(photo = Hilaria rigida, CalPhotos)
Introduction
Nurse plants are plants that protect and otherwise help seedlings of another species grow until they are able to survive on their own. Nurse plants can provide shade, protection from wind and heat, and even soil nutrients.
The Details
Hilaria rigida is a common grass in the western deserts. It forms clumps that persist year round. It grows in deserts, scrublands, and even sand dunes–areas seedlings of any species have difficulty getting established. Unshaded, soil surface temperatures can reach as high 150 F in the desert. However, H. rigida clumps shade the soil and keep the temperature much lower. H. rigida is a nurse plant for Agave deserti and other succulent plants.
Apparently, in addition to providing shade and coolness, soils under H. rigida are richer in nitrogen (fertilizer) than other nearby soils. However, one scientific paper presented evidence that H. rigida competes with A. deserti for water. Thus, H. rigida seems to provide enough shade that seedlings are not killed by heat. But, H. rigida competes with the seedlings for water. The seedlings have more nitrogen than expected, but they have to give up some water. In time, the agaves will grow large and will do a better job of competing for water.
Much in Nature is a trade-off as various species compete for precious resources. It is not clear what H. rigida gets from A. deserti.
Additional reading: Agave deserti and nurseplant, Hilaria ridiga
