DO NOT USE PLASTIC SHEETING AS A WEED GUARD UNDER MULCH IN AREAS WHERE THERE ARE TREES AND/OR OTHER PLANT LIFE. IT DEPRIVES THE PLANTS OF WATER AND CAUSES OTHER PROBLEMS SUCH AS DISEASE. USE BREATHABLE LANDSCAPE FABRIC INSTEAD OF PLASTIC SHEETING.
From Colorado State University:
Xeriscaping: Retrofit Your Yard
by J.R. Feucht and C.R. Wilson1 (5/06)
Practices to Avoid
When removing areas from turf, do not cover them with solid sheet plastic and gravel, rock or volcanic cinder. Plastics shed water and create wasteful runoff. They exclude water and essential air exchange to plant roots, increase evaporation from surrounding areas by raising local soil and air temperatures, and can cause root injury due to heat buildup.
Keep rock or gravel areas to a minimum. They tend to increase air and soil temperature. Use weed barrier fabrics (geotextiles) available in garden centers. These materials allow water penetration and air exchange. Cover landscape fabrics with mulch.
Instead of rock and gravel, consider organic materials such as wood chips and chunk bark. They give a natural look and help retain moisture, as well as hold weeds in check. Use mulch either with or without a landscape fabric to save water. Rock may be required to cover a steep slope where wood chips and gravel may wash away. In these cases, use natural river bed cobble of varying sizes. Lay rock over a weed barrier fabric in much the same way as if you were constructing a rock wall.
From the Denver City Forester and Denver Municipal Code:
Quoting the Revised Municipal Code (IN REGARD TO HASTENING THE DEATH OF EXISTING, MATURE TREES IN THE TREE LAWN BY USING PLASTIC SHEETING UNDER THEM):
Sec. 57-24. Injuring, damaging or removing trees on public property.
It shall be unlawful to injure, damage, destroy, or remove any tree situated upon the public right-of-way, or other public place within the city. (THE TREE LAWNS ARE PUBLIC RIGHT OF WAY)
(1) Any person who shall injure, damage (including by excessive pruning or topping), or destroy any tree situated upon
the public right-of-way or other public place shall promptly notify the city forester of such occurrence and shall, within such
reasonable time as specified by the city forester in an order, repair said damage to the satisfaction of the city forester.
From the Ft. Collins .org in regard to elminating mosquito breeding grounds (since plastic sheeting does not allow the water to drain into the ground, water can pool on top of the sheeting under the mulch):
How to Control Breeding Sites
Plastic in gardens to prevent weeds
Use landscape cloth that will allow water through instead of plastic sheets