Plants and Oiled Highways

MARCH, 1935 ARIZONA HIGHWAYS 11 Plants and Oiled Highways

IN THE SEMI-ARID region of Southern Arizona, moisture perhaps more than any other factor, determines where a plant will grow, or whether it will grow. If one will climb to an elevation and view the surrounding country, he will notice that by no means does the same type of vegetation cover the entire area. Along the washes a certain specie will predominate, on a slight elevation where rainfall immediately runs off, another will thrive and an adobe flat will be host to still another.

Since the coming of oiled and hard surfaced highways to the state of Arizona, a narrow strip of land, immediately along the highway, known as the shoulder, is receiving two to three times more moisture than the surrounding country. This is of course due to the runoff from the oiled and hard surface. While traveling over these highways, you have perhaps noticed the varied vegetation that has sprung up along the shoulders. True to the theory that whenever conditions become favorable for plant life, plant life will arrive, we find that a great number of species have taken advantage of this extra moisture and have made this their habitat.

Looking very closely you will notice that usually, the vegetation consists of but one or two species of plants. A keen observer will also observe that in most instances it clings closely to the further end, or sloping side of the shoulder. In many cases the plants form a line for miles along the highway, seldom spreading more than a foot to either side. Very seldom will it invade the adjoining desert. In many instances the growth is a pure stand, that is, a single specie will predominate for several miles, and then suddenly change off to different specie. The altitude and type of soil of course has a great deal to do with this sudden change. The highway grader in its work of rebanking the shoulder destroys a great amount of this growth. Where the shoulder has not been disturbed for several months, vegetation is sure to appear.

Strange Growth Observ ed Along Modern Oiled Roads

In the following paragraphs I have named some of these plants and have pointed out the location where they thrive to the highest extent Do not expect to find the following plants, or in many cases any plants growing along every mile of highway, for in certain areas conditions and factors exist which are detrimental to plant life.

Between Florence and Florence Junction, about a mile or two beyond the Gila River bridge, on Highway 80, a pure stand of Eriogonum deflexum is found growing along both sides of the highway. This growth extends for a distance of several miles. The plants are twelve to eighteen inches high, are a brownish pink color when mature. The flowers are exceedingly small and deflected downward. The specie is also found growing along the highway between Picacho and Tucson; Wickenburg and Congress Junction. Doubtless it is growing in many other sections of Arizona I have not visited.

As the first foothills are reached after leaving Florence Junction, as one travels towards Superior, the rubber plant Asclepias subulata will be noticed growing along the shoulders. This plant grows in abundance in the vicinity of Yuma. It was introduced to the Tucson area several years ago, but the winters were too cold, unless protection is afforded it dies in the winter months. The plant is from one to four feet high, with many non-branching stems from a common base. Flowers yellow forming an umbel at the end of the stem. This particular plant is of interest, as it is one of the very few plants growing in the state that has any possibilities of becoming a producer of rubber.

Another plant which has chosen the roadside as its new habitat is Capparidaceae Wislizena refracta, commonly called Alkali weed. It is an annual, and starts its growth in the late winter months, and blooms throughout the summer and fall. East of Coolidge Dam where the highway follows the Gila river this particular plant has literally lined the shoulders for a distance of several miles.

The above mentioned plants are by no means the only ones choosing this narrow area. I have mentioned the above ones as they are the most prominent, and the most easily noticed.