There aren’t many vines native to California, and few are truly evergreen. Fewer still are both evergreen and vigorous enough to cover a good-sized arbor or trellis. Calystegia macrostegia (coastal or island morning glory) is one that really fills the bill. It may drop leaves in summer if grown dry, but a little supplemental water or summer fog is enough to keep it green and growing near the coast. And vigorous? You may have to hold it back.
Widely flaring flowers of Calystegia macrostegia tend to close by late afternoon or on cloudy days.
The species is native to chaparral and coastal scrub communities of central and southern California from Monterey County south into Baja California, including mountains in the Transverse and Peninsular ranges and most of the Channel Islands.
Six subspecies with overlapping ranges vary in habit from a low, sprawling herbaceous perennial to a 20-foot twining vine with a woody base. The lobed leaves may be hairy or smooth and vary from small and almost linear to large and broadly triangular. Funnel-shaped flowers range in size from less than an inch to more than two and a half inches wide and in color from pure white to the palest pink or faintly lavender.
Such variety would seem to provide sufficient raw material to motivate breeders, yet only one horticultural selection is commonly available, and then only in specialty nurseries. ‘Anacapa Pink’, from the Anacapa islands off the coast of Ventura County, bears masses of large, pink-tinged white flowers over a season that can extend from late winter to fall. Its triangular leaves are large, smooth, and a lush dark green.
Island morning glory will clamber up, over, and through any support, including nearby shrubs, so it may need regular pruning if this effect is not intended. This somewhat tender vine grows most happily in well-drained soil and cool sun and inland will need summer water and some shade. Without support it makes a lovely mounding or cascading groundcover.