Plant of the Month: Fraxinus latifolia, Oregon Ash
Oleaceae (Olive) Family
Illustration and article by Erin Savoy
Edited by Samantha Elie
Oregon Ash, or Fraxinus latifolia, is our only PNW native ash tree. Its compound leaves with wider leaflets and grand form make it easy to identify. There are many of these trees growing in landscapes and facilities throughout the Pacific Northwest cities including Portland and Gresham, so once you get to know them you will probably start seeing them quite often. This specific species of ash grows west of the Cascades and Sierra Nevadas, from southwest British Columbia to central California along riparian corridors within forests, woodland, chaparral, and grassland ecosystems. They are most abundant in the Columbia Basin and in the tributaries, streams and valleys in western Oregon and southwest Washington. In these bottomlands we have relatively generally mild weather with cool winters and warm summers, especially at low elevation.
Generally growing up to about 25m, this tree forms broad, round crowns on large limbs, with a moderately shallow root system that is wide spreading and densely fibrous. It can grow even larger in the upper portions of its range, and will often live up to 250 years. The pinnately compound leaves have 5-9 leaflets. Each leaflet is ovate, broad, and often will show signs of rust, insects, or other disease even if on a healthy plant.
These trees are dioecious, with male and female greenish-white flowers blooming from separate plants in dense panicles, near the base of new foliage that appears in spring between April and May. Both types of flowers are small and inconspicuous, appearing before the leaves on the branch. The male flower has a 4-parted calyx and 2 anthers, while female flowers consist of a calyx, pistil, and stigma.
Autumn is the tree’s most aesthetic season, when it produces attractive seed clusters that ripen between August and September. These fruits are 3-5cm long samaras with wings that somewhat resemble half of a maple seed, and usually are not produced until a plant is at least 20-30 years old. The winged samaras are dispersed by wind and may be carried by waterways.
Oregon ash have a fairly high tolerance of drought, but are in their best conditions in moist to wet soils with lots of organic matter, such as in riparian forests that flood and have silt deposits. They can thrive with a wide range of soil types including clay, sandy, gravelly, and rocky earth with significant available moisture. Ash is so well suited to these heavy soils and poorly drained areas that it grows in areas which may be too wet for any other trees, even cottonwoods.
For this reason they are useful for revegetating wet areas that are periodically flooded. They can become long-term dominant in riparian areas along slow streams and other poorly drained areas, and will colonize wet areas in grasslands and abandoned fields as well. These forests of ash around streams provide important habitat in valleys that are otherwise cultivated farm or pasture. You will often find it alongside other riparian wetland trees such as black cottonwood, red alder, white alder, bigleaf maple, Oregon white oak, California laurel, California sycamore, Douglas fir, grand fir, and various willows. Common associated shrubs include snowberry, hawthorn, serviceberry, mock orange, and crabapple. Sedges especially like to dominate underneath ash trees.
This species is primarily propagated by seed. It will vigorously sprout from the root collar after cutting. The canopy provides structure and cover to birds and mammals, and the seeds and foliage provide food. There are many species of butterfly that are attracted to it. Riparian ash forest stands also provide food and habitat for beaver and nutria, in addition to other browsers.
There are timber uses as an important hardwood, but ash is mostly planted as a component of riparian habitats, including in forest buffers, in wetland creation, restoration and enhancement of erosion control and waterline stabilization projects. This species holds important use and ceremonial medicine for the Costanoan, Cowlitz, Karok, Kawaiisu, Mendocino, Yokia, Yuki and other Pacific Northwest First Nations.
United States Department of Agriculture; Natural Resources Conservation Service. Fraxinus latifolia Benth. Oregon Ash https://plants.usda.gov/home/plantProfile?symbol=FRLA
California Native Plant Society; Calscape. Oregon Ash (Fraxinus latifolia). https://calscape.org/Fraxinus-latifolia-()
Oregon State University. College of Agricultural Sciences – Department of Horticulture. Landscape Plants: Fraxinus latifolia. https://landscapeplants.oregonstate.edu/plants/fraxinus-latifolia
Burke Herbarium Image Collection; Vascular Plants, Macrofungi, and Lichenized Fungi of Washington. Fraxinus latifolia (Oregon Ash). https://biology.burke.washington.edu/herbarium/imagecollection/taxon.php?Taxon=Fraxinus%20latifolia
Native American Ethnobotany Database: Fraxinus latifolia Benth. http://naeb.brit.org/uses/search/?string=Fraxinus+latifolia
Pojar, J and MacKinnon, A. 2004. Plants of the Pacific Northwest Coast: Washington, Oregon, British Columbia and Alaska. Vancouver (BC): Lone Pine Publishing.