The Curious Case of Color Change in Crepe

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(And Gregarious Generalizations about Grafts)

Q. I purchased a "Red Rocket" Crepe Myrtle two years ago specifically because of the bright red blooms that were covering the plant. Last year I anxiously awaited their reappearance, but the flowers that emerged were white!! How did this happen? Is there anything I can do? Please help! Thank you,

---Lisa in Villas, NJ (about five miles from Cape May)

A. This is one of those questions where I was pretty sure I knew the answer right off the bat, as 'flowers changing color' is a common occurrence with grafted plants. So let's take a short detour and use roses—kind of the poster child for grafted plants—to explain the important topic of grafts and grafting.

Although some roses (and other kinds of plants, like fruit trees) are sold "on their own roots", many are a combination; the blooming section of one plant grafted onto the rootstock of a different variety of the same kind of plant. If the top, "grafted" part of such a plant dies, or if the grafted area is covered by soil or mulch, the rootstock will take over and produce flowers or fruits that are not even close to the ones you had hoped for.

Roses that are specifically bred for the far North pretty much have to be the 'own root' variety, as the plants need to be heavily mulched—sometimes completely covered—to reliably survive really frigid winters. But in most parts of the country, purchased roses can come either way—as a single plant from top to bottom, or as two very different roses that have been joined together.

When an otherwise desirable rose has a deficiency or weakness that makes it difficult to grow, cuttings are taken and joined to the root system of a different, tougher rose. The rose used for the root stock may be more vigorous, cold hardy, heat tolerant, disease or pest resistant or have other admirable qualities, but its flowers are not one of them. In fact, one of the best and most commonly used root stocks is the infamous multi flora rose—one of the most despised invasive plants in parts of the country where it was (ill-advisedly) installed as a 'living fence'.

With any kind of grafted plant—fruit tree, rose, tomato (a new and growing trend)—it's always vitally important to keep the graft above the soil line. Because if the graft (an obvious area with a linear bulge, generally a few inches to a foot above the root system) is covered by soil or mulch, new growth will sprout from the root system, eventually producing a different flower or fruit than the one you wanted—and often killing the part of the plant that had been producing the desired piece of horticulture.

Same thing if you prune below the graft; then you only have one plant left, and it wasn't the one you thought you had. This is a classic result when people 'whack' their roses—they cut so low they prune off all of the rose they wanted, and then wonder why they have this thorny aggressive thing in its place.

(Grafted or not, you should never whack your roses. You can—and should—cut the canes back throughout the growing season to remove spent hips and diseased areas, but stop pruning at the end of August to allow the plant to go dormant. Fall pruning invites the kind of winter injury that can kill the top part of a grafted plant and give you that new and inferior rose. Always prune roses in the Spring; and be sure to cut well above any graft.)

But—getting back to our ostensive topic—I had no idea whether this variety of crepe myrtle (or any variety) are ever grafted plants, so I contacted Monrovia, the breeder of "Red Rocket". Their response? "Nope; they're all grown from cuttings and shipped on their own roots."

Waa-wahhhh!

So I emailed back to ask if they knew of any other reason this color change could have happened. One in-house expert after another passed on the question, and it was starting to look like I was going to be fertilizer out of luck. But then the Monrovia press people found an excellent article by crepe myrtle expert Dr. Gary Knox of the University of Florida called "Seeing Red", in which he explores the breeding and development of myrtles with vibrantly crimson blooms.

After listing the best red flowering varieties currently available, Dr. Knox highlights a paragraph that's the horticultural equivalent of a 'black box' warning on pharmaceuticals. I quote:

"Full Sun Needed

Most red-flowered crape myrtles have one important quirk — flower buds that open under cloudy skies [or in shady situations] develop into pink or white flowers, not red. While some people find this attractive, most people are upset since they thought they bought red crape myrtles. Apparently, full sun is needed for full expression of the red flower pigment. Accordingly, gardeners, landscapers, and retailers should make sure red-flowered crape myrtles are placed in areas receiving full sun."

Mystery solved and lesson learned. Thank you, Dr. Knox!

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