I SUSPECT I’m not alone when I say that weather extremes in recent growing seasons have made me feel a bit like a stranger in a strange land in my own garden, wondering what will bloom when and when to do what. And most of all, wondering what madness is coming next.
I talked about managing our gardens and especially its longest-lived residents, our trees, in changing times with Daniel Weitoish, the Arboriculture Supervisor at Cornell Botanic Gardens in Ithaca, New York, where he and his colleagues care for the woody plants at the 30-acre botanic garden, plus a 100-acre arboretum and about 3,600 acres of natural areas.
Like plant people everywhere, they’ve been feeling their way through the impacts of a shifting climate and extreme weather events, and now that winters are somewhat milder, even experimenting with adding some new plants to the collection.
Read along as you listen to the Aug. 12, 2024 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 (and browse my archive of podcasts here).
extreme-weather tactics, with daniel weitoish
Margaret Roach: After I talked to you for a recent “New York Times” garden column, I have to say, I felt relieved in a way that there’s that sort of brotherhood feeling or whatever [laughter], that camaraderie, that even at your level at the botanic gardens, you guys are also having to figure your way through. And I think that’s good for other gardeners listening to hear. It feels a little unfamiliar out there at the moment.
What’s the biggest thing there over the last couple of seasons and so forth, is it the change in rain patterns or is it heat or what’s the thing for you at Cornell up in Ithaca?
Daniel Weitoish: It’s probably two main things, for me in the trees anyways. One is that earlier and longer season, so longer at both ends. And two is just unreliable water; it is feast or famine frequently. Low snow pack coming into spring, drought very early and then it might pour later in the season. So yeah, water management and just changes from the norms.
Margaret: Yeah, yeah. You said longer season at both ends. And I keep wondering what it’s like to be like a bud or parts of a plant that in the old days, I’ll say, we used to have a proper winter, and first there was a fall that preceded it that gradually got cooler, the things used to we would say “harden off,” and then at the other end they gradually wake up. And I feel like, oh my goodness, it’s like, whoa, whoa, zero to 100, zero to 100, zero to 100. Do you know what I mean?
Daniel: Literally, temperature-wise sometimes for sure.
Margaret: But also just, how does that, it must just change the protective layer, so to speak, on those parts of the plants that … they must be so vulnerable and so forth, it’s just …
Daniel: For sure, leaves that are emerging, they haven’t yet formed the cuticle to help them resist the sun. So if we have less cloud cover, a lot of heat, that can cause some foliar damage for sure.
Margaret: Yeah. So learning to read the signals: When we did the Times story, we were talking about how, I’ve been gardening a long time, and so to me I always knew, oh, around Memorial Day I do thus and such, and around… I had an association with the calendar and certain garden activities or bloom times or whatever. And that’s all kind of off, not happening exactly that way anymore. You and I talked about learning to read the signals, instead being a little more of an observer, a more careful observer. What are some of the signals you see and that you’re learning to read? Where are you taking some of your clues in how to manage things?
Daniel: Sure. We can come at that from a couple of different angles. So the signals telling us when to initiate plant care, so we’re looking and being really conscious about when we water, for instance, or fertilizing as well. We try to not use a lot of fertilizer in the landscape. But with unreliable rain, you could fertilize and have that wash off very quickly, not delivering the impact you’re hopeful for in your garden.
Margaret: And do you see certain plants that are the early signals, are the indicators, so to speak that, “Uh-oh, this is too much right now?” Because I know I do, right now, for instance, here it is, it’s what, close to the end of the first week of August, I have one Fothergilla shrub outside that’s already in its fall color, it’s stressed, it’s been under multiple stresses during this … we had very hot and periods of dry and whatever craziness, and so it’s telling me, “Hey, I’ve had enough, Mom.” [Laughter.]
Daniel: Pack it in for the season. Yeah, there’s a red maple out in the middle of a field [above], it was a recent installation, and that red maple right now is in full fall color as well. So absolutely. That early fall color, a lot of the mechanics that cause fall color can be … it’s simulated whenever there’s a drought. So yeah, that is definitely a sign that you look for.
And in that theme as well, up here, upstate New York, New England, fall color, the changing of the leaves is a very exciting time of the year. People travel from all over the place. And that is another thing that can be thrown off. Is it as predictable for folks that might need to travel to come see those leaf-changing events? It’s increasingly unpredictable.
Margaret: I remember when we did the Times piece also that you were saying that some of the dogwoods [above, Cornus florida; photo by Rob Cardillo] are early indicators, that their leaves will curl and things like that earlier, and when you see that you’re like, “Uh-oh.” And not just for them, right?
Daniel: Yeah, they’re easy communicators perhaps. [Laughter.] They almost seem to enjoy suffering to some extent. They are some of the first trees that are going to show those characteristic water-stress habits with flagging foliage, edges starting to curl. And we know that it’s a problem when that foliage isn’t recovering on an overnight. So if we have very hot nights, insufficient precipitation, we’re not seeing the rebound necessarily. So that’s another sign for us to initiate some kind of triage.
Margaret: And sometimes wilting or curled leaves and things like that, sometimes it’s something more serious than inadequate moisture, and sometimes you’ll see yellowing of leaves on a tree or a shrub and people are like, “Oh, I’m going to fertilize it, it’s hungry.” And we shouldn’t be rushing to judgment, we need to look more closely and maybe get some diagnostic help? How do you advise people not to, as I said, rush to judgment?
Daniel: Sure. We have such a capacity for care for the plants. And we can definitely smother them with that love. So too much fertilizer is something that can absolutely cause that, it can lead to a buildup of salts in the soil or it might not even be necessary, it can initiate a flush of vegetative growth that is now less protected, less resilient to insect or other kind of herbivory. So definitely.
If you’re seeing some kind of out-of-the-ordinary, like you’re saying, yellowing or marginal necrosis or death in the leaves, then a diagnostic test is a great first step. So leaning on those extension networks, nearly every state, well, certainly every state has that extension network for submitting a sample of soil or foliage, determining is it a disease, is it a fungus, is it an insect, low nutrients in the soil? And then you can do what is correct for the plant.
Margaret: Because the other thing that’s great about contacting your county extension is it’s not unusual that they’ve been hearing a similar observation from 17 other people in the area at the same time because the conditions being the same nearby and so forth. So they may actually, even from a photograph, say “Oh yes, we’ve been having a lot of reports of this and such.” You may not even in some cases have to wait for a pathology report or whatever. With the soil things, absolutely you do. So they have so much insight to share. And I think in these hard times, I think we need to really get to be best friends with our county extension agents in a way that maybe people haven’t in recent years, really.
Daniel: For sure, totally agree.
Margaret: So we have our eyes wide open [laughter], sometimes we’re not seeing good things, but we have our eyes wide open for things, for the signals. With watering, you were talking about how it’s the on or the off switch for you guys, and same here, it’s either we get 3 inches at a time in about 5 minutes or we get nothing. Are you using different watering tactics, or how do you know when to water? With the trees and shrubs I’m talking about especially. And I know they’re probably in mixed beds with herbaceous companions and so forth. But how do you decide or is everything on a schedule or how does it work?
Daniel: Sure, sure. So back in 2016, we got caught off guard a little bit with a terrible drought. And this caused us to really do some thinking. We’re in the Great Lakes Region, this is the land of freshwater. Having a deficit of it, we weren’t ready for it; we had no plan. So since then we’ve started developing some things to help our collections. One is to prioritize the most important plants in the landscape. If we have a 250-year-old oak, then maybe that’s something that we need to prioritize. Anything that’s newly planted, so something planted in the last three or four years hasn’t developed a robust root network in the face of a very uncommon drought. So those are some of the first things that we start watering.
Methods for doing so, we’ve used the gator bags that can be put around the trunk of a tree, and you fill them with water, and it slowly percs into the soil, giving a nice deep watering. That is an important aspect there as well. So we want to water infrequently, but very deeply. Rather than that everyday watering. With an everyday shallow watering, the roots, they’re not going to hunt for deeper soil, they’re going to stay right at the top. And then when that drought comes, they’re going to suffer more.
Margaret: And so I believe you guys use some soaker hoses [top of page] and other equipment and so forth, but yes, to get it right there on the root zone?
Daniel: Yeah, and the root zone can be wider than the drip line of the tree. So the extent of the branches where water would be captured by the leaves and dropped to the ground, roots can be much wider than that. So making sure that we’re going wide, we don’t want to stay right around the base of a tree. Soaker hoses, they’re great for us. These, again, slowly percolate down into the soil, we’re not watering foliage, where the water is going to evaporate before it even makes ground contact.
Timing is very important, too, so watering on the overnight when there’s no sun, it’s not causing that water to evaporate. We’re setting timers on our hoses so that that watering happens at 3 or 4 A.M.
Margaret: And people could set up drip irrigation on a timer as well if they wanted to. It’s a little more, well, it’s a lot more precise to get it set up than laying out a soaker hose, I would imagine. But that would be another way to not be spraying water into the air, and to be able to do it on a timer and so forth as well.
Daniel: Right.
Margaret: And of course mulch, that’s the other thing is I feel like I’m mulching earlier in the spring because I’m concerned that, well, first of all, everything around all my trees and shrubs is going to come poking out of the ground sooner than it ever did. But also because these bouts of dry and even heat can come so much earlier, and I want to retain the soil moisture. Are you finding that also that you’re putting the mulch down earlier and so forth?
Daniel: Yeah, you nailed it, it’s those two aspects. So mulch on the ground, that’s going to help insulate the soil from water loss and from temperature fluctuations, so very important in that regard. Doing it early is a must as well with changes in phenology, when things are breaking bud, or when plants are coming out of the ground, or when they flower, we need to make sure that that mulch is on the ground so that we’re not having to lift every leaf as we try to get that mulch contact with the ground.
Margaret: I know probably people ask you this all the time, but you see so many different recommendations. How deep is too deep or the right depth or whatever? What’s the guidance that you would give us for the depth of mulch? And also tell us a little bit about what you use as mulch, because you use a very interesting mix of materials.
Daniel: Sure, yeah. Mulching depth for us, our goals are, well, we have several goals. One is weed suppression, temperature fluctuation, so trying to hit all of those goals. Usually 2 to 4 inches of mulch is going to serve our needs. When it comes to trees, you really want to keep that mulch away from the base of the tree, that trunk flare where it connects with the ground. And with many perennials as well, any mulch against the base of it can retain too much moisture and lead to issues for the plants. So yeah, 2-4 inches usually does it for us.
The kind of mulch that we’re using, it’s all made on campus here, so it’s a combination of leaf collection that happens in the fall. Those leaves are composted and turned into leaf mold. And then that is combined with the chips that we produce from tree removals, so anything that goes through our chipper, it’s usually branch wood. We don’t have a very large chipper. So this branch wood yields a product called ramial wood chips. These have an excellent ratio of the living parts of the tree that are much more nutritious when composted, a better ratio of that material to the heartwood. So it’s just an excellent product, especially for woody plants.
Margaret: So ramial wood chips, and those are from the small and medium branches that have been ground up, so to speak, in the machine, as opposed to heartwood. Yeah, I see a lot of places recommend “arborist wood chips,” and I think: What’s that all about? [Laughter.] I guess partly that they’re free. But this is of a subdivision of that because that could also involve whole tree trunks.
Daniel: Yeah, for sure. It depends on the size of the chipper that material is going into. And for a lot of companies, it is easier to chip that material than it is to load it on a truck and transport it, so they’re going to chip as much as they can. But still it’s a very nutritious product for the plants.
Margaret: Again, when we did the Times column, we talked about, O.K., so say you have a bed that.. And I know I’ve seen this, plants that I used to read about that in the Southeastern United States they needed afternoon shade, well, in Margaret’s Northeastern garden these days I think they need afternoon shade in peak summer [laughter]. You know what I mean? It’s caught up. It seems like it’s just so much more intense. And so I wish in certain areas that I had a little, I could create some shade, modify the habitat a little bit. And you had some suggestions for ways we could do that either temporarily or in a little more semi-permanent way. And so that’s another possibility, yes?
Daniel: For sure, yeah. In the short term, if you need that shade right now, then using shade cloth is a great way to get that dappled sun, so it’s not the full energy bearing down on foliage.
Margaret: And especially with newly planted things and so forth, they’re just going to fry otherwise, and that shade cloth is a great way to give them a microclimate under there as opposed to out in the baking sun.
Daniel: Yeah, much of our plant material originates from our plant-production facility, and we have a lath house there [above]. So as you were talking about, with changes in the early season, going from winter to summer almost immediately, those lath structures are helping us harden those plants off, and if they’ve already been put out into the landscape, then shade cloth can get them there as well.
Margaret: And planting something to create shade, planting, what, an evergreen or something like that to create shade, not right away, but eventually, like modifying our garden design in a sense?
Daniel: Sure, yeah, trees; Plant more trees. That southwestern side is great, helping to give plants that afternoon shade they need. It depends on the siting of your garden. You may not want to plant a conifer necessarily. There’s a lot of solar gain that you can get when in the middle of winter you want the sun to hit your house and give you a little bit of solar gain there. So yeah, knowing your site, talking to your extension department and seeing if they can make any suggestions.
Margaret: To identify, but it’s first identifying, trying to put the problem into words, so that you can express it and say, “I have this bed of X, Y and Z and in the afternoon I’m noticing this is happening and more consistently in the summers and so forth.” So to really go around and take some notes, I feel like that’s what I’m doing, anyway, is I’m looking for what’s not working anymore. Because certain things we can’t fight at all and other things we can, we can give them some help like we’ve been talking about.
Daniel: Absolutely.
Margaret: Half the country in, when was it, late last November I think, half the country moved half a zone, 5 degrees in the winter hardiness. The USDA published their new plant hardiness zone map, and half the country moved half a zone. I moved from 5B to 6A, I think you guys moved, too, up at the botanic gardens.
Daniel: Yes, we also went from 5B to 6A. Further downtown in Ithaca, it’s another half zone warmer, frequently.
Margaret: Wow. So, I guess, with all this havoc, for gardeners one of the “silver linings,” if we could put that in air quotes or whatever, is that theoretically we have a wider palette of things winter hardiness wise that we can grow. Are you experimenting with, are you pushing the zones a little bit further and trying some new things there?
Daniel: Absolutely, yeah. I moved up here from Philadelphia, so I went from a 7B to a 5B gardening zone. And there are some things that I am just absolutely in love with. So yeah, taking a hold of that opportunity, and we’re trying a few different things. Lagerstroemia for instance, crape myrtles, we introduced that to the collection this year. It’s a tiny little shrubby one. But it flowered for us. It was planted last year and overwintered happily and flowered. So that’s pretty exciting.
And we’re going to continue trialing out, pushing it wherever we can, finding those microclimates that can support a half to a whole zone more as well. Poliothyrsis sinensis was another recent introduction. And it had received some winter damage its first and second year, but it’s getting pretty established now, I think it’ll be just fine.
Margaret: I don’t even know the common name for that, does it even have one? I have no idea.
Daniel: Yeah, it’s Chinese pearlbloom I think.
Margaret: O.K., I’m going to look it up, that’s a new one; interesting. So you are trying definitely some new things. But “trying” is the important … we can’t guarantee that just because our zone’s been changed that it’s rock solid, that we’re not going to have some disappointments. So we’re going to try.
Daniel: Yeah. And it only improves with time, too. So this winter for us, we far exceeded a 6A winter. Our low temperature I think was 3 degrees. So when we have winters like that, two or three in a row, that gives the opportunity for plants to really establish, and get a resilient root network. And then when it does return to marginally 6A, then something 6B/7A might make it, anyway, because it’s established.
Margaret: Speaking of planting new things, proper planting technique with our woody plants is going to be more important than ever. There’s no wiggle room with all these other stressors to be stressed by a bad planting job. And yet when I buy a young tree or shrub at the garden center, I can’t just pull it out of the pot and stick that root mass in the ground whole. That’s not going to work. And I wonder if you can give us a description, it’s really important to do some work on the roots first and get them in the right position, yes?
Daniel: For sure, for sure. You can save many future headaches with a well-planted plant. So firstly, planting when the plant is dormant is advised if you can swing it, so before it leafs out. When you pull a plant out of a pot, you don’t want to immediately put it right into the ground. Those roots have been in this non-natural environment, they’ve reached out for a wide root network, hit the side of the pot and started spiraling around. So I try to nearly bare-root every plant that I can, repeatedly dunking it in a wheelbarrow of water, for instance, I can get a lot of the soil off of that root mass. And then tease those roots to where they want to go, which is wider.
Margaret: Spread out, yeah.
Daniel: Depth is another very important aspect. We want that plant to be stable in the ground. Far too often we plant too deeply. We want to have the root flare, the place where the trunk begins to flare out into the roots, we want that at or just slightly above soil grade. I’ve been planting above soil grade recently because we anticipate adding a few layers of mulch for several years during that tree’s establishment. So planning for that, so that they don’t get buried on year 10, for instance.
Margaret: Well, I just think it’s really important for people to know that the level it’s at in the pot is probably not the level it’s supposed to be at in the ground [laughter].
Daniel: It’s very rare that it is.
Margaret: No, no. Well, Daniel, I hope you’re going to come back and talk to us about other things that are going on up there. Because again, I think those of us backyard gardeners need advice more and more all the time, for better or worse. But it’s great to speak to you again. And I hope all is well up there. Thank you.
(Photos from Cornell Botanic Gardens, except as noted.)
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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 15th year in March 2024. 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 Aug. 12, 2024 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify (and browse my archive of podcasts here).
