Why Tucson needs crevice gardens

If you’re familiar with rock gardening, crevice gardening is a new method that falls into that category. With rock gardening, rocks are mixed into the soil berms and are used to help create hilly areas of the garden with rocks sticking out of the soil. Essentially, they simulate rocky dry slopes, and look like miniaturized rocky hills.

The innovation in crevice gardens is that flat rocks (like flagstones) are inserted vertically into the soil to make little sheltered rock canyons for planting. This allows for a small amount of soil for plants that like fairly barren conditions — ones that grow in canyons or on the sides of steep creek beds, for instance. The rocks also act to channel water into the man-made crevices, and — if oriented correctly — can also provide some shade during our most punishing daylight hours. They also help shelter small plants and their seeds from the wind.

Crevice gardens originated in Europe and moved to other parts of North America chiefly so that gardeners in wetter climates could grow a wider variety of plants: those that liked dry rocky slopes, or those that did well in alpine conditions, for example. But crevice gardens can serve a purpose in the desert as well.

Why plant crevice gardens here? For one thing, it allows you to plant a richer diversity of plants, many of which may be running out of natural areas in which to grow — plants that like shade, for instance. Crevice gardens also give you an opportunity to have a visually interesting area of your garden, and to add dimension to a flat garden space. They allow you to create microclimates for unusual or rare plants, particularly in yards that are south-facing and very hot or bare.

As an added bonus, the rocks provide shelter for small animals such as our native lizards and various insects, such as butterflies, who will warm themselves in the cool desert mornings on the toasty rock. And, if you choose your plants wisely, you won’t need to water your crevice garden after it’s established, except for particularly hot and dry periods.

Crevice gardens are great whether you have a flat yard or a sloped one. In a flat yard, they add visual interest and create microclimates for unusual plants. In a sloped yard, they’re a great way of making use of your topography while adding a great design feature and a new miniature ecosystem.

What are some downsides? Crevice gardens may attract some types of wildlife that you may not want — digging animals, for instance. Whether this is a likely issue for you depends on where your yard is located. Walled urban yards are usually less likely to suffer from large populations of gophers or pack rats. If you have a larger property in the desert, I recommend siting larger crevice gardens far enough from your house that a few extra critters won’t be an issue. You can build small crevice gardens closer to your house. Another issue is, because crevice gardens are great for plants that thrive in bare minimum conditions, they can attract weeds. This is true of any disturbed soil, however.

Rocks with flat sides are needed, as is sandy or even gravelly soil for creating the “hill.” If you have a fairly small area that you want to turn into a crevice garden, you don’t need to obsess over getting super-flat rocks; you can create crevices with any rock that has at least one flat side. Depending on the size of the rock, you can have a fairly deep crevice or a shallow one.

If you have access to broken up concrete, you can also use that for creating a crevice garden. Repurposed concrete, sometimes referred to as urbanite, is perfect for crevice gardens because it has two flat sides. It can also frequently be easily found — either because a homeowner has demolished concrete, or because someone else in their community has, in which case it may be free. Concrete is very expensive to dump, and takes up lots of landfill space, so repurposing it in a crevice garden is an environmentally-friendly way of putting it to further use. For a great example, take a look at the blog of the Juniper Level Botanic Garden in North Carolina.

Due to our harsh climate, you will want to think carefully about the orientation of your crevices. You may want to avoid south and west-facing crevices, since these will be very hot and sunny during our summers. East-facing crevices, if blocked with soil on the west side, will have nice morning sun and warm rocks on the south side but will be shaded from the brutal western sun. North facing crevices will be shaded for much of the day and will allow you to grow plants that wouldn’t otherwise make it in a hot, sunny yard.

You can also turn the vertical orientation of your crevice garden to horizontal, and build it as a crevice wall, with the flat rocks or urbanite stacked on top of each other with sandy soil in between for small plants. It’s a great way to make a low retaining wall a visual feature in your garden. Again, orientation is crucial, and having an east- or north-facing wall will give you the opportunity to grow some plants that don’t do well in our afternoon sun.

How big should you build it? Crevice gardens have been built in containers, so build one in whatever size works for you and your garden. It can be tucked into a corner of the yard or be the main focal point of your garden. The main things to consider will be your budget, and the availability of rocks and soil.

A word of caution: don’t place crevice gardens over tree roots. The addition of soil and rocks on top of the roots are likely to kill your tree. If you have lots of trees in your yard, you can go with a small crevice garden tucked away in a root-free spot, or one that’s in a container.

There are many smaller plants native to our area that require little soil and a sheltered, rocky terrain. Small succulents, wildflowers, and grasses come to mind. If you’re in a cooler part of Tucson, you can use crevice gardens to plant some unusual wildflower species that thrive in higher elevations in our mountains.

Smaller plants are usually planted as bare-root, or nearly so. Just remove as much of the container soil as possible. You can also tuck in seeds, particularly with grasses and wildflowers. The soil should be pretty sandy; the mix will depend on whether you’re using native soil or purchased, but if in doubt, use mostly sand and gravel with a small proportion of regular soil.

Some of our smaller cacti, such as Mammillaria spp. will do well in crevice gardens. These little guys often have beautiful flowers. There are over 200 species with varying growing needs, so read up on the ones you want to plant before deciding where to put it.

Small varieties of prickly pear (Opuntia spp.) are also good candidates here. O. fragilis and O. macrorhiza are possibilities. Hedgehog cacti (Echinocereus spp.) are also great, and have spectacular hot pink or purple blooms.

Succulents such as small agave will also do well due to their tolerance of our sun and small root systems. One option is Queen Victoria agave ‘Compacta’ (Agave victoria-reginae ‘Compacta’) which stays under 12 inches wide. Twin-Flowered Agave (A. geminiflora) is another nice choice, due to its green, narrow leaves and tolerance of even reflected sunlight. Generally, ones that do well in containers will do well in crevice gardens.

You should be able to find these in any local nursery that stocks a large variety of cacti and succulents. Local options include B&B Cactus, Bach’s Cactus Nursery, and Plants for the Southwest. 

Native grasses are another category of plant for crevice gardens. They provide color, texture and movement, along with vertical accents. They are also great for birds, insects and even desert tortoises. Ones that should do well in hot, sunny locations include purple threeawn (Aristida purpurea), cane beard grass (Bothriochloa barbinodis), sideoats grama (Bouteloua curtipendula), red grama (Bouteloua trifida), Arizona cottontop (Digitaria californica), and many others.

You can read more about your options for native grasses in Spadefoot Nursery’s guide.  Look for species that grow on rocky slopes, don’t require extra irrigation, and are appropriate to your elevation. Spadefoot Nursery and Desert Survivors are good places in town to get native grasses. You can also get grass seed mixes at Native Seeds/SEARCH.

Many native wildflowers also do well when given sheltered places to grow. You can try desert marigold (Baileya multiradiata) as they like sandy soils, but don’t give them too much of a slope. If you’re higher up, try western wallflower (Erysimum capitatum), which has gorgeous orange flowers and attracts butterflies. Penstemons (Penstemon spp.) can also do well but plant them or sow seeds into east-facing crevices since they don’t like too much sun.

Native poppies (Eschscholzia spp.) also do well in little nooks and crannies; you can just disperse the seeds in the early spring or fall and tuck them into the crevices and lightly water. You can also plant Arizona poppies (Kallstroemia grandiflora), which aren’t true poppies but look very similar. Poppies are annuals but will reseed if they are happy.

Globemallows (Sphaeralcea spp.) are another great wildflower to try, and many are perennials. They come in a variety of colors, including rusty orange, pink and purple. You can also try growing scorpionweed, also called wild heliotrope (Phacelia distans), a beautiful violet-blue flower that does well on rocky slopes in the wild.

Excellent places to get wildflower seeds for our area include Native Seeds/SEARCH and Borderlands Nursery and Seed.

If you have made room for larger plants, there are lots of great candidates. For starters, brittlebush (Encelia farinosa) will do great in rock gardens even on hot sunny slopes, although it may look better with some afternoon shade. Chuparosa (Justicia californica) will give you some nice red color and will do well on any rocky slope. You can also plant desert lavender and creosote bush.

For more information on crevice gardens, check out this interview with two gardening proponents of the style, Kenton Seth and Paul Spriggs, who also have a book out.  They’ve also been interviewed in the New York Times. You may also want to take a look at the website of the North American Rock Garden Society. While they don’t have a chapter in Arizona, their website is filled with helpful tips and ways to connect with other rock gardening enthusiasts from around the world.