Your mat can be vacuumed, broomed or simply hosed off. They dry easily and quickly in the sun, it is recommended that you dry your mat completely before storing. They won't attract mildew, rot or retain a musty odour if treated and stored correctly.
Prior to washing remove any grass or debris. A soft broom working with the grain gives the best results. If required give the mat a quick hose down and for heavy stains (ie red dirt) use soapy water (mild dish washing liquid works fine) give the mat a light scrub with the broom and hose off any suds.
Lay the mat out in the sun until dry, turning once the top side is dry.
Please note that the use of any abrasive, oxidising or other such agents (bleach, Chlorine, Degreaser, Kennel Cleaner Shower Power etc) is not recommended as they could cause damage to your mat.
Remember that while your mat is manufactured as an outdoor product it is not designed to be left permanently outside in the elements. Leaving the mat permanently outside exposed to the elements will significantly decrease the appearance and life span of your mat.