If you don’t live in the center of the Tucson valley, chances are you have some sloped areas in your backyard. Slopes can be a challenge to gardeners, but they also offer some great design opportunities.
For one thing, incorporating slopes and hills into your garden design can increase the visual interest in your design. You will automatically introduce a three-dimensional element, which allows you to design the flow around it. For example, you may choose to lead your path around the hill or along the slope to provide your visitors with a visual journey. Hills and slopes can also provide you with uninterrupted views, which makes your yard look larger than it is.
Slopes also provide you with natural microclimates. North facing slopes are a great opportunity to plant vegetation that likes cooler and shadier microclimates. South and west facing slopes are trickier in our climate, as they get a lot of sun and tend to be very hot during our summers. Also, the top of a slope will be very well-drained while the bottom will tend to be moister, so keep this in mind when placing your plants. More on this below.
On the negative side, your yard will likely have erosion and drainage issues, particularly with larger, steeper slopes. You will need to design in some hardscaping to keep these issues from negatively affecting your property.
Designing for slope: hardscaping
A number of hardscaping features can help you navigate a sloped yard. The extent (and expense) of these features will depend upon the topography of your property and on what areas you wish to have accessible. Any place you’re planting non-native plants will need to be accessible by foot; larger plants and irrigation systems, as well as ponds and pools, may need access by small machinery like Bobcats.
Ramps and steps provide access for humans and pets (and also other critters). Our local building codes will specify the maximum angle of the ramp and whether the steps require a railing. Your contractor should know what is required. Steps can be made from concrete, wood, stone, recycled concrete and a number of other materials, so you can choose the look that meets your needs. Pathways can be from gravel, mulch, stone and a variety of other materials. They can be integrated into the surrounding landscaping, or clearly visually separated, depending on the look you want.
Don’t forget about lighting — especially on sloped areas and steps, adequate lighting is a key safety feature. You can read more about designing your look in my recent article on outdoor lighting.
Earth control structures
Low walls work well for small slopes and cost little to set up. You can use river rock and dry stack it up to three or four courses high. You can also purchase precast bricks made for creating walls. Walls need to stay under three feet in height if they are stacked without a foundation. You can also use individual rocks in areas where you’re planting plants to help prevent soil from running off in rain events before your plants get established.
Terraces are another option, and often have a more formal, structured look; however you can still get an informal, natural look by using natural rocks. They can be cut into a slope to help with erosion and increase the flat area. This can leave you with more usable yard. Building out smaller terraces can be an alternative to building a large (and expensive) retaining wall. You can have 3- or 4 foot-tall terraces built with rocks or low walls over a span of ground that would otherwise require a 4- or 5-foot retaining wall. Terraces can also function as seating areas in your garden.
Terracing is great in our climate because it prevents water from running off down a slope and the flat terraced areas can soak in rainfall. This prevents erosion and allows for moisture retention in your garden soil — both critical in our desert climate.
Retaining walls are needed for slopes greater than 35 degrees. These need to be built with proper foundations and require a permit. Retaining walls can be built out of stone, brick, wood or metal, so you have a lot of aesthetic options. However, I would recommend you get a professional to do this for you, since an improperly built retaining wall will fail and cost you a lot of money in the long run to repair or rebuild. They need to be correctly engineered and drained to function well.
For a quick reference to permit requirements in Pima County, check out this helpful page: Do I need a permit?
Water and drainage
Drainage is the most important thing to consider when you have a sloped yard. Improper drainage will result in erosion, destabilization of the slope or water collection and resulting plant death and mosquitos. If you have drainage issues, fix these first, ideally with the help of a knowledgeable contractor. A good place to start is the Watershed Management Group. They can take a look at your site and help you identify problems and solutions.
Slopes on your property also allow you to collect rainwater passively, which is a great feature to take advantage of. Even relatively small earthwork structures like berms and swales can help you take advantage of the rainfall without having to purchase large tanks. For more on this, take a look at my article on passive rainwater harvesting.
Slopes also may allow you to have flowing streams and even waterfalls. In the flat areas you can consider having a pond or wetland. The combination of rockwork, plants and moving water can be a spectacular addition to your yard design. For example, if you are lucky enough to have a north-facing valley feature, you can turn this into a cool, lush mini-canyon for an incredible retreat away from the heat. You can also create your own canyon feature — even one that’s a couple of feet deep can provide a protective microclimate for unusual plants, and keep water from evaporating too quickly.
Microclimates and topography
Even a relatively small hill or slope will affect how air and water move around and through that particular piece of ground, and can provide small areas of shade or orient plants towards the sun. This creates little microclimates around the different topographic features in your yard. The south and west side of a slope will always be hotter than the north or east side, for example.
Because water drains downward due to gravity, the top of a slope will always be drier, while the bottom will tend to collect moisture. You will need to keep this in mind when you’re selecting what plants to place at different elevations on your slope. Slopes affect cold air exposure, as well, because cold air is dense, and sinks and flows downhill like water. For this reason, avoid putting frost-sensitive plants at the bottoms of slopes.
You can use slopes, valleys and canyons to your advantage. If you have a slope that creates a u-shaped valley, notice what direction it faces. Even in colder microclimates (like in the foothills), south-facing valleys stay warmer, and may allow you to grow plants that would freeze on flat ground. Similarly, a north-facing mostly shaded valley will allow you to grow plants that would otherwise be baked by our hot sun, and to do it with a much lower water investment.
Finally, pay attention to the prevailing wind in your part of town so you know whether to expect your valleys and hills to be sheltered or windblown. The top of a hill will be much more exposed to sun, heat and wind, and those plants will suffer from drying out and even from the force of the blowing wind. The top of a hill in a windy area is not the place for delicately-leaved plants, nor for trees with shallow root systems.
If you have a larger property, I recommend installing at least one weather station with a wind measuring device and temperature sensors. For smaller yards, thermometers with minimum and maximum temperature recording features are indispensable in helping you plan out your garden. Most of them also have humidity sensors, which will be a big help in figuring out your microclimates.
Slope microclimate factors
- North-facing slopes, canyons, small valleys: The perfect location for more delicate plants that require part shade. However, depending on the configuration of your slope, you may still get morning and afternoon sun during the height of summer, so make sure you check this carefully.
- East-facing slopes: This is a great place to put plants that need some sunshine but can’t handle our intense, hot afternoon sun.
- South-facing slopes: These areas will get lots of sun and heat during the middle portion of the day. However, depending on the exact topography of your site, they may be shielded from the harshest western sun. Make sure you investigate this before choosing your plants.
- West-facing slopes: These get the hottest sun. It’s great for wintertime, but not so good for summer. Pick and choose your plants carefully. You may put moveable containers here in the winter, and then stick with sun-loving native plants for the summer. You can also consider shading your garden with a deciduous native tree like the desert willow which will provide shade in the summer but will lose its leaves in the winter.
- Top of slope: This will be the best-drained area. Save it for plants that like well-drained soil. Fortunately, this is most of our native plants, as well as drought-adapted plants like rosemary and lavender. This will also be the hottest area and will be most exposed to wind.
- Middle of slope: The Goldilocks zone. Drains well, but still holds on to water. Of course this is heavily dependent on the type of soil you use and the angle of the slope. Be mindful of wind and sun direction when choosing plants.
- Bottom of slope: This is where water will pool if you don’t have good drainage. This may be great — a good place to put a mini-wetland (for example on the north side where it won’t get baked by the sun). Or you may want to put some water-loving grasses or wildflowers at the base. You can also direct the water to a tree from here. Make sure the water doesn’t end up standing for hours after a large storm, though, as this will kill almost any plant that grows here. This is also the coolest area.
Now that you’ve figured out your drainage, sun and wind exposure, and topography, it’s time to choose some plants. A thorough list is way beyond the scope of this article, and in any case would depend on your individual site, but here are some overall suggestions.
First, think about the plants that will do well in your area. Native plants are a great guideline for you. Take a walk in the desert near your house and see what grows where. What canyon plants do you see in Sabino canyon? What grows on the north slope of the Foothills trail you’re on? Local walks are a great starting place.
Another great inspiration can be your own neighborhood. What plants are growing well in your neighbors’ yard? Do you like the look or design? Don’t be afraid to get to know your neighbors and ask them about their plants.
There are countless plants that you can use in your yard, but here are a few basic categories I want to highlight.
- Grasses are an often overlooked type of plant, but they can really add to the visual interest on terraces and slopes. Choose native grasses such as different species of grama or threeawn. Check out this great guide to grasses from Spadefoot Nursery for more info.
- Tough, sun-loving natives like prickly pear cacti and creosote bush would do well at the tops of hot, sunny slopes. Other options include cholla cacti, jojoba bush, brittlebush and desert lavender.
- Groundcovers are a good choice for shallow slopes because they will help prevent soil erosion. For example, trailing dalea (Dalea greggii) is a great native groundcover which blooms with purple flowers in the spring and summer and is very drought-tolerant. It’s a great choice for south-facing slopes. Another tough little plant is damianita (Chrysactinia mexicana) which grows in little mounds and has beautiful yellow flowers.
Again, the best strategy is to look around at nearby natural areas and see what is happy growing where. You can take a guided hike with some plant lovers who can identify plants for you. You can also take a tour of the University of Arizona Arboretum to learn more about some of our local plants. Other great places to think about microclimates and local plants are our various local gardens: the Tucson Botanical Gardens, Mission Garden and Tohono Chul. Finally, don’t forget to visit the Pima County Master Gardeners demonstration garden, which can be toured on your own or guided. All of these places are filled with volunteers and docents who will be happy to share their insights with you.
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Do you have any gardening topics you’d like to see covered in the Tucson Garden Guide? Email me at [email protected] with your suggestions and questions. Thanks for reading!