ETYMOLOGY
irregular from Latin umbra (shade, shadow) + -acious
EXAMPLE
“… About the same period also an approach was made, by altering and levelling the ground, and planted with elms, sycamores, and chestnuts on each side, which have already become very umbracious, and will in the course of a few more years, when he who planted them rests beneath their shade, form a stately avenue. …”
From: The History and Topography of the Isle of Axholme
By William Brocklehurst Stonehouse, 1839
Lexicophilia 