At first glance the hills in the park look like a typical annual weed/grass dominated landscape. The hills turn green following the onset of winter rains and fade to yellow and brown as the landscapes dries in the summer. However upon closer inspection there are many native species that are remnants of plant associations that once covered the landscape. The four native plant associations are Southern California Black Walnut woodland, Coastal Sage Scrub, Valley Grassland, and Riparian.
The Southern California Black Walnut woodland has the Southern California Black Walnut (Juglans californica var californica) as the dominant tree and other trees and shrubs such as Blue Elderberry (Sambucus mexicana) and Toyon (Heteromeles arbutifolia) as secondary species. In undisturbed landscapes, the trees either form a closed canopy mostly on north and east facing slopes or open canopy savannahs on the hotter, drier south and west facing slopes. In Ascot Hills Park, the tree species exist primary in the flatter slopes of the valley where possibly they were retained for shade. On the hillsides relatively few trees exist except where recent restoration plantings or where new seedlings are re colonizing slopes now that the detrimental practices of livestock grazing and plowing have been ceased. Few of the under story species are present except for Coffee Berry (Rhamnus californica) and Wild Cucumber (Marah macrocarpus).
The Coastal Sage Scrub association is the most diverse in plant association in the park, however, this community is highly fragmented and impacted by past land use and management practices. The plants in this association are mostly low growing shrubs, annuals and perennials that offer a year around supply of flowers and seed, providing food for wildlife. These plants predominantly grow on the more xeric and warm slopes with south and west aspects. In Ascot Hills Park today, plants are mostly found in small remnant patches that were too steep to graze or plow, however pioneering species can be found to be re colonizing in many parts of the park following the ceasing of grazing and plowing. Examples of species in this association include California Sage brush (Artemesia californica), California Bush Sunflower (Encelia californica), and California Buckwheat (Eriogonum fasciculatum). Many of the re colonizing species have “weed” in their common names. In this plant association these include: Narrow-leaf Weed (Asclepias fascicularis), Deerweed (Lotus scoparius), and Doveweed (Eremocarpus setigerus). This association also includes a few shrub trees such as Laurel Sumac (Malosna laurina) and occasional interspersing of Black Walnut and Toyon from the Walnut Woodland.
The Valley Grassland association includes annual grasses, perennial bunch grasses, herbs, and wildflowers. The native valley grasslands originally covered much of the dry, flat to mildly sloped areas of the park, however this community has been mostly replaced with nonnative grasses and weeds. The conversion from the native grasses resulted mostly from preferential grazing of livestock along with the introduction of annual grasses and weedy species from Mediterranean areas. Currently the native bunch grasses and associated herbs and wildflowers are found in few remnant patches that were too steep tograze or plow. The native bunch grasses found in the park include Purple Needlegrass (Nassella pulchra) and California Brome (Bromus californica).
The Riparian association is found along the stream in the park and is part of the Southern Willow Woodland Scrub. The vegetation in this association include winter deciduous trees and shrubs that have roots that grow into saturated or near saturated soils along streams and drainage channels. These species include Mule Fat (Baccharis salicifolia), Black Willow (Salix goodingii), and Arroyo Willow (Salix lasiolepis). Black Walnut and Blue Elderberry from the Black Walnut Woodland association are also found in the upland areas just outside of the wetted stream. At Ascot Hills Park the Riparian association has been altered by introduction of many exotic species, including Blue Gum (Eucalyptus globula) and Mexican Fan Palm (Washingtonia robusta). In the park development many additional species were planted, including broad-leaved, winter deciduous trees such as Fremont Cottonwood (Populus fremonti), White Alder (Alnus rhombifolia), and Western Sycamore (Platnus racemosa).