Plants on the Move

  • Difficulty Rating: Beginner
Plants on the Move

Some plants are just tougher than you think.

I know a gardener who moved from Boston to Virginia simply because he wanted to live in USDA Hardiness Zone 7, a mild-weather swath of the country where stately Southern magnolias flourish. In Zone 7 (minimum winter temperature 5 to 10 degrees) he could grow magnolias, camellias, figs, and other plants too tender for the freezing winters in Boston (where the minimum winter temperature can drop to 5 degrees below zero). The move was a big success, and his garden is a showplace, but gardeners who have their own roots more firmly planted are discovering that cold-tolerant hybrids are allowing them to change the look of their landscapes, too — without moving.

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Southern magnolias are a good example of what is going on. Technically, these handsome evergreen trees are hardy in Zones 7-9, and shouldn’t grow any farther north than Texas, Arkansas, Tennessee, and Maryland. But gardeners are always pushing the limits. We experiment with plants and hardiness wherever we live, and Southern magnolias, with their enormous, luminous white flowers, have proved to be pretty adaptable. In the Kansas City area, a cultivar called ‘24 Below’ has survived brutal winters; ‘Bracken’s Brown Beauty’ and ‘Edith Bogue' are both surprisingly winter hardy, too. According to the renowned plantsman Michael Dirr, ‘Edith Bogue’ was introduced to New Jersey from Florida — and it has turned out to be so hardy that a new generation of gardeners can now appreciate the charms of the Southern magnolia, far outside its natural range.

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Hybridizers working on cold tolerance have introduced camellias, gardenias, crape myrtles, figs, and hydrangeas to cold-winter gardeners who, in the past, could only appreciate these plants on vacations in mild climates. William Ackerman, who died at 89 last year, was a pioneering plant breeder at the National Arboretum; he introduced more than 50 cold-tolerant camellia hybrids. Ackerman’s work was based on a group of camellias that survived bone-chilling winters in the Washington, D.C., area in 1978 and 1981. His hybrids (‘Winter’s Charm’, ‘Winter’s Beauty’, and ‘Winter Star’ are three good ones) are known to survive to -15 degrees.

Cold hardiness is relative, of course. Growing camellias in Kansas City or Des Moines is always going to be a challenge — and to gardeners who are willing to pamper young plants, wrap them in burlap against biting winter winds, and risk losing a bet against nature, it’s worth it.

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To improve your chances with a marginally hardy plant, look for the toughest cultivars. Modern mop-head hydrangeas (‘Endless Summer’ is the best known of these) survive winters in Zone 4 (minimum temperature - 25), and produce buds every summer on new growth. Hardy crape myrtles withstand winters in Zone 6, where the temperature can drop to -10. If they die to the ground in a really tough winter, wait until you’re really sure and then use sharp loppers or pruning shears to cut off the dead stems; new shoots will come up. Two gardenias, ‘Frostproof’ and ‘Kleim’s Hardy’ are rated for Zone 7 but deserve a look if you’re a Zone 6 gardener willing to take a chance. If you love boxwood but can’t grow aristocratic English box, look for cultivars called ‘Green Velvet’ or ‘Chicagoland Green’, both great plants in cold climates.

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A marginally hardy plant may be able to survive an occasional quick cold snap, but perhaps not temperatures below normal for weeks. The Hardiness Zone map is based on long-term averages, not isolated extremes.

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Make room for these plants in the most forgiving spot in your garden, protected from cold, desiccating winter winds. Mulch the ground around them to insulate against frost. Prune in spring or summer, if necessary — but not in fall, because you don’t want to stimulate new growth just as winter arrives. In general, take especially good care of them: a plant that goes into winter in good health has the best chance to survive the challenges of the season and come back strong in the spring, ready to make you look good, too.