Our oak collection at the Washington Park Arboretum is nationally accredited through the Plant Collections Network, administered through the American Public Garden Association in partnership with the US Department of Agriculture. Oaks are one of our four accredited collections at the Arboretum, the others being maples, magnolias, and hollies. Gardens with accredited collections collaborate on research including taxonomic studies, conservation programs and pest and disease management. Twenty-two Botanic Gardens in the US have accredited oak collections, and having these collections spread over the continent allows for a greater representation of the family across different climates. We currently have a total of about 340 oaks representing about 90 different species, cultivars, varieties, and subspecies. Much of this collection is concentrated around the north of the Arboretum between Azalea Way and the Arboretum Loop trail where the large canopies of oaks make for a shady summer destination.

One of the many gems in our oak collection is Quercus graciliformis, Slender Oak or Chisos Oak, which is endemic to a very limited area in the Chisos Mountains in western Texas. There are less than 250 trees, and they are only known from within a ten square mile area within Big Bend National Park where they grow in dry oak woodlands at the bottom of canyons. There is thought to be some suitable habitat across the border in the Mexican state of Chihuahua, but detailed surveys are still needed. The small size of the population in the wild makes this species especially vulnerable to extinction. The largest group of Slender Oaks occur in one canyon, where a severe fire could threaten them.

This small semi-evergreen tree is one of the species targeted for ex-situ conservation efforts at botanic gardens like ours. Ex-situ conservation protects populations of threatened plants outside of their natural range to preserve the genetic material of the species. Botanic gardens across the country have worked together to collect acorns from all the known wild populations of Slender Oaks and now grow them in their gardens. If a catastrophic event like wildfire destroys these wild populations, the genetic material from these botanic garden specimens can be used for restoration purposes. Our two specimens are not from known wild sources.

We have two Slender Oaks in our collection, both along the Arboretum Loop trail at the north end of the park. The largest, in grid 40-1W, was received in 1990 and planted in 1996 and is now a lovely tall and slender tree. The second tree in grid 42-1W was received from Cistus Nursery in Oregon in 2015 and planted in 2019. It is establishing well and is still small. Both of our specimens were received as Quercus canbyi, Canby Oak, and the taxonomy of the two species has been much debated, although currently the consensus among botanists is that they are separate and distinct species. Canby Oak acorns mature in one year whereas Slender Oak acorns require two years to reach maturity. We’ll have to keep an eye on our two trees and watch closely to see if any acorns mature to solve the mystery!

Photo credits: Joanna Long, Manager of Horticulture at the Washington Park Arboretum