[1][2] Howell is considered one of the top three self-taught botanists of his era for the Pacific Northwest; the other two being Wilhelm Nikolaus Suksdorf and William Conklin Cusick.

[5] His father was a doctor who had taught him some Latin and science, but he mostly educated himself while farming along the Clackamas River after leaving Sauvie Island.

[2] An aquatic plant sent to Harvard botanist Asa Gray in 1878 was named Howellia aquatilis by him in the brothers' honor.

[5] Today his collections are in many American and European herbaria, with a large set at Oregon State University.

[2] Lacking funds, he borrowed type and hand-set the book a few pages at a time, taking them to Portland to be printed.

It was purchased by Howell's brothers Joseph and John in 1873 from James and Julia Bybee and was adjacent to their parents' home.