Jennifer Schultz Nelson
Extension Educator, Horticulture
It seems like whenever I travel outside of our Zone 5 climate, I get a case of what I like to call "zone envy". There's always a plant or two at my destination that would look perfect in my garden back home, and further research usually reveals the disappointing news that the plants I'm admiring will only survive winters much warmer than central Illinois.
On our honeymoon in Florida last year, my husband and I noticed a small shrub that seemed to be everywhere. What captured our attention were the brilliant true blue flowers that covered each shrub. True blue flowers are a rare sight in the plant world, and we thought this plant would be perfect for our orange and blue Illini garden.
We found the identity of this gorgeous blue-flowered plant was plumbago. My husband insisted we had to be able to grow this plant. I was more cynical, thinking it was probably not hardy in Illinois.
A little research upon returning home revealed that sure enough, the plumbago we saw in Florida, Plumbago auriculata, is only hardy in subtropical climates. In the U.S., that limits its range mostly to parts of Florida and California. I concluded our Illinois garden would never see a plumbago plant.
Boy was I wrong. While shopping for perennials early this spring, a blue flower caught my eye. The tag read "Hardy Plumbago", and it also said it was hardy to Zone 5. Was I seeing things? Was it wishful thinking? Was it some new cultivar of the plumbago we admired in Florida?
Luckily I wasn't seeing things. The hardy plumbago plant is Ceratostigma plumbaginoides. It is in the same family as Plumbago auriculata, the botanical family Plumbaginaceae. At one point in time they were both categorized in the genus Plumbago.
The plumbago family contains about ten to twenty individual species, native to warm temperate to tropical regions of the world. The plumbago we saw in Florida, Plumbago auriculata, is native to South Africa.
Another common name for Plumbago is Leadwort. The genus name is derived from the Latin name for lead, plumbum. This is probably because the blue flowers of many species resemble the color of lead, but there are also accounts of plumbago being a folk-remedy for lead poisoning.
We can learn even more from the Latin names for plumbago and hardy plumbago. Plumbago auriculata, the tropical species, can be translated as Plumbago–'with lead' and auriculata–'with ears', referencing the leaf shape. The –oides suffix in Latin names means 'like' or 'similar to'. So for hardy plumbago, Ceratostigma plumbaginoides, its name means 'a Ceratostigma that is similar to plumbago.'
In its preferred tropical climate, Plumbago auriculata will grow anywhere from three to ten feet tall, depending on cultivar and individual site conditions. It rarely becomes invasive in most gardens. Hardy plumbago, Ceratostigma plumbaginoides is really more of a groundcover, typically growing from sprawling rhizomes only to a height of about twelve to eighteen inches. The sprawling rhizomes result in a spreading growth habit which can become invasive in some gardens.
Both Plumbago auriculata and Ceratostigma plumbaginoides prefer rich, well-drained soil that is slightly acid. Both perform best with regular watering, but once established will tolerate some dry periods.
The flowers for both species are very similar–five petals on top of a tubular flower, resembling the flowers of phlox. Shades of blue vary in both species depending on the cultivar.
Plumbago auriculata remains green and flowers most of the year in its preferred tropical climate. There really is no variation in foliage color throughout the year. It can survive some freezing temperatures, but not extended freezes. In my opinion, Ceratostigma plumbaginoides has an advantage of not only being hardy to Zone 5, but it also has some interesting foliage colors. The new growth is burgundy, and the foliage develops a nice coppery-bronze fall color, perfect for my orange and blue Illini garden!
I did find Plumbago auriculata sold locally this spring, sold as a 'specialty annual'. The first place I saw it, it was a very large plant, about four feet tall. I was intrigued until I saw the price-- $90. In my opinion, that is way too much to pay for an annual, even if you could figure out a way to overwinter it indoors.
A few weeks later, I found Plumbago auriculata again, this time at a local "big box" discount store while grocery shopping. These were smaller, about two feet tall, but only cost $6. That was much more my speed. I brought one home and it looks great in our Illini garden. I know it's not hardy, but I may try digging it up and bringing it inside, or mulch it heavily and hope for a mild winter. Since it only cost me $6, I figure I can afford to experiment!