What is a Bioswale?

I get this question each and every time I tell a client of my landscape design business that I'm recommending one for their property.  No one seems to have heard of these powerful little grading gems, but everyone should know what they are, how to make them, and where to place them.  The reason for this is that they allow for storm and irrigation water to infiltrate below the soil surface.  This hydrates the soil profile and recharges the water table.

In places like Los Angeles County, where I live, unsustainable and damaging land planning has desiccated the landscape, focusing water directly to the ocean instead of using it on the land that would filter as well as use it to cover itself in plants.  Bioswales are so effective when made and sited correctly that they can be a main tool in our kit for preventing drought.

Yes, that's right, by capturing water in the ground we enable trees to draw moisture to the land in which they are growing.  Trees have many outstanding and vital properties, what some appropriately call 'ecosystem services,' not the least of which is attracting rain.

So what is a bioswale then?  A swale is simply a shallow linear depression in the ground, set on contour, to collect and infiltrate water into the ground.  A bioswale is a planted swale.  I specify both for my clients often, to help them use less irrigation water but mainly to help recharge the ground water and support plantings as part of an integrated permaculture system set up to recover ecological function across a site.

What is a Bioswale and How Do You Build One?

Once people understand what a swale is, they always ask me how to make one.  It needs to be dug on contour, that is parallel to topographical contour lines for sloped land.  If you have a flat property, you don't have to follow this rule.  A swale needs to be no more than 4" deep and 18" wide (they can be any length) in order to capture and hold water.  In areas that receive torrential downpour storm events, this still works.

I recently was asked by a landscape design client of mine in Beverly Hills about whether they fill up or need to be deeper on her steeply sloped property.  I explained that the purpose of swales is not to try to capture every inch of rain that falls on a property because the amount captured and stored in the earth will mean significant improvements on her site.  Furthermore, I explained that the area down slope of a swale receives the stored moisture and so becomes hydrated.  It is an ideal location therefore to plant trees and smaller plant species.

On larger acreages, you can dig swales with a tractor, expanding their dimensions to deeper and wider, but on residential and suburban properties a shallow depression is adequate.  A variation for flat parcels is a shallow circular or oval depression.