Last year, just before Christmas, a polar vortex brought sudden sub-zero temperatures to many states in the South, wreaking havoc on gardens and landscapes that were not designed for that kind of cold. To discuss the damage that has been apparent so far and what could reveal itself come spring, my guest this week is horticulturist Troy Marden.
Troy has lived in Tennessee for 25 years and his career has included many kinds of horticulture work, including working at nurseries and botanical gardens and designing landscapes. In 2017, he launched a new venture, Troy B. Marden Travel, a tour company that takes small groups on visits to the great gardens of the world, and small, private gardens that rarely open to the public. He’s also the author of the “Southern Gardener’s Handbook.”
The last time I saw Troy, we ran across each other in England at the Chelsea Flower Show last year. Troy points out that the gardening world is quite small, so it’s not unusual to run into people you know at international gardening events.
Troy’s Gardening Origins
Troy grew up in north central Kansas on the prairie, and he has gardened all his life. His grandparents gardened — and his parents vegetable gardened to varying degrees of success.
He recalls that the first seed he ever planted was in his babysitter’s flower bed when he was 3 years old. His babysitter had a big silver maple tree. “The silver maples have those huge helicopters that fly down in the spring,” he says. “And to a 3-year-old kid, they were just the most fascinating thing I’d ever seen. And somehow I had this innate sense that if I poked them into the ground, they would grow — and they did. And her flower bed was full of them, and she said to my parents, ‘Come get these things.’”
All these years later, two of the trees are still standing in his parents’ yard.
Troy’s early interest in growing was stoked by participating in 4-H and FFA, the group formerly known as Future Farmers of America. Starting at 13 years old, he worked at a local nursery and continued working there until he moved to Tennessee when he was 22 years old. “I had a little leg up on a lot of people because my career started 10 years before anybody else’s normally does,” he says.
Troy’s Travel Company
Because Troy works in the travel industry, 2022 was spent making up for two years’ worth of lost opportunities due to COVID. Many of the people who had signed up in 2020 and 2021 and had their trips postponed were finally able to go. Now in 2023, he’s starting fresh.
“People are ready to travel, but I think there’s a little hesitancy out there from time to time,” he says. “I don’t think we’re fully back yet, but we’re on our way.”
He takes small groups, just 15 or 16 people most of the time, to visit private and public gardens around the world.
He notes that gardeners like to share, and that’s how he’s been able to leverage social media to access private gardens that would not accept busloads of visitors. With just 15 visitors rather than 55, it can be an intimate experience, and the private gardeners don’t have to worry about their garden being trampled, he says.
The Polar Vortex
Troy and I were among the Southeast gardeners affected by the polar vortex around the turn of the year, him in the Nashville, Tennessee, area and me in the Atlanta, Georgia, region.
“Tennessee really was right in the crosshairs of this, and I know Atlanta got hit,” Troy says.
He lives up high on a ridge, about 40 minutes outside of Nashville, which sits in a “bowl” where the weather can be quite different.
“What I had at my house was a more than 50° temperature drop, from about 50° on that Thursday morning before Christmas, or that Thursday lunchtime before Christmas, to 4° below zero in eight hours,” Troy says.
Plant cells are full of water, and water does two things when it freezes, he says: It crystallizes and expands.
“Those sharp pointy crystals puncture the cell walls, and when they thaw out, all of that water leaks out of the cells, and that damage is literally what we’re seeing on plants like Aucuba that have just turned completely black and plants like skip laurel that have just turned completely brown,” Troy says.
The punctured cells died in an instant once they thawed, he said. It was evident in some plants, like Aucubas, the next day — though Troy thinks if he cuts his 15-year-old Aucubas back to the ground they will grow back up from the roots.
“The nice thing about some plants like that, when they’re well established, they’ll push back up very quickly,” he says.
Aucuba may be hardy to zone 7, but it is a subtropical plant with a big, very fleshy leaf, Troy says.
Then there are plants like boxwood, with small, hard leaves that have waxy surfaces. Troy’s were just fine for three weeks, but then orangey-brown lesions began to form, and within two or three days, the leaves began to turn brown and shed.
Some clients have boxwoods that appear to be just fine with no signs of damage, but boxwoods that were already stressed due to drought, leafminer, or some other affliction have not fared as well.
As for skip laurels (Prunus laurocerasus “Schipkaensis”), they began to present damage within a week,
“I have some that are 15 years old and 15 to 18 feet tall at some clients’, and we’ve scratched the bark all the way to the bottom of the plant and cannot find anything alive,” Troy says.
Skip laurels, also called schip laurels, are one of the main screening plants used in Nashville, and Troy hasn’t seen one yet that he thinks is alive.
Otto Luyken laurels lost their leaves, but their stems are still green, so he has hope they will leaf out in spring. This is despite them being the same species (Prunus laurocerasus) as skip laurels.
“Some of the damage was very immediate and very visible, and I think we’re still seeing damage appear today,” Troy says of the polar vortex, adding that he predicts more damage will become apparent through the spring. “There are some things that are still maybe hidden to us.”
He points out that it was easy to notice damage on evergreen plants because they had foliage on when the damage occurred. For deciduous plants, Troy was worried about seeing bark split in Japanese maples, crepe myrtles, dogwoods and other thin-barked trees, but says he’s been lucky so far. However, if there is residual damage to those trees from the polar vortex, it will become visible when the plants put their energy into leafing out in spring.
“For some plants that are already struggling, unfortunately, we may see more plant death when things try to leaf out because they’ll push out that big burst of energy to try to put on leaves and that’ll be all they have left,” he says. “So I don’t think we’re done. And I hate to be the bearer of bad news, but I also feel like all of us in the industry need to be honest with ourselves about where we are in all of this, and we need to be really honest with our clients and with the public.”
Troy knows one nurseryman who expects he lost between 6,000 and 8,000 hollies, among other damage.
Hollies are tough plants, but the damage they tolerate varies from variety to variety. Holly hybrids like Robin, Liberty, Oakland and Mary Nell got hit really hard, while a Nellie R. Stevens holly growing right next to them was fine.
Troy has heard stories from the 1980s, before he moved to Nashville, that a big freeze froze crepe myrtles and 40-year-old Nellie R. Stevens hollies completely to the ground. “They came back up from the base eventually if you were patient enough,” he says.
Then in the 1990s, a freeze defoliated all the hollies, and some did not leaf out again until June. “But they did finally leaf out,” Troy says.
These examples emphasize for Troy why gardeners should do the scratch test: Scratch the bark with a thumbnail or knife and see if it’s green underneath. If it is, the plant is still alive.
If you see bark split in a plant, especially thin-barked plants like Japanese maples and dogwoods or in a plant that is pushing its zone like Azalea, that means the cambium layer has frozen and died. That is the layer that moves the water and nutrients up and down the plant.
Wind, and How It Damages Plants
“When a broadleaf evergreen plant goes dormant for the winter, its root system also goes partially dormant, and so it can’t take up water as effectively as it does during the growing season,” Troy points out.
Take for example the southern Magnolia, with its broad leaf surface.
“We had 50 to 55 mile an hour winds here in Tennessee, and so when your root system is frozen and you can’t draw up any water, or you’re semi-dormant and you’re not drawing up as much water but the wind is blowing 50 miles an hour and you’re getting evaporation out of that leaf surface. That’s another thing that we’re seeing is this foliage just freeze-drying. Because you’re in a wind tunnel.”
“By the time we had dropped below zero here, the wind had died down, but that afternoon, while we were in the teens and the single digits, as the temperature was dropping, we had 50-to-55-mile-an-hour winds. So the wind chill at that point was well below zero and so not only are we getting actual physical damage to the plant from the freezing of the cells, we are getting wind damage. We’re getting windburn. And the plants are freeze-drying.
Troy also notes that he had no measurable rainfall from July 1 to the middle of October, so his garden went into winter in a drought-stressed state.
“The drought has played a factor. Plants were already under stress here, then this happened.”
He also thinks if the temperature dropped over the course of 48 hours or 72 hours rather than within eight hours to -4°, the plants would have been more resilient. A plant can quickly prepare itself for an event like the polar vortex if it has a warning, he says. “It will voluntarily decrease the concentration of water in its cells, which bumps up the amount of sugar, and sugar water doesn’t freeze or it doesn’t freeze as quickly.”
Native Plants and the Polar Vortex
When Troy visited a nursery post-vortex with both American hollies and non-native red hollies, the American hollies looked much better. But where he lives, where the wind was whipping 55 miles per hour, natives like Illicium floridanum (aka Florida Anise bush) are just as brown as the nonnatives in his yard. On the other hand, the Illicium has green stems and will likely leaf out in spring again.
“The jury is still out on native plants,” Troy says. “Did they survive better? Perhaps some of them did, but I don’t think we’ve seen the full extent of the damage yet, just like we haven’t with some of our non-native ornamental plants.”
None of the wild eastern red cedars alongside the road near Troy are brown at all, but some of the cultivars used in landscapes are showing damage, he notes.
A Big Loss
Troy had begun digging up his plants in anticipation of a move within the next year. He has between 600 and 700 plants in pots right now, but after the polar vortex, he thinks he’s lost about a third of them to freeze.
“A lot of the woody stuff probably will be okay. A lot of the things that grow from a bulb or a rhizome probably will be okay — the peonies are gonna be fine,” he says. But he anticipated he lost about half of his daylilies, which have a soft crown.
“The good news is, in my breeding program, the stuff that lived, it’s going to be really, really hardy,” he says.
What to Expect
Leather-leaf-type Vibernums defoliated after the polar vortex even though they are rather evergreen in Nashville. Troy thinks there will be some deadwood to remove but the plants will survive. “But I’m not going to cut anything until I’m sure that it’s dead,” he adds.
Cryptomeria took a huge hit. Big old trees — 25 or 30 years old — did not have a green leaf left on them. “Typically you would say a conifer, if it defoliates, is not going to leaf back out from, quote-unquote, dead wood or bare wood,” Troy says. In this case, he’s unsure what’s going to be revealed come spring.
Leyland cypress (Cupressus × leylandii) and hinoki cypress (Chamaecyparis obtusa) both took heavy damage. But Sawara cypress (Chamaecyparis pisifera), including the popular cultivar “Golden Mop,” took no damage.
“What I always say is that a plant that I’ve lost is a gardening opportunity. You know, it hurts sometimes, especially if you lose something that’s special to you or that you’ve paid a lot of money for. And this is gonna be costly, unfortunately for a lot of people, it’s going to be costly for homeowners. It’s gonna be costly for professionals.”
He notes that some plants may not be as readily available this spring.
Troy anticipates many of the macrophylla-type Hydrangeas will grow back from the base, perhaps after some really hard cutting. And they may not have very many blooms because they bloom on old wood.
“Endless Summer and some of those new varieties that flower more readily on their newer growth, you may get some blooms late in the season. I think the paniculata Hydrangeas, like limelight and all of those popular ones, they bloom on their new wood. So even if you have to cut them to the ground, you’re going to get flowers this year,” Troy says.
“As we’ve been going around and doing the bark test, paniculata Hydrangeas seem, at least up here, to have weathered this pretty well. They are pretty green underneath the bark.”
Nashville is right on the edge of Camellia territory, so only the very hardiest Camellias can grow there.
“They have defoliated entirely up here, but most of them that I’ve seen are scratching a little bit green under the bark, so we’re going leave them and we’ll see what happens,” Troy says.
Preparing for the Next Polar Vortex
“We’re dealing with Mother Nature, and Mother Nature is always in control no matter what — always going to win. And as gardeners, we just kind of learned to accept that,” Troy says.
Anti-desiccant doesn’t necessarily help with anything at a cellular level, but it will protect the surface of the leaf and maybe give you a couple of degrees of freeze protection,” he says.
In Florida when a deep freeze is coming, citrus growers will turn on their sprinklers to protect their crops.
A lot of people think that that is because ice is an insulator, and that actually is not quite true,” Troy says. “Ice doesn’t insulate.”
What does happen is that when a water molecule freezes, it gives off heat.
“So that’s why in the citrus fields in Florida, they run the water the entire time that they are below freezing. … That act of water turning to ice gives off just enough heat to keep things from freezing,” he explains.
“As industry professionals and as homeowners and as gardeners and people who love our outdoor spaces, I don’t think we can give up on plants that we’ve lost,” Troy says.
He says this is, he hopes, a one-off weather event that should not dictate what he does going forward. For example, red hollies have been good to Nashville garden designers for almost three decades, and he doesn’t plan to give them up.
He notes that many native plants are very site-specific, so he urges caution before making misleading blanket statements about replacing cultivars with natives. A plant that might be native in one part of your state will not necessarily do well where you intend to grow it.
I hope you enjoyed my conversation with Troy Marden and have gained a better understanding of the polar vortex’s effect on plants. If you haven’t listened yet, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
Did the polar vortex damage your garden? Let us know in the comments below.
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