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The Mexican petunia is a low mounding perennial with long dark evergreen leaves and purple-blue flowers that cover the plant from spring into summer. This species grows 12-15 in. high and to 24 in. wide with a compact habit. One popular cultivar, R. b. 'Katie', is smaller with a mature size 12-15 in. in height and 15-18 in. wide. These plants are valued for their attractive foliage and large purple petunia-like flowers.

This southwestern native is highly ornamental and useful for small spaces in Inland Empire gardens where it grows best with regular water and protections from hot sun exposures. It is cold tolerant, surviving temperatures below 20°F, and maintains good foliage character throughout the year.

Water Needs

The Mexican petunia is well adapted to partially shaded microclimate locations in the Inland Empire where it grows well with normal winter rainfall and regular summer water. It is a colorful flowering accent shrub for courtyard and southwestern style plantings in small spaces and in group plantings. Monthly watering from spring through fall will enhance its spring flowering cycle and help maintain good foliage character. It should be noted there are several winter months marked by an asterisk (*) when rains can provide sufficient moisture and irrigation is not needed. The high and low range of moisture indicates it can grow with varying amounts of water depending upon exposure conditions and size.

Irrigation Schedule and Graph

Moderate Water Use Plants

Irrigation Schedule 4

  Jan* Feb* Mar* Apr May Jun Jul Aug Sep Oct Nov* Dec*
Runs per Month 0x to 3x 0x to 3x 0x to 3x 2x to 3x 2x to 3x 3x to 4x 3x to 4x 3x to 4x 2x to 3x 2x to 3x 0x to 3x 0x to 3x
Inches per Run 1" 1" 1" 1" 1" 1" 1" 1" 1" 1" 1" 1"
Inches per Month 0" to 3" 0" to 3" 0" to 3" 2" to 3" 2" to 3" 3" to 4" 3" to 4" 3" to 4" 2" to 3" 2" to 3" 0" to 3" 0" to 3"

Range of supplemental summer water: 17"-24"
Range of supplemental winter water: 0"-15"

  Jan* Feb* Mar* Apr May Jun Jul Aug Sep Oct Nov* Dec*
For more information on how to use this Irrigation Schedule and Graph, follow this link.

For information how to calculate your irrigation system’s schedule and precipitation rate, please follow this link.

Plant Properties

Plant Type: Perennial
Foliage Character: Evergreen
Habit: Mounding, Compact
Growth Rate: Moderate
Height: 12 in. - 15 in.
Width: 24 in.
Water Needs: Moderate 4
Foliage Color: Dark green
Flower Color: Purple
Flower Season: Summer, Spring
Soil Adaptations: Well-draining soil, Sand, Clay
Exposure Adaptations: Morning sun, Heat, Frost, Partial sun
Function: Rock gardens, Hummingbird plant, Flowering accent plant, Container plant, Borders, Small spaces


Fast growing plant that can be invasive in the garden. If planting, consider putting a root barrier around it to contain its growth. Pull or cut out stems as needed (D).



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