APA Local Controller described a runway excursion event when traffic required to use Runway 35L in lieu of 35R due to mowing operations; the reporter indicating mowing policies should be changed.

Narrative

We were on a north flow with Runway 35R closed for mowing operations; which in itself is a hazard because it was closed. A Cirrus that was receiving flight following from approach called up approximately 10 miles south and was told to report 3 miles south straight in for Runway 35L and that traffic was holding in position; and continued inbound; the wind was ranging from 120 to 070 at 10-20 KTS. The Cirrus had the ATIS and knew of the weather. The Cirrus landed and while rolling out he was told to exit when able and contact Ground Control; I observed the Cirrus try to make B12 and was seen north of B12 in the grass. Wind shear could have been an issue; as our wind instrument is north of midfield and the Cirrus never made it down that far the runway. Another factor could have been the closing our main runway for mowing operations; the extra width of the main runway might have prevented this. This was not the only wind event of the day and we are in desperate need of wind shear equipment. Recommendation; wind shear equipment; not closing the runway to mow the grass; actually being able to use the runways; as normal during the middle of the day when it is hottest and most needed.