Marin Master Gardeners
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How to bank some water in your garden

February 16, 2019
Diane Lynch

We’re approaching the end of our rainy season, but it’s not too late to contemplate ways to capture some rain for future use. Though we had a little summer in January after a nice rainy spell, the hope is always to get more rain before spring.

Some years ago, I loaded my car with trash cans, placed them in my yard and started saving rainwater. I could get into July without turning on the irrigation, but it was hard work. I’m fortunate to have a drainage rill that runs through my garden into an open 1,100-gallon catch basin. I would pump the water into the cans, cover them to keep out mosquitos and selectively pump or bucket it back out into the garden. I ended up with a damaged shoulder and the mosquito abatement woman who came to monitor the basin took an instant dislike to the trash cans all over the place, though they had no larvae. And then there was the general untidiness.

So now I’m older, lazier and fill fewer of the cans, but I still pump the catch basin to water the garden and in recent years the stream started trickling all summer, making me the happy recipient of water from my uphill neighbors.

Marin Municipal Water Department estimates that Marin residents use about 88 gallons of water per person per day, with half going into the garden during the warmest months. In California we have a vast, but fragile water system dependent on rain, snow pack and groundwater. Through our periodic droughts, we’ve begun to think of ways to save water by planting selectively, laying the garden out in hydro-zones, planting according to similar water requirements, using drip irrigation and mulching to retain moisture.

Landscape architects used to design to channel rainfall as quickly as possible to the street into storm drains, which feed into streams, rivers and bays.  Many homes had (and still have) gutters connected to French drains to facilitate water removal. Current thinking is to strive for as many permeable surfaces as possible. Driveways can be paved with permeable material. Garden paths can be gravel or decomposed granite, rock or pavers set directly in the soil, which allow water to filter slowly and sink in to the water table.

Consider creating some passive water storage in your garden. You can do something as simple as reworking the angle of a hillside bed so water seeps into the hillside rather than running off — this will encourage deeper root growth and make water available later into the spring, along with replenishing the water table. Specialized cans capture rainwater via the guttering system and have more secure lids than run-of-the-mill trash cans. Depending on the lay of your land, if you raise walkways in the garden you can create beds that will capture rainwater. You can dig small basins uphill from trees and plants, which will allow more rainwater to penetrate.

A bioswale or rain garden is more involved but still easy for a do-it-yourselfer. Simply speaking it’s a pond-like indentation in the soil with gently sloped sides to capture water and slow it down to allow for removing debris as it trickles into the soil; it would typically be landscaped with granite or limestone riprap or vegetation. It can be underlaid with aggregate and pea gravel to create a retention zone. These can be installed on slopes or in lawns.

You can create a storm water gathering system by installing a large tank, typically concrete or plastic, and some rudimentary plumbing. Harvest rainwater from your roof to fill it. An amazing amount of water flows off any roof. In a perfect world, you’d set the tank on the uphill side of the house to allow gravity to distribute the water. Tanks come in many shapes and sizes.

To get started banking water on your property, go out there in a good rain, carefully watch where the water goes and map where you think you could adjust capture runoff. Relatively small steps can have a big impact for your landscaping and feed the aquifer, too.

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