Summer 2023

Climate Change: Improve your garden’s ability to handle too much rain!

by Robin Mitchell

This past winter, California had record precipitation in the form of both rain (in the San Francisco Bay Area, as well as other places) and snow in the mountains. And the previous years we experienced drought.

Many homeowners experienced flooding and erosion in their gardens, which were not designed for such large amounts of rain. Is it possible to make your garden more resilient when the weather delivers that much water?

There are a few ways to improve your garden’s ability to handle too much rain, including

• Sculpt the garden into berms and swales
• Build a dry creek bed
• Build a rain garden

All of these methods are designed to slow the water down as it travels across your landscape, so that it will have more opportunity to sink into the soil rather than running off the property, which can lead to soil erosion and flooding.

The first thing to do when deciding to include any of these methods in your landscaping is to observe where the water from rainfall naturally flows, and where it collects. This will help determine where these water catching elements can be placed to be most effective.

Berms and Swales
Berms and swales are “earthworks” that shape the soil to slow the water running across the landscape so that it can infiltrate into the soil rather than running off. A swale is a shallow channel that is designed to slow water down and a berm is a built up area downslope from the swale that keeps the water in the swale from continuing downhill. Berms and swales follow the contour of the land and are perpendicular to the slope, so that they catch rainwater as it travels down the slope. In our area, “vegetated” swales are ideally planted with California native plants that can tolerate periods of wet and dry soil.

Dry Creek Beds
Dry creek beds are similar to swales except that they are lined with rocks rather than with soil and plants. They can be very aesthetic design elements in a landscape in both dry and wet conditions, although they do require maintenance to keep the rocks from being covered in leaves and soil, or invaded with plants.

Rain Gardens
Swales and dry creek beds can direct the water to a “rain garden”, which can be an interesting garden element even in dry years. Rain gardens are depressions in the landscape that can receive and hold stormwater runoff, and allow that runoff to slowly infiltrate into the soil, keeping the water on site. A rain garden is filled with plants that can flourish in both moist and dry conditions, similar to a vegetated swale, as they will be filled with water during the rainy season but will dry out during the summer. This allows the rainwater to be stored in your soil, where your landscaping plants can use it over time, rather than having that water flood other parts of your garden (or house) and flow into the street. Rain gardens also create important habitat for bees, butterflies and birds1.

The publication Coastal California Rain Gardens contains detailed information about the benefits and construction of rain gardens, including information about rain gardens in clay soil, which is a predominant soil type in many Bay Area gardens. It also has a list of plants that can tolerate the wet and dry conditions of a rain garden.

Homeowners can construct simple swales, berms and rain gardens, but if your landscape has significant slope to it, or needs larger rainwater control infrastructure, you should talk to a professional who can advise you on the best approach for your grounds.

Design considerations

The UCANR publication Coastal California Rain Gardens has good information about installation and maintenance of rain gardens, particularly pertaining to site selection, proximity to structures, and avoiding breeding mosquitoes.

References

UCANR Rain Gardens website
Contains lists of plants for rain gardens in different areas of the state

Coastal California Rain Gardens
UCANR Publication 8531, October 2015.

Rain Gardens – A New Approach
by Alice Cantelow, UCCE Master Gardener, El Dorado County, 2017.

Other websites that may be of interest

Rain Gardens
CalRecycle, State of California

Rain Gardens and Swales
Napa County Resource Conservation District