houseplant tuneups (and some plants to covet), with longwood’s karl gercens Leave a comment

COULD YOUR houseplants use a tuneup after a hard winter indoors? I know mine will need it, from re-potting, to light pruning, to full-scale rejuvenation in some cases, so I wanted to get expert advice. Karl Gercens has been growing houseplants since age 5, and the last 25 years he’s been doing that not just at home, but also in the historic Longwood Gardens Conservatory in Kennett Square, Pennsylvania, where he’s the conservatory manager.

In that time, Karl estimates he’s seen more than a million pots of display plants, many of them species we think of as “houseplants,” move through its galleries as different seasonal shows were staged. Each one has to be exceptional to make the grade, the best varieties (like a variegated Thanksgiving cactus, above) and also grown to perfection. So I know he has the answers on both fronts: on which plants to choose and how to care for them.

Read along as you listen to the March 6, 2023 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).

houseplant tuneups, with karl gercens



Margaret Roach: You’re a little plant-mad, aren’t you? [Laughter.]

Karl Gercens: I think that would be an understatement. I thought everyone was like me. I thought everyone collected plants from the age of 5. I thought everyone filled every windowsill and their car was covered in dirt, but maybe not everyone’s like that.

Margaret: [Laughter.] We should say, for people who haven’t been to Longwood, that even at this time of year you have exhibits on. There’s one called Winter Wonder [below] on right now, isn’t there?

Karl: Absolutely. And being from Mississippi, growing up in a warm climate, moving to Pennsylvania, I thought that was going to be tough. But when you come to a place like Longwood where it’s perpetual springtime, especially in the conservatory where there’s something in bloom 365 days of the year, winter has really become my favorite season. We have flowers blooming that are completely unusual to those of us who only garden outdoors. And it’s absolutely inspirational to see something that is not local, to see something that kind of takes you to a different part of the world.

Margaret: Yeah. I mean, I think it’s a must stop for anyone interested in plants, as a destination. So houseplants, as we call them, those of us who don’t have a conservatory [laughter]. So step one in houseplant success is probably trying not to impulse-buy just based on the pretty face of some the picture that we see online, but maybe doing some advanced homework to see if it’s a good match for us. Is that the idea?

Karl: Oh, it’s so terribly true. Oftentimes we go to a grocery store or a garden center or any place we buy plants, and we fall in love with it. And if we are one of those people that say, “I’m going to make this work,” do you really know what that means? Are you going to completely turn your life upside down? Are you going to change your light, your humidity, your temperature just for a plant? So maybe we need some self-reflection first, and get to know yourself, and get to know what conditions you have.

And I think the first step is, what am I looking for? What is light? Everyone says, “Oh, my house is bright,” but what is bright? “Oh, my house is warm.” Well, what is warm? So really trying to clarify what those conditions are, so that we can match a plant to you, and you truly will have success.

Margaret: So I mean, some of the toughest things, and I was saying in the introduction about winter, which is especially hard because we have the heat on … And I mean my thermometer and humidity gauge in the house, I mean, sometimes when the heat’s been on all day, it’s like in the 20-something percents of humidity. I mean, it’s really dry, right?

Karl: It totally is. And oftentimes, too, when I’m changing my sweatshirt and you just feel the static electricity, that’s a heads-up that you probably got some dry air going on in that house. And I was just talking to some of my students yesterday, and they were asking you, “How do I grow this specific philodendron, or this palm, happily inside my home?” And we were talking about ways to add humidity to your environment, and certainly the biggest way is a whole-house humidifier. But even with that, you’re barely going to hit 35 percent, 40 percent.

Adding humidifying trays, those large 12-plus-inch pebble-filled trays underneath all of your plants. Grouping plants together. But maybe even creating a plant room. So if you have a spare bedroom inside your home, maybe you’re turning the heat vent off and it’s just a little bit cooler than the rest of the house. And then maybe you do have all of your plants in one spot as opposed to spreading them out through the entire house.

Margaret: Yeah, that’s a good idea. And are there also some plants that stand up better? I mean, if we have to just face facts and that’s what we have, at least part of the year, is quite dry, are there some that are more durable in the face of low humidity?

Karl: Well, talk about low-hanging fruit. So that’s exactly right. Let’s start with those plants that don’t need all of that care, that don’t require us to turn our lives upside-down. And certainly the easiest plants are the ones I think with the funnest names. When you think about cast iron plants, well, that pretty much tells you what it’s made of. It is almost indestructible.

Margaret: Tough [laughter].

Karl: And I mean, this is a terrible name, but I learned it as a child, the mother-in-law’s tongue, which perhaps has been renamed snake plant, but the Sansevieria [above, at Longwood] are certainly super easy inside your home. And when you have a plant that thrives in your conditions, first of all, it looks beautiful, but you also feel a little bit better in that it’s not only producing some fresh air for you, but you feel like you’re being successful. So start with those plants that are almost going to guarantee success. And as you build up that confidence, then you can branch into things that a little bit more challenging.

Margaret: So I know from … we just recently did a “New York Times” garden column together, and also from just seeing the displays and what’s in the collection at Longwood and so forth, so when you get a … I think cast iron plant; is that Aspidistra, Is that correct?

Karl: It is, it is.

Margaret: Yeah. And then … or a snake plant, a Sansevieria, Karl doesn’t just go order any old, any old, right? You still ask to turn it up a notch, because I mean, after all, this is a world-class show you’re putting on. So tell us a little bit about how we up our game with even these toughies like that.

Karl: Well, yeah. So many times too, I had a previous coworker that sometimes snubbed his nose at houseplants. He’s like, “That thing looks like it’s from the 1970s.”

And I said, “Well, interestingly, the 1970s are coming back. People are liking the fashion styles and even some of the decor that came from it.” But with these houseplants, we don’t just have to have the plain-Jane green version. We can have truly exciting things. More value is incorporated into anything if it’s harder to find. So put a little effort forth, seek and find. Go to some of your favorite garden centers, your favorite websites, as we mentioned, networking through social media, to get those cultivars that have a little zing to them.

So things like Aspidistra can have a beautiful white stripe, or maybe even a white airbrushed tip. The cultivar ‘Snow Cap’ [above] is one of my favorites, and that’s readily available from places like Plant Delights Nursery in North Carolina, where they sell it as a garden plant. But for really the rest of us, it has to be a houseplant. And it is absolutely incredible because each leaf looks like it’s been hand-colored, and you get that beautiful soft white brushing at the tip. It’s not necessarily a hard edge, it’s not necessarily a speckle. I mean, it really is artistic.

And that’s just one of my favorite selections, cultivars, again, those things that you won’t necessarily find at the average plant shop, but if you seek them out, they’re certainly out there to be found.

Margaret: I know you gave me the tip, speaking of the social media, you told me if I wanted to find some of the things that you’ve talked to me about in our recent conversations for the Times article, you said, “Well, put that name, that plant name of the genus and species, so to speak, and/or the common name, into an Instagram search.” And see who all the total nuts are, the people who are obsessed with it. Find the societies and the groups and the individuals who have given themselves a name having to do with that or who post pictures of that type of plant all the time. And those are your real insiders. And then network with them to learn about unusual varieties, but also sometimes tips on where to get them, right?

Karl: Absolutely. I have found so many like-minded people through social media just by searching for a single plant name. There is a asparagus fern, which I think we featured in the Times article, which has this incredible, bright white new growth, and I-

Margaret: Like an asparagus fern? Is it a foxtail? Is that the one?

Karl: It is, yes. The foxtail asparagus fern [Asparagus densiflorus ‘Myersii’, above]. And that plant, I was just in lust over it the first time I saw it at the Philadelphia Flower Show. And I wanted that plant so badly. I’m like, “Where can I purchase one of these?” I was willing to pay $2,000 for one of these things [laughter]. I had just decided it’s going to be my entire paycheck. I’m just going to make this happen. And nowhere could I find anything for sale. So I might’ve been just doing some lamenting on Facebook, and just sharing with whomever was following me at the time that, “I saw this plant at the flower show. I really would love to find a piece of it. I’m sad that I can’t get it.”

And literally a person only 30 miles up the road messaged me and said, “I have a piece that I can give you.” And I literally almost fell off my chair. And it was that aha moment where you realize that you just have to ask. You just have to put it out there and say what you want, whether I’m seeking this out or I need some help with this, and the world of social media, it’s I think miraculous in that you’ve got these people from around the world that are willing to jump in and tell you what you need to know.

Margaret: Right. There’s one plant you’ve been growing, I guess you could call it a rubber tree or something, that you’ve been growing for a really … that’s the common name … a really long time. I think you’ve had them since probably childhood, different ones, but that you have an exceptional example of. So sort of as exceptional compared to the straight species, the plain green version, as that foxtail asparagus fern with its white new foliage was compared to the plain green one. So do you know what plant I’m talking about [laughter]?

Karl: Oh my goodness, do I ever. All my little children. I have so many plant passions. And of course if you back it up, my coworkers tease me that I have this thing for variegation. I said, “Wait a minute. It’s not just variegation. I have a thing for color.” And I really love … I mean, red’s my favorite color, and purple, and orange, and yellow.

And actually white is my least favorite color. So when you think of variegation, we typically think of a green and white. So technically, my variegates need to be of those more rich and intense colors. And certainly there is a rubber tree … there’s a cultivar called ‘Ruby’ that when I saw it for the first time, I mean, literally a warm rush must have gone over my body. And I was just thinking, “This is the ultimate plant. It has got color from top to bottom.” It has that color 365 days of the year.

A phrase that someone had told me years ago is “flowers are fleeting, but foliage is forever.” And that is so true, because if you can fall in love with foliage that truly excites you, whether it be red, pink, striped, swirled, speckled, every time you look at it, you’re going to get that pleasure. And I get that when I look at my rubber tree. That cultivar, ‘Ruby’ [above], is just one of my favorites.

And as a houseplant, it grows really well. Certainly with any Ficus, they’re going to have a change in leaf when they move, meaning they’re probably going to drop some leaves if they go from bright to dark or wet to dry or warm to cold. But once they even out in your life, they’re probably going to grow so well that you’re going to have to think about rejuvenation or pruning. And that’s when I became pretty daring when it came to how I handled my houseplants. I mean, this rubber tree is getting 5 feet tall in my 7-foot-tall bedroom. Something’s got to go, and it’s not going to be the entire plant.

Margaret: So let’s digress that into some care, because I mean, we wouldn’t do this in the dead of winter, but spring is approaching before we know it. Plants are waking up, and noticing the longer days of more intensity of light, and so forth. So there is a time for some pruning, some cleaning up, some repotting. You do some hard rejuvenation with certain kinds of plants-

Karl: I certainly do.

Margaret: So like a Ficus, it could do it. If it was just too big, you could give it a hard cutback. Is that what you’re saying?

Karl: You certainly can. And I find so many times the people I’m dealing with, even in my community or even with some of the students that I’m teaching, that we’re often afraid to do pruning. And while I think that’s probably true on woody plants that are outside, we don’t need to be pruning our maples really hard … I mean, that’s horrible to be cutting the tops out of some of our woody trees. That’s just horrible. But as these houseplants go, we should think of them really more as perennials.

And with our perennials, even like Caryopteris, you prune that back to a nub in the fall or even in the spring, and it completely regenerates from that. And you have a perfectly natural-looking plant again. And I think that is the same with so many of our houseplants, including almost all the Ficus. I’ve had great success with rubber trees, with the fiddleleaf fig, with the weeping fig, to the point they literally touch the ceiling.

And I’m not buying a bigger house just to grow the plant, and I’m not going to make a bigger pot for it. So you have to make it work in the space and in the pot.

Margaret: So other species of plants that might be good for this? Like pothos, I would imagine you could do a hard cutback if it’s just looking scraggly and nasty or if it’s just stringy. [Above, a yucca that has regrown after a hard cutback at Longwood.]

Karl: Oh, it’s so true. And oftentimes, people seem like they’re proud of their pothos when it starts to creep over the edge of the bar and up around the windowsill. I’m like, “Oh, no, no, no. We do not want this plant vining through our house.” It needs to be full and bushy, and almost look like it did when you bought it.

So cutting those things back sometimes to even 6 inches from the surface of the pot is perfectly fine for pothos and even some overgrown ficus.

But you were alluding to the fact that spring is coming and the light is getting more intense, the temperatures are getting warmer, so our plants are starting to respond more. And I would oftentimes say that April, May, are great times to prune our houseplants back pretty hard and let them start to rejuvenate through the warm days of summer.

Margaret: So not palms; I know that wouldn’t be good. Who else could I do this with though, so some other examples? Could I do it with some of the Dracaena, or …

Karl: Oh, so true. So true. When you buy your Dracaena, sometimes called corn plant, they’re those giant canes that were grown in warmer areas. And you already look and they have a giant cut, and they’ve got probably three stems coming off of them.

And Dracaena are so easy to grow inside the home. I find that those things, when you purchase them, maybe they’re 10, 12 inches’ worth of foliage, but then it stretches out to 2 feet,3 feet, and it just starts to look silly. So you can literally prune those things back to within 2 inches of where they had started growing, to the point there’s no foliage left at all. And they will completely regenerate from that.

And not only will they look full and lush, even within the first season, you have a chance now to play with rooting some of the cuttings you’ve taken off of there.

Margaret: Really?

Karl: We took some Dracaena and Ficus, very fun to try to propagate. See if you can start a new one, share those with friends and neighbors. [Above, Karl’s favorite Dracaena ‘White Aspen’ at Longwood.]

Margaret: So that trunkish-like thing that I’ve just cut off, I do what to propagate it [laughter]?

Karl: Definitely. So with the Dracaena, let’s just say you had a two-foot stem you chopped off. I would take it down to about 12 inches at the top. Strip off some of the foliage at the bottom, and you’re going to see those leaf scars where you pull the leaves off. And then if you have a humid environment, maybe that’s going to be a container with a plastic bag over it, you can leave the foliage intact and just push it down in that barely moist media, peat-based with some perlite, some charcoal. Keep it moist, keep it warm, and those things will root within a month or so, as long as it’s nice and warm.

Margaret: Oh, that’s crazy.

Karl: And you’ll be the crazy plant lady having all kinds of small baby plants here, there, and everywhere. But they make great gifts when people stop by. And you can share your passion, and you can share your story with someone else. And in turn, they’ll have a story to share with the next generation as well.

Margaret: I think you can do that, not exactly what you just described, but you can do a hard cutback rejuvenation of Schefflera also. Is that another one that’s a good one?

Karl: Schefflera‘s great. That’s another one that really grows well inside my home. There’s two species that we oftentimes deal with. The smaller leaf version they sometimes called arboricola, and then the larger-leaf version actinophylla. And they’ll have cultivar and trade names that go with them. But I have definitely played with Schefflera over the years, trimming them back to a nub, talking 2 inches from the soil surface. And they completely regenerate, nice, full, lush, bushy.

And it just makes you feel fresh and your plant has come back to the way it was. If only we could turn back time ourselves and have no longer our gray hair and our body aches, and if we could just rejuvenate ourselves like that. Wouldn’t that be nice?

Margaret: Yeah, I’d like to get a good cutback. Yeah, definitely. That’s pretty funny. Excuse me. So one that I want to ask you about, because you have this incredible looking sort of holiday cactus, this variegated one. In the course of being a parent to a holiday cactus, whether it’s a Thanksgiving cactus or a Christmas cactus or whatever, what should we be doing through the year for that to make it happy and bloom and so forth? What’s the sort of protocol? And what’s the name of your variegated one that I’ve seen in pictures?

Karl: Oh, my goodness. That variegated Thanksgiving cactus [above], the cultivar is called ‘Norris Variegated.’ And I got that from a wonderful plant guy up in Maine. Lordicultural is the name of this company [a list of sources, including Lordicultural]. But he has 300 cultivars, 300 named cultivars of holiday cactus. I didn’t even know there were that many out there. I mean, I’ve been doing plants since I was a kid. I thought red, white, peach, pink. It blew my mind that … first of all, I fall in love with the flowers. They’re just beautiful when they bloom in the winter.

But again, if you’ve got foliage that is equally as interesting, the cladodes on there can be this beautiful, buttery yellow. And one might be half-yellow, one could be solid yellow. They’re kind of a grayish-green. I mean, each one’s different.

And of course, as you know with holiday cactus, they have almost like those finger-like stems that come out, and it just looks like there’s movement. And from a distance, the thing looks like it’s blooming even before it has flowers on it.

But working here at Longwood, I was unaware that those things would reliably bloom twice for us. So they always bloom in November, hence the Thanksgiving cactus is the most popular one. And we would just leave those baskets hanging in our conservatory and they would re-bloom in March, complete heavy flowering, just as if it were Thanksgiving.

Strangest thing. I have no idea. Tried to figure out the science behind it. We know that we can manipulate holiday cactus to flower when we want them to. It really is a change in light levels in the fall. So once the nights get longer, that triggers them to bloom. Once it starts to cool off a little bit, that triggers them to bloom.

So I have found that inside my home, I won’t get reliable flowering around Thanksgiving if I have it in a room that I’m living in. Because you go in there and turn the lights on and the plant thinks that it’s a long day, it’s summertime, and it may not be triggered to flower. Or perhaps in our homes we have that constant 68 to 72 degrees, and the plant’s like, “Oh, it’s summertime. Why should I go into flowering mode?”

Margaret: I see. O.K.

Karl: So in order to trigger your Thanksgiving cactus, oftentimes I would say the guarantee is to put it into that unused bedroom where the light’s not going to be turned on in the middle of the evening, and perhaps that room where you’ve turned the heat vent off and it’s actually going to chill down a little bit. Maybe that bedroom has a north-facing window that you sit your pot on the ledge and just the natural coolness of the window will help trigger that into flowering.

Margaret: And you’re saying put it in there all year round, or put it in there at a certain time of year?

Karl: I think certainly in the fall. So between September and November, that’s the trigger time for those. But then once it finishes flowering, you can bring it back out into your regular living spaces, only because … And I have not been able to make my Thanksgiving cactus rebloom in March at home. It only does it at Longwood. And again, I don’t know what the trigger is. I’m going to have watch what’s happening.

Margaret: You have the magic mojo there at Longwood.

Karl: We’ve got some pixie dust, I guess.

Margaret: I want to just get some of the basics for sort of … Like you fertilize all year, and I had always stopped in the winter in the low light season. But so you do sort of a weakly-weekly? Is that what you do year-round, you fertilize?

Karl: That’s generally what I would suggest, because inside our homes, it’s that constant temperature, so that’s not slowing the plants down. And while we could have lower light levels, you might supplement with some grow lights, but just having that really weak solution all the time, I think, is pretty good.

And remember, in between fertilizing your plants, you should always be leaching that soil out. So take it to the sink, better yet take it to the shower, and just rinse those leaves off and let that water flow through that media so it cleans the soil out, because fertilizer is made of salt, the salt can start to build up. And whenever you’re washing that excess away, it’s just making it ready for the next round that the plant could then take up.

Margaret: Right. And similarly, I mean, water-wise, if you have a water softener in your house, that can add some salts to your water, so that’s not good water. What water do you recommend for watering houseplants?

Karl: It is so easy to just put a bucket outside, or if you have a large trashcan that your rain gutter can flow into. But even as a kid, I would collect rainwater. And I grew up on a farm. We had clean water around us, no problem. But rainwater, it’s magical. It really is. And that stuff is so good for plants.

Certainly if you’re leaving your bucket outside, it’s going to be probably pretty cold. Just scoop some up, bring it inside, let it warm up to room temperature. And it’s almost like revitalization. It’s the fountain of youth for plants to let that rainwater come and clean out the soil and really start things afresh.

Margaret: All really good advice. And the thing that I’m mad at you for, Karl, is that I’m going to have to go on a houseplant shopping binge [laughter] because you make me want all these variegated versions, and so on and so forth. But thank you for sharing all these tips and also some of the suggested varieties. They sound gorgeous.

Karl: Absolute pleasure. I wish you luck in having room for all of your new houseplants. It’s a wonderful addiction to have.

Margaret: Yes. And happy Winter Wonder show as well.

Karl: Thank you so much.

Margaret: I’ll talk to you again soon, I hope.

(Photo credits: Ficus elastica ‘Ruby’ by Karl Gercens; Aspidistra ‘Snow Cap’ by Plant Delight Nursery; all others by Becca Mathias for Longwood Gardens)

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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 13th year in March 2022. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the March 6, 2023 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).

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