Permeability in the Garden

Permeable Beauty

Jeffrey Bale

Pennsylvania Blue Stone laid on 1/4 minus gravel

The more that I delve in to the art of making gardens, the more I am convinced that the ultimate landscape is one that is fully immersed in the natural realm.  There is something about a garden that can breathe, and absorb the moisture that falls upon it that feels good to me.  I can sense the life force that results when natural systems are supported so that they can thrive.

Garden symposiums seem to be focusing more on environmental issues these days, especially in California where the wise use of water is becoming not just important, but essential.  I was asked to address the topic of permeable pavements at a Pacific Horticulture Symposium in Los Angeles.  Permeability doesn’t sound like the most exciting topic, but when you think about it, most of the life in a garden is underground where we cant see it.   When we learn to nurture that, we benefit the environment that sustains us.   It is not just a matter of engineering to get water to drain in to the ground, it is an opportunity to provide for the earth and enrich the architecture of our gardens at the same time.  I’ve selected images of the best examples of work that I’ve built or seen around the World that show how pavement can be permeable and beautiful at the same time. 

This stone path has gaps creating stepping stones and increasing permeability

It has always been important to me that the Earth be allowed to breath and replenish, even when paved.  This is seldom accomplished in typical hardscape, where driveways cover enormous areas, walkways drain on to sidewalks and in to the gutters on the street, and patios are impervious slabs.  If you were to look at an aerial view of your home, how much square footage is made up of house roof and impermeable lifeless pavement?  It is a large amount of space we occupy at the expense of natural ecosystems.  The runoff flows in to storm drains carrying pollutants that collect on surfaces such as oil and airborne grit, and without biological systems to filter and purify it, the runoff degrades waterways.

A gravel garden in Florence, Italy

Where I live in Portland, Oregon you can see specially engineered curb swales that have been installed in some neighborhoods to catch the runoff from streets.  The water that would normally go down the storm drain is allowed to be absorbed in to the ground. The swales become wetlands during the rainy season.  Healthy wetlands are some of the most productive on Earth.  So what was once asphalt is becoming a crucible for life.  Oil and dust are cleaned from the water by microorganisms and plants, rather than pouring in to the river, so the negative impact that we have by paving large areas is reduced.

This Street Swale in downtown Portland collects runoff from both the sidewalk and the street

That moisture is made available to the roots of trees, and local aquifers are replenished.  People are also being encouraged to disconnect their downspouts to reduce runoff in to storm water  systems, which during heavy rainfall sometimes cause the city’s sewage system to overflow directly in to the Willamette River.  The city has pledged to meet a federal deadline to stop that from happening.  The most responsible and proactive way to do that is to increase permeability.

Some street swales are better looking than others.  These do a nice job of addressing the issue of being able to park and still collect water from the street and sidewalk

When it comes to the garden, run off can be most easily put back in the ground by having gaps in the pavement  that allows the water to reach the soil.  This doesn’t necessarily have to be gaping swale that you can fall in to.  A simple couple of inches of open space at regular intervals can absorb average rainfall.

Gaps in this mosaic pavement allow for permeability

The easiest way to make permeable pavement is to use permeable material, like wood chips, bark, or gravel.  I’ve seen filbert shell paths that are crunchy and interesting.  But I’m a stone man, so I tend towards gravel.  If you are going to drive on it, it should probably be crushed.  If you are going to walk on it in bare feet, it should probably be rounded and smooth, or it will stick to your feet and feel sharp.  For soft gravel patios I like to use 3/4 inch round rock.  It is available at some bulk garden supply centers and stone yards by the yard, or you can fill buckets to haul it.  I dig an area out flat so that I can spread it at least 3 inches thick and have the top be at the desired grade.  I rake it smooth and hose it down to clean it.  Instant patio!  Pea gravel, which is smaller and more common, is an option too, but I find it sticks to the feet and in the soles of some shoes and gets tracked around.  The 3/4 round rock tends to stay in place, and is prettier.  I never put landscape fabric underneath as it doesn’t work all that well and is ugly when it pulls up to the surface.  It also creates a barrier that prevents organisms from moving through the soil, decreasing soil health and diversity.

3/4 inch round rock patio and path

My main garden area, which is quite small, is paved in these small pebbles.  I love how clean it is.  I can wash things on it and it drains right in to the ground.  Self sowers come up along the edges but not where there is traffic.  I mix in pebbles I have collected in my travels to add color and interest.  The gravel dries quickly, and I like to lay Oriental carpets over it in the summer to recline on with pillows.  It makes a great space to stretch and do yoga, or lounge with friends.  Its also nice that the  legs of garden furniture will settle in to it so you don’t have wobbly tables and chairs.  And I’ve even brushed bright blue paint on the pebbles and raked them in to add a dash of color.

Carpets on my round rock patio in summer

All over Europe, the use of crushed stone is very common in gardens for pathways and drives, allowing water to make it’s way in to the soil.  It is far less common here in the U.S., where people seem to be obsessed with tidiness.  I find it much more appealing than white concrete, which is way overused here.  You can top dress it with a fresh layer of gravel if it is worn away, and you can smooth over depressions in the grade if puddles form.  Decomposed granite is common in some areas, but 1/4-10 crushed basalt is probably the best available  material  in the Pacific Northwest where I live.  It will make a firm surface but doesn’t have fine particles in it so it wont totally hard pack, allowing water to pass through.  Gravel can be slippery on steep grades, so I recommend terracing with steps if using it on slopes.

Gravel Terrace, Boboli Gardens, Florence Italy

DNA Molecule Parking Strip Mosaic

When paving an area with stone or brick, laying the material in sand and not mortaring the joints allows water to reach the underlying soil.  When moisture and air can get to the ground, life has a better chance of manifesting itself.  Live soil is far better than dried out, oxygen starved soil for the root systems of trees and shrubs needing to capitalize on the soil under their canopies, which are often times paved.

If pavement is solid, like a cement pour, or stone set on a sub slab or mortared together, then gaps or holes in the pavement are the best way to allow water to drain in to the ground.  It is common for pavements to be unbroken, but even a 1 inch gap will allow water to seep in.  I use wooden form boards when I build with mortared stone that can be removed after the work cures.  The design benefit is that expanses of pavement can be broken up to make them feel less imposing and softer.  The eye will stop along the length of a path rather than heading down a runway like stretch.  Pavement pads that connect sidewalks to the street curb will drain in to the ground if there is a gap left between them.  If repairs need to be made to sidewalk or curb then the pad wont need to be disturbed, and tree roots are less likely to push up from underneath because water and air can reach them.

When I replaced the once solid pavement entry to this house I created gaps in the step pads to allow runoff to drain in to the ground.

A walking labyrinth I built in Puget Sound has gravel filled gaps between the circuits that allow moisture that falls on the surface to drain in to the ground, reducing the impact of such a large pavement.

Path and Small Patio in Portland

Patios can harbor plants to soften their look.  I use these gaps like expansion joints, allowing small plants to take hold and make lines of green in the pavement, or I fill them with pebbles set in sand.  My pebble mosaic patio at home has sensuously curving expansion joints that have become linear gardens unto themselves.  If you want to plant groundcovers in the gaps, I recommend removing the soil and filling the gaps to 1/2 below the finish grade with a good planting mix and planting in that.  The drainage will be improved and the plants will establish better.

Driveways are usually the most offensive part of a home garden, especially in the suburbs where parking for numerous cars seems to be a requirement.  I’ve worked on several homes where the driveway took up more than half of a front yard, dominating these homes and looking more like a parking lot.  They often crack, because the expanse of pavement is too big.  Few people can say they have an attractive driveway.  By breaking up the area with gaps, cracks are less likely to form, and water can drain in to the ground to the benefit of ground dwelling organisms and root systems.  When pouring a new driveway, add forms that are the full thickness of the pavement and can be removed, and then fill the gaps with permeable material.  There are pavers on the market that can be laid for driveways to be permeable as well.  The trick is laying them so that they aren’t ugly.  A pleasing shape and creating a complimentary pattern are the tricks to solving this problem.  Try to be as imaginative with materials as well as utilitarian. Use the touch of an artist rather than the Army Corp of Engineers.  Concrete strips that only cover the area for tires to run on with an open strip of lawn or low ground covers used to be far more common than today’s expansive driveways.  If you want to make openings in an existing slab, a concrete cutter can be hired to cut lines and remove sections of pavement at relatively modest expense.  Doing this where the driveway meets the sidewalk or street can significantly reduce the amount of water that flows down the drain.  The gaps could be planted with tough low growing herbs rather than turf grass, which is far more appealing than vast areas of pavement.

Permeable Driveway Pavers

You might just start hanging out in your driveway rather than just parking on it if you make it more appealing to look at.  When you hose off your patio and paths, the water will have somewhere to run besides a drain.  Life will have a chance to flourish, our waterways will be cleaner, and your garden will become a healthier, vital, and more beautiful place .