Climate change affecting plants and trees in home gardens

Climate change touches the natural ecosystem in countless ways, and its impact can be seen in our own backyards — and front yards as well as community gardens. Temperature changes and weather events are affecting which plants are thriving in people’s gardens and which are dying. Weston Miller is a community and urban horticulturist at Oregon State University and part of the Master Gardener program. He joins us to tell us more about what plants, shrubs and trees are adapting best and how to best approach gardening in the midst of ongoing temperature changes and extreme weather events.

This transcript was created by a computer and edited by a volunteer.

Dave Miller: From the Gert Boyle Studio at OPB, this is Think Out Loud. I’m Dave Miller. We turn now to climate change and gardening, because seasonal shifts and extreme weather events are affecting which plants are thriving and which are suffering impacts that people can see in lawns and yards and gardens. So with the spring gardening season upon us, we thought we would turn to Weston Miller. He’s a community and urban horticulturist at Oregon State University and he’s part of the master gardener program. Weston Miller, welcome.

Weston Miller: Thanks for having me, Dave.

Dave Miller: And I should say that if folks have questions for you about how gardening should adapt or how trees or shrubs will do in the changing climate, you can give us a call right now. What are some of the specific aspects of climate change that we’re talking about here in terms of impacts on gardens and trees and lawns?

Weston Miller: Right, Dave, thanks. I would say folks probably remember back in June we had the heat dome and it got to 113-115 degrees or so in Portland. And a lot of plants suffered last summer from that. And then it turns out that many of them, the damage is still unfolding now, as the plants start to leaf out or grow new leaves this spring. So heat dome. Also, I’m sure folks remember the ice storms that we’ve had recently, other extreme weather events, windstorms et cetera. And really what we can expect with climate change is that the frequency and the intensity of extreme weather events is going to increase.

Dave Miller: I imagine that gardening is dynamic, and that you’re always learning new things, even if you are a part of the master gardener program and you’ve been doing this for decades. But how much of what you’ve come to see as givens have been in one way or another upended because of a changing climate?

Weston Miller: It’s true that one of the best aspects of gardening and landscaping is it’s a lifelong learning process, and I suppose things are changing a little bit in terms of hardiness zones, for example. The US Department of Agriculture has shifted their hardiness zones in the last number of years to account for warming [and] changing climate. Also just in terms of first frost and last frost, those dates are less predictable than they used to be. Also the amount of heat that would accumulate during the year and affect the ripening of fruits and what not, that’s also changing. So it might change the kinds of fruits, grapes, things like that that people can grow in Oregon, because we’re getting more heat accumulation during a given growing season.

Dave Miller: I want to play you a voicemail that we got from Allison in Southeast Portland: “Five years ago my wife and I bought a beautiful old homestead house, just off of 82nd Avenue, with a rhododendron in the yard that was almost as old as the house. It’s got to be 70 or 80 years old. It’s basically a tree. Climate change has really given it quite a wallop though. And last summer’s heat dome event really kind of put the icing on the cake. We’re likely to lose our beautiful rhododendron. We lost a lot of important garden plants. We’re very serious gardeners. So this year we’re starting to think about native California plants and native eastern Oregon, high desert plants, things that are more adapted to extreme climate events, both hot and cold. We were really hoping to do Pacific Northwest natives, Willamette Valley plants, but it’s proving that a lot of those maybe won’t be able to withstand our changing climate.

So starting to think about plants from other places.” Weston Miller, I’m curious what stands out to you and what you heard from Allison?

Weston Miller: Allison’s observations are spot on. Rhododendrons in particular have had a hard hard time in recent years. We’ve had an extended drought. Rhododendrons are plants that like to grow in part shade and oftentimes people plant them in full sun. So those are the ones that are going to suffer the most. Um, I would advise watering an older rhododendron more during the summer to try to get it through. Any leaves that are damaged now could be cut off, removing it is certainly another option. And if people are going to replace plants, it does make sense to first of all choose plants that are drought hardy, and that can be native plants and native plants to California and other regions. Also plants that are from the Mediterranean region, say like lavender and rosemary, would be really great choices of plants that are adapted to really hot, dry summer conditions.

Dave Miller: Native plants in the past, by virtue of the fact that they have developed here and this was their home, they would have made perfect sense. Is there a point at which the climate will have changed enough that the plants that have thrived here for millennia or longer, no longer make the most sense?

Weston Miller: What I would advise is that folks still go with native plants and that they think about watering them a little bit more. So folks have a sort of misconception that native plants don’t necessarily need water, but I would very much water any new plants that people buy even Oregon natives first several summers, at least to get them well established. After that time they might get a little bit ragged during the hot weather, but they probably will survive fairly well. I would really recommend a drip irrigation system for people’s new plants. Soaker hoses would be a great example of that. That would give them an easy way to provide their plants with substantial amounts of water.

Dave Miller: Arthur has called in from Portland. Arthur, go ahead.

Caller [Arthur]: Good morning. Good afternoon. Thanks for picking up. Listen, I used to be a professional, so-called groundskeeper/gardener myself, and I’ve retired from that profession. But that whole idea of using gas-powered lawn mowers, weed eaters and backpack blowers, to just maintain at a certain height is so antiquated and so anti-environmental. And one thing about this global warming, I noticed that there are certain weeds when my job was to take care of grass in Oklahoma. I noticed that certain weeds that had previously only grown in Texas, have now migrated north to Oklahoma due to temperature rise. And you know, because it was my job to spot these weeds. Things are very weird, creeping like your expert was talking about, like the plants are creeping northward. I’ll take my answer off the air. I think this is a fascinating discussion. Thank you.

Dave Miller: Thank you. Weston Miller, how do you think about lawns?

Weston Miller: Well, I’d say lawns have their place. They’re great for when you want to have kids playing. And if you want to play croquet and things like that. [I] recommend people have a small lawn that they can maintain well. It actually turns out that lawns accumulate carbon in the soil at a rate that’s more or less equal to forests regenerating or fallowed agriculture. But we do emit carbon and other greenhouse gasses through maintenance activities. So [if] folks are gonna have a lawn, I’d recommend using a push mower. With human exercise to do the mowing, or battery powered, or an electric mower. And that way we can avoid a lot of the emissions associated with maintaining a lawn. And then the other comment the listener had was about weeds, and we can definitely expect that weeds will proliferate with warmer conditions. Weeds are aggressive plants that like to grow in disturbed soil and with warmer temperatures, they’re going to be just that much more aggressive.

Dave Miller: How do you feel about letting lawns go brown in the summer? IE, not watering them?

Weston Miller: Not watering lawns is certainly one way to do it. If you want to have a lush green lawn, you want to water it. If you’re okay with [it] going brown, that’s okay too. I think I would also recommend that people think about reseeding their lawn on a regular basis to try to keep the grasses competitive, and then also using a little bit of fertilizer in the fall, especially, and also the spring, to help to keep the grass as strong as possible. Such that it can out compete all the weeds that like to grow in the lawn area.

Dave Miller: If you’re just tuning in, we’re talking right now with expert gardener Weston Miller from OSU, about how climate change is changing gardening and tree planting and lawn care. If you have questions about what you should be planting in our changing world or how you should be thinking about gardening in different ways, you can give us a call. We’d also just love to hear if you’ve already been making your own changes based on your experience with the soil and with the seasons. Gloria has called in from Portland. Gloria, go ahead. Caller [Gloria]: Hi, good morning Dave.We have a small yard, but lots of trees and bushes and the ones I’m concerned about are my hydrangeas. They are showing no signs of green and one of them is over 20 years old. The other one is a baby. It’s only five years old. I’m tempted to just not dig them up or trim them until about April or May hoping that maybe they will spring back.

Dave Miller: And you’re basically wondering if they’re goners?

Caller [Gloria]: Right.

Dave Miller: All right. Weston Miller, how long should she give them?

Weston Miller: Gloria, that’s a great question. Hydrangeas were particularly hit hard by the heat dome last summer. They really suffered and with plants I would generally take a wait and see approach. Give it another month or two. They will likely spring back from the root systems, and if they don’t, then you’ll know, and you’ll be able to dig them out and plant something else in that area.

Dave Miller: Gloria, what’s it been like to not get flowers recently?

Caller [Gloria]: [Laughs] Well, I have an old daphne plant and it is doing just fine, and so I’m clipping daphne. If you have any idea what it is, they smell incredibly wonderful.

Dave Miller: It remains one of the marvels of the natural world in the Northwest, given that I came from a place in the Northeast where nothing smells good in February, there’s just more dumping of snow. So yeah, I am obsessed with daphnes. And we’ve been talking about … actually, Gloria, thanks so much for that call. Weston Miller, as Gloria noted, her daphne has been doing well. I’ve seen plenty of them that have been smelling and looking good recently. We’ve been talking about, whether it’s hydrangeas or rhododendrons or other plants or shrubs that have been suffering in various ways. But I’m curious about the flip side: which plants have been doing well recently?

Weston Miller: That’s a great question, Dave. For example, I like to grow pineapple guavas. They’re a subtropical plant and they survive in terms of how cold it gets here, but they don’t necessarily fruit very well. And I’ve noticed that pineapple guavas are doing a little bit better for me. So anecdotally, that’s one example of a plant that’s sort of on the edge of producing fruit in our area, but with a warmer climate, it’s certainly going to do better. So in that way I’m hedging my bets.

Dave Miller: You know, we’ve been talking a lot about plants and shrubs, but let’s turn to vegetable gardens. How should people be thinking differently about vegetables in our changing world?

Weston Miller: Great question, Dave. Regarding vegetable gardening, I would say that along with weeds, we can also expect that insects will do better. Insects are temperature dependent, and if it’s a little bit warmer, they might have an additional generation per year, or otherwise emerge a little bit earlier. So that’s one thing I would recommend folks just do: really rigorous bug patrol on their vegetables and look under the leaves and find things like aphids and imported cabbage moths and to squash those. Also, I would expect plants like broccoli might be a little bit harder to grow. They are typically planted in the spring and if the plants get stressed, they put up their flower stock and bolt. So that might happen if it gets really warm in, let’s say like early May or something like that, after the plants have been planted. I would expect that kind of variability to occur. Planting several different varieties, choosing varieties that are particularly adapted to warmer conditions. That might be the case with lettuce, where there are varieties that do better during the summer than other varieties. I think folks just need to monitor their site, take notes, try to change things year after year in terms of their vegetable gardening and perhaps change their expectations a little bit.

Dave Miller: Individuals are not the only people thinking about this. So are  business owners. We got a voicemail from Austin, who runs a landscape design/nursery company called The Green Seed. This is what he said: “Last year’s heatwave really got me thinking about what plants are really gonna make it here in the future. Many of our classic Northwest plants really aren’t adapted to the heat and drought: Rhododendrons, hydrangeas and some of them, even looking at certain conifers, David’s viburnum, they really got scorched in that heat wave last June. Some of our native plants really are not even adapting well to the climate change. This Western red cedar is one that’s really struggling with drought. This heatwave really decimated them last year. It really makes me wonder about the viability of even these native plants for wildlife. And do they have a future here? So with our business, we’re really shifting our focus of our design and nursery to plants that are heat tolerant. And drought tolerant. If it can’t handle 115° heat, or requires large amounts of water, maybe it shouldn’t be planted. We’re also looking at California garden plants for inspiration. They have long dealt with these conditions and many of the plants that thrive there also thrive here. Some plants that I’ve observed that really handled the heat wave and drought last year with heat, are Gaura,  Zauschneria, also known as California fuchsia. ceanothus, manzanita, hebes and penstemons.”

Weston Miller, what do you think about those suggestions? California fuchsia, ceanothus and manzanita among others.

Weston Miller: Those suggestions are all spot on. They’re California natives and they’re going to do well in a little bit warmer climate. They all certainly do well here. And also the comment regarding Western red cedars and also Douglas firs. Those are native trees and they are definitely having a hard time, and showing dieback and whole trees are dying. So I would recommend folks keep an eye on trees that they have on their property, and if needed, to contact an arborist to either provide consulting or to take it out if the tree has died. For established trees like Western red cedar and Douglas fir, if folks want to keep those alive, they should probably provide them with some extra water during the summer.

Dave Miller: Jeff has called in from Portland Jeff. Go ahead.

Caller [Jeff]: Hi, I’m a beekeeper and I recently tore up my yard, and I’m looking for ground covers. I want to replace the grass with ground covers that are drought resistant but also bee-friendly.

Weston Miller: There’s a fabulous book called Meadowscaping, that’s put out by the West Multnomah Soil and Water Conservation District, and they have great plans for incorporating grasses and other native plants. Yaro would be one example that are gonna provide habitat for pollinators, and also look reasonably attractive, and just be a great replacement for your lawn area.

Dave Miller: Weston Miller, I want to go back to something you said at the beginning, that we’re talking about a lot of different kinds of extreme weather events, not just drought and heat domes, but also potentially flooding in the winter and spring or ice storms. How do you think about preparing for all of these very different kinds of events?

Weston Miller: Well, I’d say with ice storms, if people have trees on their property, they might consider getting an arborist to come and and limb them up and otherwise remove branches so that they don’t take on so much weight when we do get the ice storms. For me, I have a relationship with my arborist and I know I’m looking at my trees and have a long-term plan to deal with what I know is coming in that way.

Dave Miller: We’ve gotten calls from people who seem to have been doing gardening for a while, but where do you suggest people start if they didn’t grow up gardening and don’t necessarily even have a lot of land, but just want to get some dirt on their fingers?

Weston Miller: Well, the City of Portland has a great community garden program. So do other cities and churches and locations like that. And I would just say if folks do have gardening questions, they can call their local OSU extension service master gardeners. We’ve got volunteers who are trained and ready to answer people’s gardening questions, from simple questions like ‘when do I plant peas’ to more complex questions about choosing plants.

Dave Miller: Finally, Lee has called in from Portland. Lee, go ahead.

Caller [Lee]: Hi, thank you. I have a question for Weston about recommended techniques for watering. When you’re either preparing for or in the middle of the heatwave. I’ve read conflicting advice, so I’m just wondering what is best practice to make sure that your garden is going to get through as unscathed as possible.

Dave Miller: Thank you. Weston?

Weston Miller: That’s a great question. Overall I would recommend, especially with vegetable gardens, don’t let the soil dry out. It’ll be really hard to rehydrate it. Try to pay attention to the weather, and water in advance of heat events, and then water the soil and not the plants, and really water deeply such that the water is getting down into the root zone of the plant.

Dave Miller: Lee, thanks for that call. And Weston Miller, thanks for taking all of these calls. Weston Miller, thank you.

Weston Miller: My pleasure, Dave, thanks for having me.

Dave Miller: Weston Miller is a part of OSU’s master gardener program. He teaches workshops on climate change and fire risk and gardening.

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