A Gardening Journal

The Best Season Ever: 'Gloire de Marengo' Ivy Grows Up

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At nearly eight feet high, this potted standard of Hedera canariensis 'Gloire de Marengo' ivy is an immense showstopper at the end of a long pathway. It's never so lush and wild as in early Fall, after a Summer of basking in dappled sun and heat. In the rush as well as relaxation of Summer, I didn't get to the clipping, so streamers of new growth have cascaded nearly to the ground. I also didn't notice that some other growth had emerged this season, and in the opposite directon: Up. This ivy had finally begun to grow as an adult.

  

See at the top of the standard's round head? Two new stems are pointing upward.

 

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These upward-pointing stems aren't just young juveniles (so to speak), whose slender and fast-growing growth will begin to cascade when a foot or two long. Instead, these are young stems of adult growth—young adolescents (so to speak). And their habits and life trajectory are radically different than those of the juveniles. 

 

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In details large and small, adult growth of ivy is a different animal than juvenile. Its stems are thicker and more rigid, hence the upward orientation. And the distance between leaves is shorter, so the stems lengthen more slowly. Some juvenile stems cascaded five or even six feet just over this past Summer. While adult stems do lengthen, they rarely exceed four or five feet even after many years.

 

More strikingly, adult stems don't self-cling, because they don't form the rootlets that enable juvenile stems to adhere to almost anything they come into contact with. The only reason my standard does produce cascading streamers is that new juvenile growth—which I allow to emerge only from the standard's head, not the supporting trunk—finds itself suspended in space, with nothing to adhere to. These young stems haven't formed roots because they haven't touched anything. In contrast, adult stems project outward and upward, and with a rigidity that brings bushiness and bulk to what had been a comparatively lax mass of growth. Indeed, the only reason I have been able to form this standard's large head—of what began as floppy juvenile growth—was through faithful pinching back of that growth, leaving behind just the stubs to thicken year by year even as they remain juvenile. The standard's trunk—its oldest juvenile stem—is now so thick that the training stake is no longer needed: The standard is self-supporting.

 

In the picture below, you can see that the petioles of adult leaves are green, whereas juveniles are rosy pink. Adult leaves are a bit smaller, too, although their coloring is about the same. In both adult and juvenile leaves, the variegation of new growth is green and cream; the unusual slate-blue color doesn't emerge until a given leaf is fully grown. Young green-and-cream-variegated juvenile leaves are at the bottom-left, with mature blue-and-cream-variegated juvenile leaves above and below them. At the top of the adult stems, the foliage is variegated with just green and cream. The lower leaves on these stems emerged earlier in the season, and have matured sufficiently to change to blue and cream.

 

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The most defining characteristic of adult growth—sexual maturity—isn't yet in evidence. Mature ivy flowers in late Summer and Fall, so the lack even of buds by late October shows that these stems won't be mature enough to flower this year. Perhaps by next Fall, they will produce ivy's typical spherical heads of bizarre small "Those are flowers?" blooms. (Here are the blooms and berries of another of the adult ivies in my collection.) 

 

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Dramatic as they are, these aesthetic and sexual changes are only check-offs on an ivy's bucket list. When ivy begins to produce adult growth, other options and puzzles emerge that are as much existential as visual. First, adult growth emerges only from juvenile growth. While only adult growth flowers and produces seeds, those seeds only germinate as a new generation of juveniles. The conditions that cause a given stem to stop producing juvenile growth and start producing adult—that bring on that stem's puberty, as it were—are not well understood. At the least, years must pass: Juvenile ivies often continue to grow as juveniles for many years, even decades. I'm not aware of any cultural changes—in fertilizing or exposure, or the provision of greater root room via potting-up, say— that hasten the onset of puberty.

 

One factor that does help is to allow the ivy to climb—but only so high. As with my standard, adult growth seems to form most often only after the juvenile stems have climbed to the top of the available support. For a while, they keep extending past that support (and soon start to cascade) but the lack of both adherence and continued upward progress seems to foster the shift to adult growth. 

 

Perhaps one reason is that adult stems don't cling, and are also more rigid. On both counts, they could be more suited to growing upward and outward into open space, because the particular growing conditions at the top of the support are quite different than those even a foot lower. There's no more elevation for climbing stems to gain, and no more climbing to do. Further, wind at this highest elevation is likely to be stronger, and could more easily damage long slender lax growth than shorter thicker denser growth. Ivy has a strong sense of touch: Contact with a suitable climbing surface encourages formation of the adhesive roots as well as quickens the upward growth of the adhering stem. Perhaps the vine can also feel when its stems have lost touch with their support, as happens when stems of juvenile ivy overshoot the top. Forming adult stems might then "feel" much wiser.

 

The emergence of adult growth and, more importantly, the flowers it produces, is truly big news to pollinators. For years or even decades, they had ignored the ivy in its juvenile phase: Without flowers, what was the point of a visit? To form the first adult flowers at the very top of the plant might ensure the quickest and broadest notice among the pollinator communities. When additional flowers are formed farther down (which gradually happens), the pollinators are already used to paying a visit, so will easily discover those lower-altitude flowers even though they might be in the shade, or hidden by growth of neighboring plants.

 

Pruning is one factor that's likely to delay puberty. After all, it's the classic method of, at once, removing adult growth and encouraging formation of juvenile. Pruning is what keeps "silver dollar" eucalyptus from switching to production of its long-leaved adult foliage. Similarly, by cutting back smoke bushes and empress trees, production of immature (but showy!) juvenile foliage is assured.

 

Would pruning of adult ivy stems stimulate production of more adult growth, or more juvenile? It's likely that, as long as the pruning leaves some adult growth, new adult stems would emerge from dormant buds on that portion. It would be risky to assume, however, that new adult stems would emerge from juvenile growth below. 

 

That said, once even a single stem of a given plant of ivy has begun to produce adult growth, the rest of the plant slowly comes round to the idea: More and more stems switch from producing new juvenile growth to producing adult and, after many years, it's likely that production of juvenile growth would be abandoned. But removal of all adult growth would encourage re-emergence of juvenile stems from the base of the plant or even the roots. If I decapitated my standard, then, juvenile stems would sprout from the trunk and below. Given the long wait for appearance of adult growth in the first place, and its slow increase thereafter, any such "adulticide" would be quite a misfortune.

 

At any age, ivy retains its ability to originate new side growth from points all along the stems. Once adult growth has started—typically, at the very tip of a given stem—that tip's upward race of self-clinging juvenile growth stops. Apparently, then, so does the character or effectiveness of the growth-inhibiting hormone originating from the tip: Stems tipped with adult growth continue to send out side branches from farther and farther down but, eventually, they are of adult growth, not juvenile. I know of a house in Essex, CT, with a pair of large plane trees in its front yard. Both have ivy climbing up their trunks thirty feet and more. Over who-knows-how-many years, the ivy has been producing adult growth that, by now, emerges even from stems nearly at ground level. Each tree trunk is at the center of a cylinder of evergreen growth about eight feet in diameter.

 

Year by year, then, it's likely that my ivy standard of Hedera canariensis 'Gloire de Marengo' will produce less and less juvenile growth. There will be fewer long streamers by October, but more stubby and stiff adult growth that adds to the density and size of the head. Fall through Winter, the canopy will be wreathed more and more thickly with flowers and the berries that follow.

 

The standard is about a decade old, and its transformation to a fully fledged adult might take a decade more. That would be twenty years from a cutting that was trained up a pole, to an adolescent standard whose new growth was all juvenile streamers, to one whose growth is all stubby-but-flowering adult stems. Adult ivy has an indefinite lifespan; with reasonable care, this standard's adulthood will last many decades longer than that of its creator.

 

 

Here's how to grow this essential variegated ivy, as well as pictures of how comparatively slender this standard was when this posting was created back in Spring of 2012.

 

Here's a look at new growth of 'Gloire de Marengo' that began emerging when the plant was still in the overwintering greenhouse. The light was somewhat less strong there—both because it was still Winter, and because the greenhouse's translucent surface absorbed some—which had striking effects on the new leaves' pigmentation and size.

 
 
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