily grazed by cattle in the Voelcker dairy farms days and probably up until a few years before the city purchased the property.
Whitebrush is truly a remarkable plant that you will be hard pressed to find at the north side parks! We get a lot of questions on this plant when it is blooming and the air has a sweet aroma to attract bees and butterflies from far off. It is a true gem of the park!
Wendy Leonard, PHP Park Naturalist Wednesday, 23 January 2013