On a visit to the coastal area around Beaufort, South Carolina, I was blown away by the beauty of a very tall plant covered in powder-blue flowers. Some of the plants were taller than me, with flower heads that reminded me of Phlox. It was love at first sight, and I resolved to plant some in my own garden.
Unfortunately, I later learned that the plant, Cape Plumbago (Plumbago auriculata), is cold hardy only in Zones 9–11. In my Zone 8 garden, it could be grown only as an annual.
Now I am on the lookout for a similar plant called Hardy Plumbago (Ceratostigma willmottianum), which is reputed to be hardy in Zones 7–9. It offers much the same beautiful blue flower color but matures at a more manageable four feet tall. As an added bonus, it is considered deer resistant.
On a recent visit to a local arboretum, I discovered a low-growing plant covered with intense blue flowers. I was surprised to learn that it, too, was called Plumbago (Ceratostigma plumbaginoides).
This Plumbago grows only about eight inches tall but spreads readily to form a thick groundcover. It is difficult to find flowers with this intensity of true blue, making it especially valuable in the garden. As a bonus, the green foliage takes on shades of bronze and red in fall, adding another season of interest.
Ceratostigma plumbaginoides is cold hardy in Zones 5–9. At the cooler end of its range, it is deciduous. In my Zone 8 garden, it may retain some of its foliage through the winter. New spring leaves have tints of red or bronze.
Hardy groundcover Plumbago is not demanding about soil texture or acidity, although it prefers well-drained loamy or sandy soil to heavy clay. It grows well in full sun to partial shade, and its blue flowers attract bees, butterflies, and other pollinators.
Consider using it in a pollinator garden or rock garden, or anywhere you need a low-growing groundcover with a long season of interest. It can also be useful on a steep slope where mowing is difficult or impractical.
This lovely Plumbago, growing near Beaufort, SC, was eight feet tall and covered with flowers and pollinators. It is not cold hardy in zones cooler than 9.
A groundcover Plumbago, Ceratostigma plumbaginoides. I plan to try this one as an alternative to Ajuga.