Have you ever heard of predictive planting design? "That’s nothing new under the sun" you might say, and that is entirely fair: Much, if not most, of planting design involves itself with predictions of the future, even if only via simplistic estimates of how large different plants get and how long they’re estimated to live. For example, an understanding of which crop plants are viable to grow where and how is a key requirement for agriculture, otherwise people will starve. Another good example would be the forestry industry, which can model forest growth under different local conditions and management plans with relative finesse. Predicting the development of ornamental plantings is nothing new either, really, as planting design often incorporates detailed guidelines for the optimal spacing of plants with regards to their mature sizes, and drawing up schedules of flowering times is used to plan successions of colors and shapes. The framework of naturalistic planting design - or designed plant communities, as I’d rather call it – has also worked hard on integrating change and long-term vegetation development into ornamental planting practices. This includes designing and managing for predicted goal states at different time spans, maybe 5, 10 and 20 years to the future. Much of the literature on designed plant communities also likes to include a reminder of not becoming too bummed out if the predictions fail - At least as long as they don't fail spectacularly*.
Securing the information needed for accurate or accurate enough predictions is akin to finding a needle in a haystack. The amount of effort spent depends much on what kind of an error margin can be accepted. In the previous two posts on planting for stormwater management I've tried to elaborate a bit on different factors that play into planting design in general (see monster mindmap), and how you might approach the planting design process in a project setting (goals-context-means). Since both of the sketched approaches are essentially context-specific and extremely knowledge-intensive, it should be obvious that I don’t believe in one-size-fits-all prescriptions on how to make reasonable predictions for planting design. Instead, I think we could try to approach vegetation development through different themes or factors. This way, we might be able to make predictions based on some of the more easily foreseeable aspects of plant life, at the very least.
The first theme I've decided to tackle is climate, but on a relatively shallow and practical level. The highs and lows of temperature, the length of the growing zone, the amount of rainfall and its distribution throughout the year are some of the obvious climatic factors that influence how plants grow and reproduce. Plant species have numerous physical adaptations that have made them viable competitors in their specific climates and sites. Unfortunately, these adaptations can instead become burdens when a plan is moved into another part of the world with different kinds of environmental conditions. Understanding how climate will influence plants is key to being able to predict how vegetation will develop, as a mismatch between a plant and its climate often spells swift death.
Different kinds of global, national and regional systems have been developed to predict how far a plant can be moved without compromising its will to live. As an example, we'll check out some of the growth zone systems used in the Nordic countries, what they can be used for, what they lack, and what other climate-related considerations play into vegetation development. To begin with, we’ll look at how two different trees relate to the climate, since there’s usually ample information available on them. Lastly, we’ll look at herbaceous perennials, since these are at the heart of designed plant communities – and many of them are notoriously unpredictable.
But why start with climate? Well, let's start with some thought experiments. Say I wanted to plant a date palm (Phoenix dactylifera) here in Eastern central Sweden. Actually, I'll buy a male and a female tree, since date palms are dioecious and don't produce fruit alone. Dates are tasty, and the trees look cool. But my imaginary crystal ball spells doom for the trees, and that prediction comes true with the early winter frosts in late October. Good news: My prediction was accurate, and next time I'll know not to plant dates outdoors where I live. Bad news: My date palms died, and my money has gone to waste. But why did the trees die? Uuuuhhh. Maybe the plant tissues froze and broke? Maybe date palms are evergreen, and losing their leaves suddenly left them without the energy to regenerate? Maybe the root systems rotted in the wet soil? It could also always be that the Nordic sunshine is insufficient to maintain the plants… Or maybe the answer lies in a combination of all these factors.
Why did I assign a doom flag for the date palms since the very beginning? Well, I have never seen any individuals outdoors in Northern Europe***. I also knew that date palms have their origin in the dry regions of the subtropic zone, which has a completely different climate from my own location in the northern temperate zone. But how different are they exactly? The United States Department of Agriculture’s (USDA for short) Plant Hardiness Zone Map, which has become increasingly popular among horticulture and landscape professionals, can be of help here, as it will give us information on the extreme cold tolerance of different plants. Date palms are classified to survive in USDA zones 8-12 (minimum temperatures in the past 30 years between -12,2 ⁰C and +15,6 ⁰C), while I'm in zone 5-6 (-28 ⁰C and -17 ⁰C). The most basic climate assessments were enough to show that the odds were never in the date palm's favor here.
We could try again with one of the hardier varieties of date palms. The odds of the trees surviving outdoors are still close to zero, but we can help the trees by planting them into well-draining soil and by protecting the growing points (palm hearts) at the top of the trees. It turns out that we get several mild non-winters with only singular days which are below zero degrees Celsius in a row, and so the trees don't freeze either. Against all odds we have managed to raise two date palms, so we can expect to taste home-grown date fruits soon. The time for harvest never comes, though. The reason for this could be that the trees are maturing at a slower rate than they would in their original tropical and sub-tropical climates, making us wait longer until they actually can produce proper quantities of functioning flowers and bear fruit. Date palms apparently can only be pollinated at temperatures of +35 degrees Celsius, which in my area happens very rarely (thank goodness). Having that temperature coincide with the blooming time would be a miracle, but let's entertain that thought as well. The next hurdles are that the fruit do not ripen unless there are enough warm, rainless days… Yeah, that's where we'll have to give up. Even the least demanding varieties of date palm require 1,5 times more heat in a growing period than an average year in this area provides, and as we get double the annual rainfall of prime date-producing regions there's a good chance the fruit will get wet and rot. The prediction of date palms failing in the current climate, even with considerable strokes of luck, still holds (although I would say that winterless winters are a definite misfortune, far outweighing the positives of being able to farm your own dates).
So date palms are a no-no. Let's stop with the thought experiments, then, and pick a tree that we know will grow in my area. What else says "Northern temperate region" better than the silver birch (Betula pendula)? Most people consider them beautiful, and they can be found almost everywhere in the country since they tolerate a wide range of soil and moisture conditions. In Sweden, there are a few varieties available for landscape uses, mainly fk JULITA E and fk SAVOLAKSIE. Apparently there's also a sort called 'B-MW', and then there are silver birches without a specified type. Which one should we choose? "Fk" stands for "frökälla", seed provenance or literally "seed source". A known seed source means that the plants under that name share common traits that make them especially valuable for specific uses, but they are not subject to the strict rules for uniformity that characterize cultivars would be. "E" stands for "Elite-plants" that are considered to be the best selections and cultivars of their kind in Sweden. Names in CAPS denote the specific name of the seed source, and names in 'citations' are reserved for cultivars. Basically, what we can infer from this breakdown is that the three available types should have certain advantages or disadvantages when compared with each other.
According to Tönnersjö tree nursery, the main differences between these types are their winter hardiness. The Swedish winter hardiness zones are determined through a wider variety of climatic factors than the USDA system is, although it is also largely dependent on identifying areas with similar temperature ranges across years. In the end, the Swedish system is still based on its original use as a guide to where and which apples** should be grown where, and so the system has a great degree of opacity. This preoccupation with apples is also the reason why the northern and montane areas in Sweden were not included in the system until the 1940's, when the Swedes had to conclude that there is a point to growing woody plants besides ones that bear fruit. Like the silver birch for forestry and ornamental use, for example. So out of the plants we are interested in right now, the variety 'B-MW' is the least hardy (no zones given, but recommended for southern Sweden), fk JULITA E is the second hardiest (zones 1-4) and fk SAVOLAKSIE that will work for most of the country (zones 1-7).
So fk SAVOLAKSIE is basically the best variety, right? With its broadest hardiness? Why sell the two other types at all? While the silver birch has considerable plasticity that allows it to adapt to a variety of circumstances early in its development, it is also a difficult species to transport in a north-south-direction. Basically, what this means is that the parent trees are likely to produce good quantities of site-adapted seed: seeds from short-summer climates will also be primed for late leafing out and early winter hardening process, which is made visible by early onset of autumn color. If we move a silver birch whose internal clock is genetically optimized for short summers to a climate with longer summers, the leafing and hardening schedule of that specific birch specimen will not adapt. This will reduce its ornamental value in its new context, while also maintaining the slower growth rate that would be typical for short growing seasons. On the other hand, a silver birch from a longer summer climate is likely to leaf out too early, resulting in frozen leaves and cracked bark; and it will probably start preparing for winter too late, which might induce further freezing damage in a variety of tissues. In the worst case, moving birches father south or north have an increased risk of premature death, disease and deformity.
Not all species are as sensitive to transplanting in a latitudinal direction as silver birches, but this specific example really highlights the role of genetics in climate adaptation: Choosing a genotype (provenance or cultivar) that will behave predictably in the given conditions is just as much about the assumed minimum of the temperature range and growing season length as it is about the maximum. There are limits to how far it is reasonable to optimize genotypes for specific sites (i.e. seed from local sources is not automatically the only good choice, much less necessarily the best choice), but completely neglecting genetic variation in climate adaptation is always a bad choice.
And now, we will finally talk about perennials! While the USDA hardiness zones are used for both woody material and herbaceous material, the Nordic systems (SWE, NOR, FIN) only classify woody plants. This is partially because of the history of these systems, but also because most perennials spend winters underground, ideally covered in snow, and are thus subject to less cold than woody plants. It is also more difficult for perennials to start leafing out much earlier than suitable, because the warmth cues and water availability are dependent on the top layers of soil baring and thawing. Well. This was the case before, anyway. Regularly frozen soils and reliable snow cover during the winter have become less of a rule than the exception in lowland, coastal and southern parts of the Nordics. Under these circumstances, much rides on perennials' capacity to tolerate fluctuating temperatures and soil moisture levels.
The Swedes modelled their first hardiness classification for perennials partly on the idea that regional differences in climate would make or break perennials, and so they decided that A stands for perennials that can be used anywhere in the country (Alchemilla mollis, Aquilegia vulgaris, Anaphalis triplinervis), and D is for plants that are only likely to thrive in the warmest parts of the country, and even then under cover and in well-drained situations (Crocosmia x crocosmiiflora, Gaura lindheimeri, Carex testacea) . Since even the categories B and C include recommendations for good drainage and winter cover (B: Brunnera macrophylla, Astrantia major, Calamintha nepeta, C: Chelone obliqua, Echinops bannaticus, Lavandula angustifolia), I think that this system continued to perpetrate the idea that perennials are sensitive, finicky and labor-intensive. The new system breaks up with both thoughts of perennials' regional hardiness and their assumed status as the primadonnas of the garden world, and is, in fact, predictive in its nature. So now A stands for a plant that is very likely to survive any winters under a variety of site conditions (still A. mollis and A. vulgaris). But! If an asterisk is added (A*), it indicates that good drainage is required for a predictably good performance (A. triplinervis). Plants in the D-category are considered lost cases, no matter what, and should be only used as annuals in Sweden (funnily enough, parsley and Satureja douglasii are the only plants included as an example of this. All of the D-class examples above have been excluded from the list). So what about the categories B and C in this new system? This is where it all gets very interesting. Enter our last example, the purple coneflower (Echinacea purpurea).
The purple coneflower is possibly the most famous of North American prairie species in Europe. As Noel Kingsbury says, “everybody loves a daisy”: it’s an easy to approach plant with a slightly dampened pink color and a stereotypical flower shape. It is beloved by pollinators, and many birds eat its seeds in the winter. It has also been used in alternative medicine to prevent and treat colds, which is also a factor that adds to its good reputation. Additionally, it’s a darling of the horticultural industry, with many fantastic-looking cultivars in almost all shapes and colors imaginable. No wonder that it has become wildly popular in home gardens and urban areas alike.
Personally, I hate this plant. I hate it. The pretty flower is not enough for me to compensate for its overall lackluster habit, boring leaf architecture and late leafing out. I think that the complex cultivars are cute enough in cut flower arrangements, and that’s where they belong – not in the garden, thank you very much. Above all, to me it’s the poster child for unpredictable perennials, the pure species seeding prolifically in some gardens but disappearing in a few meager years from most locations – especially true for the more complex cultivars. Sometimes a colleague has emphatically suggested (read: kindly coerced) me to include it into a planting design we’ve been working on. At times like those, I have three tools for curbing the unpredictability that the purple coneflower represents: 1) I always choose the variety ‘Magnus’ or ‘Little Magnus’, as these are the only ones our Grand Old Man recommends. 2) I use them in relatively small amounts, and place them in across larger areas in small groups of no more than 3 individuals to cut my losses if and when they decide to quit on me. 3) I try to secure good drainage in the planting bed construction to coax the plants to persevere for just a bit longer.
My suspicions of the purple coneflower are confirmed by the new Swedish winter hardiness classification for perennials: No Echinacea variety is deemed to be reliably hardy, even though it can reportedly survive up to -39 degrees Celsius (USDA zone 3a). Approximately half of the classified varieties are considered to be B*, which means that they usually do survive winters in places with good drainage. This class includes the basic pink ‘Magnus’ and ‘Little Magnus’, but also the white cultivar ‘Virgin’, the yellow ‘Cleopatra’, and even the bizarre-looking cultivars ‘Green Twister’ and ‘Irresistible’. The rest of them are C*, which denotes that they -might- survive, maybe, sometimes, in well-drained positions. Oudolf and Kingsbury (2013) also report that Echinacea species and cultivars have varying lifespans, E. pallida possibly living up to 10 years, and some of the E. purpurea cultivars not living past 5 years. This is also a good reason for spreading my risks (i.e. the coneflowers) sparsely in a mixture of other, more long-lived and reliable plants in an attempt to stabilize the planting as a whole.
I assume that the purple coneflower does manage to bloom in time even further up north, since its flowering time is said to be from July to September. But I wonder if the seeds have enough time to ripen during the short summers? If not, there will be no offspring that might replace the original individuals once they die. Otherwise, the common problem for North American prairie species is that their schedule is quite different from their North European counterparts: Many of the commercially available prairie plants leaf out very late and bloom even later, so in short-season climates they might never have the time to bloom. I think many aster species are not recommended in the northern parts of the Nordics for this very reason, and most prairie grasses have limited useability even in the southern parts. Similarly, they might not harden and die back early enough, costing them precious resources that would help them through the winter. These considerations are important for successful plantings with predictable performance and development, and they cannot be interpreted from their hardiness ratings.
The basic takeaway of this post is, as the Norwegian Garden Society (Det Norske Hageselskap) so aptly puts it: "Stol ikke blindt på klimasonene". Climate factors influence plant behavior in a multitude of ways which are not always readable from climate-based classifications of growing zones. Understanding which plants will live or die based on how cold it gets in the winter is a great start for any plant selection scheme, but you cannot make a reliable planting design without understanding how the climate will influence plant size, health, flowering or fruiting, among other things. Considering the regional and local climate is such a necessary step of zooming out before zooming in to the specific project site, that many aspects of it become automatic if a planting designer frequently works within areas of the same climate. What this usually means is that planting designers become better at predicting vegetation development in their “own” climate than in areas that they’re less familiar with. In those cases, it’s a good idea to go back and do as a beginner does: check the patterns of temperature, the timing and amount of rain, the length of the growing period, and any other pertinent climate factors. And then compare the results with the adaptations of the plants you have available for selection. Chances are that you will find ways to improve the predictability of vegetation development in your planting design.
Footnotes:
*As for how "acceptable failure" is defined within the paradigm of designed plant communities is a complex question, which I hope to get back to after I've submitted my first manuscript.
***No, the potted palm tree in the picture above doesn't count, they take it into the greenhouse for the winter.
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