Workers for the Arizona Department of Transportation and possibly other agencies, as well as some private developers will use tank-mounted trucks to spray a bright green liquid slurry on recently graded areas or along roadsides in areas of recent road construction. The application of this green spray is called hydroseeding or hydromulching. The two terms are often used interchangeably, but hydroseeding is the application of a slurry of water, wood fiber, native seeds (usually grasses), fertilizer, and a green dye. Hydromulching may or may not include seeds, but uses a similar slurry with the addition of a stabilizing emulsion or tackifier to help protect the soil from erosion.
These areas of green dirt are especially common along the Catalina Highway. When I saw this sign on a bright green, recently sprayed hillside, I just had to get a photograph.
The green dye is added to make the sprayed areas visible and help the operator insure even distribution.
I was unable to discover exactly which native plants and grasses are used for the Arizona Department of Transportation hydroseeding, but private construction projects during the winter often choose wildflower hydroseeding. These wildflower mixtures usually contain California Poppies (Eschscholzia californica) and Desert Bells (Phacelia campanularia). The hydroseeded California Poppies are a deep orange color, while the subspecies of California Poppy native to the Tucson area, Eschscholzia californica ssp. mexicana, is more of a yellow-orange color.