Sylvan Lake is located at 8,500 feet in the montane and lower subalpine life zones. The major plant communities on the slopes above the lake are aspen and montane coniferous forest. Lodgepole pine and Douglas fir dominate the montane coniferous forest community on moist slopes, while ponderosa pine and Rocky Mountain juniper occupy drier slopes.
The aspen forest is floristically diverse, with aspen, narrowleaf cottonwood, Colorado blue spruce and lodgepole pine in the overstory, and serviceberry, snowberry and a variety of wildflowers in the understory.
Near the reservoir, introduced grasslands are dominated by smooth brome, orchard grass and timothy. This area was planted for hay production in the past. Mesic sagebrush shrubland and mixed mountain shrubland occupy a small amount of the park. Diverse wetland, riparian and aquatic plant communities have become established around the reservoir and in West Brush Creek above and below the dam.