As landscape architects and planting designers, most of us have been trained in basic practices of site analysis. We map and analyze site features such as topography, circulation, views, and existing site elements. The scale of detail that we document depends on what we are tasked to do on the site.
When you’re planning to use plants as your primary design medium, you will need to pay attention to the aspects of a site that affect their thriving and survival. Small-scale differences in topography and aspect can have a significant influence on what species thrive. If you’re planning to use a lot of herbaceous plants, you need to observe at an especially detailed scale – on a meter by meter (even foot by foot) basis. Identifying where those differences occur and how they interact will be essential to creating a successful design.
The long-term success of projects by planting designers such as Piet Oudolf, Isabelle Greene, James van Sweden/Wolfgang Oehme, and Christy ten Eyck is rooted in their deep knowledge of different plant species and how they behave. Building that knowledge base requires time and attention, but you can get started by explicitly identifying the physical aspects of a site that will influence your plants’ success.
Gardeners get to know sites intimately through observing and interacting with them over time. Professional planting designers don’t have that luxury. We’re on the clock. We need to be as efficient as possible with gathering the information that we need.
When I walk a site prior to design, I look for factors that affect availability of plants’ 3 big needs: light, water, and nutrients. Other factors – such as management – will affect what plants will grow well, but I don’t need to observe those on site. The aspects that I absolutely need to observe and document on site include the availability of:
Light availability occurs at the scales of climate and site.
If you’re practicing in multiple geographical regions – maybe designing for a climate with which you’re not familiar – you’ll need to understand the annual patterns of light (and temperature) in the region where you’re designing. Temperate (and colder) climates have strong seasonal variation in the availability of light. Subtropical and tropical climates tend to have less seasonal variation in light availability. Identifying the overall annual light cycles will help you choose appropriate species for your design.
At the site scale, I find it useful to distinguish areas of full sun (6 hours or more of direct sunlight), partial shade (less than 6 hours), and full shade (no direct sun). In partial-sun conditions, aspect (north/south/east/west) and time of day that sunlight is received are highly influential. North and east-facing areas tend to receive gentler morning light, while west and south-facing areas receive the harshest rays of the sun when it’s hottest. (This is inverted in the southern hemisphere) Any shaded condition is complicated by the degree of shading – tree species differ widely in how much light their canopies block from reaching the ground. Building shade tends to be very dark. If you have the opportunity to observe or get site documentation at the brightest time of year (summer, northern hemisphere) versus the darkest, do it. Find out as much as possible about how the light shifts over the year on your site in order to choose appropriate species.
You also need to consider how light levels will change over time. If you’re planting trees, you may be tempted to plant shade-loving plants underneath. However, at the time of planting and for 10-20 years following (depending on the size of the trees), the groundcover around your trees need to be sun tolerant since the canopy coverage will be minimal until your canopy trees mature.
Water availability also occurs at scales of climate and site.
Understanding the yearly (and multi-year) patterns of precipitation will inform your planting design. Plants have typically evolved in response to specific annual patterns of precipitation. In temperate climates, moisture availability mostly matters during the growing season. In the midwest and eastern US as well as throughout mainland Europe, precipitation is relatively consistently available during the warm season. The Gulf Coast of the United States as well as the Caribbean has a summer-wet, winter-dry precipitation pattern. The West Coast has a summer-dry, winter-wet precipitation pattern. These moisture availability patterns affect which species grow well where. Identifying your overall moisture availability pattern will help you identify which species are likely to thrive.
At the site scale, variations in topography are often the best way to identify how water flows across a site – where it’s more abundant and where it’s scarce. If it rains on your site visit, celebrate! You’ll be able to see how water actually moves across the site. Looking at the plant species which are already growing on site is also a good way to identify moisture distribution. Dry-loving plants are a clue that an area is dry, while wet-loving plants clue you in to the opposite. If there are strong seasonal variations in precipitation, try to get photos of the site during wet and dry seasons in order to see the extremes of moisture on the site.
Too much water is equally damaging to plants as too little water. Most plant species cannot tolerate their roots being submerged for more than a few hours. Water completely submerging the roots prevents plants from accessing oxygen and they will suffocate (Missouri Botanical Garden has an article explaining how this works). If portions of your site regularly have standing water, you will need to look for wetland and marsh species that are adapted to those conditions.
You can address a lack of water with an irrigation system. Depending on the infrastructure of the region that you’re working in, this may be a useful solution. As someone who designs as much as possible with site conditions, rather than fighting them – I tend to design plantings that will require irrigation at establishment and only in severe drought conditions following.
One big challenge in tropical and subtropical regions, especially in regions with a specific dry season (summer-dry like the west coast/Mediterranean or winter-dry like south Florida and the Caribbean) is that lawns require frequent and abundant watering during dry season to stay green. This is often highly detrimental to adjacent plant communities which require less water. If you’re designing next to lawns that you know will be watered regularly (regardless of your personal aesthetic or moral stance), make sure to use species that won’t be harmed by the frequent watering.
While plants are able to manufacture much of the energy they need from sunlight and water, many do have specific nutrient needs from the soil. In the ethos of trying to work with site conditions as much as possible, rather than fighting them, I generally try to choose plants that will thrive in the existing soil conditions rather than trying to change the site soil to suit different species.
Three aspects of soil determine which species thrive: soil texture/particle size, pH, and organic matter.
The soil texture/particle size is the first thing you’ll notice when looking at or feeling the soil. Soils are typically categorized by texture as sand, silt, and clay – or loams, which are roughly mixed between these different types. You can learn how to distinguish different soil types using visual inspection and the hand squeeze test with these videos from Purdue: “Soil Basics:Texture”. Sandy soils, with their large particle sizes and gaps between particles, tend to lose nutrients and water quickly. Clay soils, with small particle sizes and gaps between particles, hold on to nutrients and water.
pH is an important factor in plant health because the pH of the soil affects how plants are able to absorb nutrients from the soil. pH refers to hydrogen ion concentration in the soil – it’s significant because it affects how soluble minerals and nutrients are in the soil, which determines whether or not plants are able to absorb them. On the south Florida sandbar where I practice, our natural soil pH is very high (basic) which means that it’s very difficult for plants to absorb the nutrients they need, even if they’re present in the soil. Applying acidic fertilizers can help unlock existing soil nutrients for plants. In any case, it’s best practice to select species which are more adapted to the pH conditions of your site rather than those with highly different nutrient needs.
While the basic texture of soil is based on the underlying rock from which it was formed, organic matter content also has a strong influence on what species. Organic matter helps increase levels of soil moisture and microorganisms. When you think of “good soil” for growing vegetables or seasonal bedding plants – something dark brown and easy to work – you’re thinking of a loam that’s high in organic matter. On a potential site, high organic matter soils would usually be present only in greenfield sites that were recently forest or prairie. If you’re trying to create a plant community that mimics an ecotype with naturally high organic matter in the soil, such as a temperate forest or bog, you will want to amend your soil with organic matter.
A relatively new trend in planting design is that of choosing to design with species that are adapted to low-nutrient conditions. Perhaps counterintuitively, many desirable flowering plants actually thrive in low-nutrient conditions. Numerous designers throughout the world are experimenting with growing plants in gravel and/or sand. As someone who works on an island that is essentially a giant sandbar, I sand garden by necessity rather than by choice. One of the benefits of designing low-nutrient requirement plant communities is that weed competition can be significantly less problematic.
If you’re having issues with nutrient availability and soil, you may want to check the pH of any water sources for irrigation: wells and municipal water supplies can have different pH from the soil on your specific site, which over time will affect nutrient availability for species you’re planting.
The potential aspects of a site that you could document and consider in design are endless. But if you’re starting to get serious about designing planting, availability of these three things that a plant needs – light, water, and nutrients – are a good place to start.
This piece is specifically about what I, as a practicing landscape architect with a focus on planting design, look for when I’m approaching a site that I am tasked with designing. Aesthetic and management considerations deserve a piece of their own. When you’re designing dynamic plant communities – where plants are intended to move around on their own and change over time – you need to consider their life cycles and methods of reproduction, as well.
I don’t always get this luxury, but I prefer to see a site in at least two conditions – preferably once when it’s sunny and midday to get the sun/shade patterns, as well as once when it’s raining to see how water flows across the site.
The goal for all site observation is to be able to use it to inform your design strategy. Depending on your design goals, you will need to change the site to varying extents. That may include adding irrigation, adding topsoil, removing contaminated soils, establishing a fertilizer regime, or any other number of horticultural care practices. But without a base understanding of your site conditions, your plant selection and care recommendations will fail to support your design intent. Learning to see the physical qualities of a site and how they shape plant performance will inform your practice and help you to create design strategies that fulfill your aesthetic and experiential goals.Published in