Climate resilient gardening

As a retired climate scientist, and current gardening fanatic, I’m often asked “What should I grow that will survive coming climate change?” My usual articulate response is to expouse something like “Uh ….”, or possibly “Um ….”, and then later hope that I didn’t drool. The difficulty is that trying to answer the what-to-grow question is vastly complicated, even without the uncertainty of climate change. Different species of plants have different light, soil, moisture, wind, pollination, and pest requirements. Heck, different strains within a species can have different requirements. Layer on top of that the unique characteristics of a planting site and the habits of the gardener, it takes some real thought to give good planting advice. Sprinkle in the uncertainty of climate change and it’s a wonder that there are indeed some rules of thumb that can help us to create gardens that will thrive into the future.

Let me emphasize that the uncertainty of future climate patterns is not just uncertain, it is wildly uncertain. There is a whole cascade of uncertainties including the scale of future emissions, the earth system’s response including possible tipping points, and the complexity of local climates. We can query climate models predictions through various websites where results of many state of the art climate models are made available, such as at NOAA or the IPCC. But at the end of the day, the future is unknowable. That said, the climate models are our best guess of the future climate.

One likely emissions path is that net emissions reach zero by, at the latest, 2075 and become negative thereafter. While many nations around the world have committed to net zero by 2050 to try to keep warming below 1.5 degrees C, our national policies are not on track for us to meet this deadline. So perhaps 2075 is a reasonable guess. On average, the zero-by-2075 models, the IPCC’s SSP1-2.6 model runs if you must know, predict that the US’ east coast will warm by 1.9 degrees C by 2040 relative to pre-industrial times. Warming in this region will then flatten out around 2.4 degrees C by the end of the century. Northern Europe would expect to experience slightly larger temperature rises of 2.2 and 2.7 degrees C for the near and far parts of the coming century. Both areas expect more precipitation, about 5-7% percent more. But we gardeners would be remiss in celebrating the increased rainfall for most of it is expected in winter months and as more intense rainfall events. Simply put, under this scenario, the US’ east coast and northern Europe can expect warmer, wetter, and wilder weather, with drier summers. Very roughly, these results scale up, and down, depending on whether we have more emissions, or fewer.

Coping with this future climate is a pretty big ask of the plants we put in the ground. They will need to be able to withstand more drought, more intense rainfall events, warmer summers, and, as it turns out, significantly warmer winters. Our best strategy for planting would then be to nurture plants which are tough. Plants that we can put into the ground and largely ignore until they are looking lovely and we can admire them. The tougher they are, the more likely they can survive climatic shifts. And the larger their genetic diversity, the more likely they will survive dramatic climate shifts, as different strains may have characteristics that allow them to survive. Who would these magical superstar plants be? Natives. And, for the genetic diversity - straight species natives - none of those fancy cultivars which are often clones of one desirable plant limiting their contribution to genetic diversity.

As native plant gardeners know, native plants are the easiest to grow. They are already adapted to our climate and soil, as well as to our climate’s fluctuations. This is especially true of natives that have been living in the wild near us, the so called local-ecotypes. Natives in general, but especially local ecotypes, are the most likely plants to survive in our garden setting. They won’t need watered, fed, or mulched, once they are established. They also don’t need soil amendments as we plant them, for native plants co-evolved with the local soil. Native plants further benefit gardeners because they host native wildlife, in particular predators of garden pests. True, native plants can also host the pests themselves, but in a healthy ecosystem the predators will keep the pests at bay. And natives, of course, provide food and shelter for our native wildlife more broadly. Supporting our local ecosystems benefits our own garden as we’re all part of an interlocking life system.

In addition to planting natives, the other golden rule of climate resilient planting is to plant a diverse range of plants. Indeed, for all shocks, and quite possibly for all systems, diversity rules the resiliency roost. This is because if some plants/stocks/peoples/schemes succumb to a given stress others may survive, indeed some may thrive. For the garden, this means diversity in species - goldenrods and bee balm and oaks - but also genetic diversity with in a species. For instance, it is beneficial to plant paw paw trees from different lineages, which is likely to give you more fruit anyway. Research shows time and again that diverse ecosystems are far more likely to survive and adapt when faced with stressors. Monocultures can be wiped out in a season - think the potato famine, the loss of 3 billion chestnut trees on the US’ eastern seaboard in the 1900s, or the banana’s doom.

Moving on from resilient plant choices, there are also some gardening practices that can help our gardens to thrive into the future. If the carbon concentration in the atmosphere continues to increase until 2050 / 2075, in addition to warmer average temperatures, we expect both more summer dry spells and more intense downpours in non-summer months. Dry spells mean many plants will struggle to stay alive, while downpours can lead to soil erosion and loss of soil nutrients. To address both of those issues we can plant densely and nurture the soil.

Planting densely is just what it sounds like - putting as many plants as possible into the ground - the less bare soil the better. The more plants in a given area the greater the root mass and this alone will help to retain your soil. Planting densely also reduces water stress, perhaps somewhat counter-intuitively. Deeper rooted plants bring water closer to the surface and thus can moisten the soil column during dry periods. But that’s not the only way dense plantings reduces water stress, a thick canopy also reduces evaporation because soil temperatures are lower and because whatever water is evaporated tends to stay within a dense leaf canopy. A moist canopy environment means there is less of a difference in the water vapor concentration between the inside and the outside of the leaf. And therefore less water is transpired via the plant to the atmosphere. So the net result of dense planting is less water loss, less water stress, less runoff, and reduced soil erosion.

The appreciation of the benefits of dense planting is growing for vegetable growers as well as flower gardeners. We are now encouraged to grow cover crops, like clover, in bare winter beds and in the growing season between our vegetable rows. Such cover crops can be dug into a planting hole, thus adding organic matter, and, in the case of clovers, adding nitrogen to the soil. A massive advantage of dense planting is the great reduction in maintenance time - far fewer weeds, no need to mulch, and a reduction in watering requirements. Dense plant is a key management strategy for climate resiliency. Part of dense planting’s magic comes from its role in protecting, aerating, and moistening of the soil column. It therefore contributes to the well being of the soil’s microbiome.

The microbiome with the soil is of paramount importance to plants’ health because the soil fungi provide plants with many important nutrients. We can further nurture this underground ecosystem by practicing no dig or low tillage gardening. The old technique of forking over a garden bed is loosing ground (cringe). Turning over a garden bed not only brings weed seeds to the surface, but it also accelerates the loss of organic matter from the soil - important for water and nutrient flow and retention. Digging up the soil also physically disturbs the soil microbiome thereby reducing mineral and nutrient uptake by plants. Not to mention, digging over a garden bed is hard work. Taking care of your soil is another top management tip for a climate resilient garden.

And a further resilient gardening technique, hinted at above, is to garden with the wider biosphere in mind. We can protect the critters in our garden and beyond by avoiding pesticides and fertilizers, leaving the leaves, and encouraging wild areas. The base of many food webs are the critters that live in plant litter. We can not overestimate the importance of the local pockets of diversity especially in light of a recent estimate that we’ve lost 70% of wild animals in the past 5 decades. That’s f’ing disturbing. The least we can do is to put up with a bit of untidiness. It may take some getting used to, but after a while your sense of what is beautiful may shift as mine has. I now see the garden litter as crinkly and beautiful in its own right, and not just because it means I get to see more bluebirds, indigo buntings, bald eagles, barn owls, red tailed hawks and so forth in my garden.

We can expand this connecting-with-the-wider-biosphere topic and try to have our gardens link to our neighbors gardens, and of course beyond that. This can either be a physical link of wild areas or a species link. For instance, if all my goldenrod dies off one year and I’m close to another native plant area, it’s likely that goldenrod will naturally return to my garden. This connectivity widens the gene pool options - which is what gives living beings options for surviving changing conditions. And connecting my garden to the greater ecosystem won’t only benefit my garden plants, its good for the bugs, birds, mammals, reptiles, and amphibians that roam about our neighborhoods and frankly just need more places to live. There are several ongoing efforts to connect ecosystems such as the National Wildlife Federation’s Certified Wildlife Habitat scheme.

All the previous tips, natives and diversity, planting densely, protecting the soil, and wildlife friending gardening, will help no matter what the climate is like. But each of them are becoming more important as plants endure more weather changes. The last piece of advice I have for you is different, it is predicated on the temperature continuing to rise. The minimum winter temperature is an important determinant in where plants can grow and indeed both the USDA’s and the RHS’ plant zones are determined by what minimum winter temperature a plant can withstand. According to the emissions scenario we explored earlier, minimum temperatures are expected to rise by almost 5 degrees C by the end of the century. Thus we’d expect regions in the eastern US and northern Europe to shift by one full USDA plant zone.

A planting zone shift has important ramifications for long lived trees, of course. But minimum temperatures are also essential for many of our popular wildflowers and perennials, which we wildlife friendly gardeners have dreams of setting on a self seeding cascade well into the 22nd century. Many seeds, including dogwoods, redbuds, and black-eyed Susans, need a cold period in order to break their seeds’ dormancy and to start germination. So if we plant a tree or a wildflower that needs temperatures to go below a certain threshold, and warming means my region is unlikely to experience that minimum temperature in coming decades, that plant is not a good long term investment. Therefore, we’d be wise focus on planting locally native plants which also are currently thriving in regions to our south (and to the north in the Southern Hemisphere). I love a new phrase and I call this equator-facing. To chose eligible natives, you can consult plant distribution maps (US or UK) or check that a given plant thrives in planting zones that are one warmer than yours.

That’s a lot of strategies to make our gardens more climate resilient, but you just need to remember one golden rule. Nurture biodiversity. Biodiversity means both many different kinds of plants and just more plants. Burgeoning comes to mind. That covers planting densely, practicing wildlife friendly gardening, and of course planting a wide range of plants. And my wise subscribers will know, to nurture biodiversity we can nurture the soil and we can encourgae our haven's connections to the larger ecosystems surrounding us.

So, get out there and plant something. Something native. Something diverse. Something dense. Something wildlife friendly. Not quite as catchy as “Something old, something new” … but you get the idea.