Buckthorns are modest trees with with great ecological values. They are not often planted in our gardens as they bear tiny flowers but they are important trees for wildlife.
Alder buckthorns are small trees which you will not find in the average garden centre. 
The flowers of Alder buckthorns are small, inconspicuous and greenish white in colour. I can imagine this description does not make you ecstatic right away but bear with me for a few more minutes because this modest tree has a lot to offer.
It usually grows to about 5 metres at most so it will suit the smaller gardens as well. It flowers from May until September, sometimes even until October which makes it a very valuable plant for pollinators. It grows best on acidic soils but my experience is that they are easy to grow. Many beekeepers love this plant as it offers both pollen and nectar during a long period of time. There is a mining bee, Andrena fulvida that depends on this tree. Bumblebees and butterflies often visit this tree and in woodland, where this tree can be found, it is often one of the few plants that provide nectar. In my garden, many Early bumblebees, Tree bumblebees, honeybees and buff-tailed bumblebees feed on this plant.
Purging buckthorns usually grow on calcareous soils and are more rare in this part of the country. 
Another reason why you could consider planting this tree is that it is the only food plant for the Brimstone butterfly, together with purging buckthorns. If you live in the right area, you may get lucky and some Brimstones might visit your garden and stay there.
Below a picture of a caterpillar that looks like a small twig; part of the Geometridae family.
In autumn, the berries turn black and are eaten by several bird species. Its autumn leaves are golden yellow.
Wildlife garden 
Okay, you have convinced me that my bees will LOVE this tree…it’ll grow we’ll in the acid soils of the Oregon Coast AND attract some other wild pollinators…where can I buy one?
After being a frugal veggie gardener for about thirty years (“why grow flowers which you can’t eat when they use precious water and space?”) I started keeping bees and changed my tune. Now it’s…
YOU CAN NEVER HAVE TOO MANY BEE-LOVING FLOWERS 🙂
I am not sure where you can buy these in Oregon but go for the local buckthorns as they are better suited for the climate there. We have a nursery that deliver plants to local governments and they also deliver to customers. Perhaps you can check with beekeeping organizations in your area.
Yes, in my garden, the bees such as bumblebees stay around and move from flowers to for example the courgettes when they are in flower or the apple trees. A very good site is buzzaboutbees.net
Your post is really interesting for me. I did not realise Blackthorn is what I call sloe because I can recognise the fruits. We have a lot that have become very bushy on the edges of our garden and we cut down as the thorns are so vicious. I do like the blossom and I did not know about the special Andrena bee. I will now find one I can transplant to another part of the garden and let it grow taller so I (and the bees) can enjoy the flowers but avoid the thorns.
The purging buckthorn has the thorns, the Alder buckthorn does not have any thorns. Blackthorn I grow in the garden too but that is a different plant, prunus spinosa.
Rhamnus frangula, (Frangula alnus) is the Alder buckthorn and purging buckthorn is Rhamnus cathartica – Common (or Purging) Buckthorn. Blackthorn is also a good plant for wildlife, foodplant of hundreds of micro and macromoth which in turn are food for young birds. So much to discover in the garden 🙂
I planted a Buckthorn in my garden about 6 years ago as I wanted to choose UK native trees, I did not realise what a good thing I had done for bees at the time. I am so pleased I chose it; currently it is alive with bees from first thing to 9pm! It is such an unassuming looking tree, no one would think anything of it to look at, but the bees know otherwise – I could swear they look happy as well :-). This is a lovely blog, found it by accident looking up why the bees loved the tree so much.
Thank you so much for the reply! Yes, the flowers are inconspicuous but are nevertheless very rich in nectar and the shrub also provides pollen but it seems that it is more of a nectar source. On warm, humid days after a shower, i had dozens of honeybees foraging on this tree. The blackbirds love the berries. Thank you once again.