In late summer, the brightest red color in my garden comes from the flowers of Erythrina x bidwillii, a hybrid coral bean bred in the 1840s. Although it was bred at a botanical garden in Australia, the parents of E. x bidwilli are a South American species, E. crista-galli, and a North American, E. herbacea. Surprisingly, genes from the subtropical E. crista-galli make this hybrid easier to grow and bloom in my garden than E. herbacea which is actually native to North Carolina.
On a trip to the South Carolina lowcountry in early May eight years ago, I noticed spikes of bright red, tubular flowers growing from nondescript clumps of foliage. These were the flowers of E. herbacea, obviously adapted for pollination by the ruby throated hummingbirds that migrate into the region earlier in the spring. E. herbacea reaches northern limit of its natural range in the coastal plain of North Carolina, so I expected that it could be grown in my garden with, perhaps, a little protection in winter.
I obtained some seed and found that they were easy to germinate, as long as I nicked the very hard seed coat with a sharp knife (NOTE: The bright red seeds are extremely poisonous. Keep away from children). The seedlings grew fast in the greenhouse and when planted outside, they have survived the winter cold as long as I put them in a sunny, well drained spot. However, none of the outdoor plants have ever flowered.
The problem seems to be that E. herbacea, like Hydrangea macrophylla and some other garden shrubs, blooms on old wood. That is, it flowers on young branches growing off the stems produced in the previous year. While the roots of my plants survive the winter buried deep in the soil, the exposed branches are destroyed by freezing temperatures. Each spring, the plants produce a new crown of branches, but they never retain any mature stems that could flower.
In the greenhouse, the plants flower beautifully, as long as I give them a dry rest and don’t prune too vigorously (though some pruning is necessary to keep the long, spiny stems under control). An added bonus is that in the frost-free greenhouse, I can raise the large, tuberous base of the plant above the surface for added interest.
E. crista-galli is generally rated hardy to about zone 9, so it is unlikely to survive in my zone 7 garden. However, it has the useful characteristic of blooming on new wood. With E. x bidwillii, I get the best of both parents. The roots seem reasonably hardy, and although the above-ground stems die in winter, new growth flowers reliably each year. I usually get a few flowers in June and then a larger flush in August/September continuing intermittently until first frost.