Q. I'm working on a project that would create the world's largest American flag, using blooming trees to create the design from an aerial view. I've been leaning towards red and white crepe myrtles for the stripes as they have a long bloom time, but haven't been able to pin down a tree with blue flowers. (Everything I search under 'blue blooms on trees' looks violet.) I'm willing to forgo using trees for the blue field plants if I must. But my biggest concern is having all three colors in bloom at the same time of the year, and the longer the bloom the better. Can you guide me in my search?
- ---Chuck in Russellville, Tennessee
The first plant that came to the mind of Holly Shimizu, Executive Director of the U.S. Botanic Garden in Washington, DC, was Blue Atlas Cedar (Cedrus Atlantica). No, it doesn't produce blue flowers—but it is a tree, and a number of selections of this outstanding evergreen have needles that are truly blue.
Cedars should do well in that area (Northeastern Tennessee), the trees would always be blue (very appropriate as the blue in our flag stands for vigilance and perseverance), and they'd be joined by the red and white parts when the crepe myrtles bloom in summer. The blue of the cedars might seem a little subdued comparatively, but fireworks can seem subdued next to a crepe myrtle.
Holly's other idea was to delve into the world of crepe myrtle breeders to see if anyone is trying to move the purple-flowered types more towards the blue end of the spectrum, specifically directing me to Margaret Pooler, a research geneticist at the National Arboretum (also located in our Nation's Capital) who has been involved in mapping the decorative trees' genetic structure.
Unfortunately, she reports that "the genes that control flower color in crepe myrtle can't produce any kind of a real blue; the purple/lavender varieties are as close as you're going to get."
Scott Aker, head horticulturist at the National Arboretum, says that his top choice would be blue-flowering Rose of Sharon—pruned into the tree form that these hibiscus family members so easily achieve with some sharp intervention. "The flowers are big, really blue, and should be in bloom at the right time," he notes. (Back when I was the editor in chief, a story we ran on blue flowers in ORGANIC GARDENING magazine identified the variety 'Blue Bird' as a great blue Rose of Sharon.)
But when I personally think of big blue flowers in the summer, I think of hydrangeas. The bloom time is fairly consistent with crepe myrtles, the flowers are HUGE, and it's easy to 'make' the blooms of the most common types blue by keeping their soil acidic, which I do with a yearly application of peat moss covered with compost. Hydrangeas tend to max out at six to eight feet in height, but there are dwarf cultivars of crepe myrtle that could be kept trained to around that size without committing the 'crepe murder' that would be needed to keep full-sized trees in check.
The USBG's Holly Shimizu liked my hydrangea idea so much she said she was sorry that our listener had specified trees. "You could get a really nice flag out of hydrangeas alone," she notes. "The white-flowered ones are always white, and the standard types can be made to flower either red or blue by manipulating their soil pH. And they're all the same type of plant, so you know they'd all be in bloom at the same time—and for a good long time."
And it would probably be a lot less expensive of an endeavor. I just took a serious look at an image of our flag, and that's a lot of lumber if it's all colored out in trees. It would take 50 white crepe myrtles just to make up the stars—at one crepe per state—and a huge amount more if you're using trees to make the much longer (and more densely planted) six white stripes; around a thousand more white ones by my eyeballing and estimating—and that's only if you get the planting distance close to perfect. And you'd need around 15% more red blooming myrtles to make up the seven red stripes of the flag.
And then there's the potential need for replacement plants. That's where I think the hydrangeas really shine; you can pop new shrubs in the ground a lot easier than having to uproot and replace a tree. A little support for the big flowerheads, a little trellising to clean up the outside of the stars (to make them look like real five-pointers from the sky), and you'd get solid, bold color from about the same number of plants, but at much less cost.