Texas Mountain Laurel, Silver Peso
Sophora secundiflora
Texas Mountain Laurel 'Silver Peso' (Sophora secundiflora) is distinguished from the traditional “green” Mountain Laurel by its pale, silver-gray, pubescent (fuzzy) foliage. It is typically used, in desert landscapes, as a flowering, fragrant evergreen shrub. Sophora's are well adapted to high temperatures, well drained alkaline soils and full sun. They typically grow quite slowly eventually reaching a mature height of 15 to 20 feet and 8 to 10 feet wide. Its slow rate of growth to mature height has led some to categorize it as a large shrub/small tree but its most common application is as a shrub. Report out of Texas suggest that the initial first 2 to 3 years are slow growth are the result of trees establishing a deep root system and that growth accelerates after this rooting period.
The form is usually low branching or multiple trunked with a dispersed to fairly dense canopy of fuzzy silver-gray leaves. Sophora's can be naturalized to survive on average annual rainfall alone, in some desert settings. This practice will further slow their growth rate and is more reasonably implemented once they are well established and put on some growth. Supplemental summer irrigations are usually required in central and southern Arizona. The highest rate of growth is achieved on established Sophora's when they are planted in full sun and given deep, monthly irrigations during spring and summer. The leaves are compound (made up of smaller leaflets) with 7 to 9, round, 1 inch diameter leaflets. With proper pruning, Sophora's can be trained into the form of a small, multiple trunk tree. Be aware that excessive pruning can inhibit flower production, as Sophora’s produce flowers only on one year old wood, and add to slow growth. Trunks range in color from dark gray to black.
Established plants are hardy to 0 degrees F. In March and April 4" to 8" strikingly fragrant, bright purple, drooping clusters of wisteria-like flowers are produced. These flower clusters are very fragrant with a smell often described as “grape soda.” Flowers fade fairly quickly and by midsummer give rise to 3" to 4" fuzzy, tan, seed pods. The seeds are large, 3/8" to 1/2" diameter and dark orange in color. While seeds and flowers can be toxic to children and pets, the actual risk is usually exaggerated. The risk is limited as the seed pod and seed coat are extremely hard and nearly impossible to crack (typically requiring pliers or a hammer to fracture). Placement and maintenance of this shrub should take into account the concerns about the flowers and seeds.
Sophora's can be used as individual flowering shrubs, in groupings to exploit the silver-gray foliage as an accent color in the landscape, purple flowers and remarkable fragrance or as a screen or hedge planting. It mixes easily with other desert landscape plant materials and adds a unique color and texture to the landscape. It will tolerate planting in or near turf or in more native areas. It tolerates reflected summer heat and can be planted against walls and structures to soften their appearance or add a touch of passive solar insulation to structures. Sophora's are thorn-less and completely pedestrian friendly.
The only insect pest of any consequence, on either the traditional or Silver Peso Mountain Laurel, is the larvae of the Pyralid Moth that feeds on leaves, young twig growth and immature seed pods. Mixed control has been achieved by an application of Bacillus thuringiensis (sometimes called BT) a widely available, commercial biological control agent. Control with BT can be variable and frequently requires multiple applications. Consistent control was seen with a soil applied, Granular formulations of Imidacloprid based insecticides. Proper maintenance can enhance the growth and increase the popularity of this remarkable yet under-appreciated flowering desert shrub. It has been our experience, in the production of this tree in nursery conditions, that Silver Peso is not as heavily infested by Pyralid Moth and that some selections appear to be immune. Apparently immune selections are being further tested and incorporated into our “Variety AZT” cloning program.
Patience is a virtue and in this instance handsomely rewarded. Sporadic availability and the relatively slow growth rate have limited the use and popularity of this extraordinary shrub. We currently have an excellent selection of Silver Peso available in 25 gallon containers. Both the Green and Silver Peso Texas Mountain Laurel are unique among large desert adapted shrubs with striking leaf form and texture, conspicuous, rich purple color flowers and an intoxicating fragrance. Their versatility, beauty and horticultural qualities make it a prefect addition to desert themed and more traditional landscape designs.