Managing Water: Flooding & Drought

Climate-Conscious Gardening

Join Master Gardeners in the Arlington/Alexandria unit of Virginia Cooperative Extension in a series of monthly articles in 2021 as we explore the topic of climate change and practical actions individuals can take in their home landscapes in response. 

By Elaine Mills, Extension Master Gardener
All photos by Elaine Mills, except as noted

In March, we provided guidance on building healthy soil as a critical strategy for resilience in the face of climate change. This month, we look more closely at the relationship between soil and water and discuss other techniques for managing water in home gardens.

As explained in the webinar “Healthy Soil, Healthy Plants, Healthy Planet,” soil with good permeability reduces the climate risk from both extreme weather events and extended periods of drought. Large and intermediate soil pores allow for good drainage of stormwater, preventing runoff or ponding. Small pores between soil aggregates hold water tightly, making it accessible to plants during dry spells. An important side benefit of proper infiltration is the protection of water quality as sediment, chemicals, and other pollutants aren’t carried into neighboring streams and, ultimately, in our region, the Potomac River and the Chesapeake Bay.

Approaches to Flooding

Holly Shimizu, conservation horticulturist and former director of the US Botanic Garden, advises homeowners to focus on water in their gardens, paying attention to how water flows through the yard during a heavy rain event and where wet spots remain afterwards. Especially on slopes where water is running off, measures such as terracing or bioswales can be incorporated into the landscape to slow the water down.

A Master Gardener uses terracing to prevent erosion in her garden.

Terracing shortens the length of steep slopes into a series of shorter, more level steps, allowing heavy rains to soak into the soil rather than running off and causing erosion. A variety of masonry or natural materials, including fallen tree branches, can be used for building the terraces. Swales, which are broad, shallow, vegetated indentations of about six inches, can function like small stream beds, slowing water, directing it, and allowing it to percolate into the groundwater. These shallow channels that run along the contours of a slope can be a low-cost maintenance option.

Bioswales slow water and allow it to percolate

Rain gardens, also known as bioretention areas, are another attractive landscape element that allows rainwater and snowmelt to be absorbed into the ground. Sited downslope from downspouts, these gardens feature a sand layer and a soil layer filled with plants. Rain gardens are designed to capture and hold runoff on site in a ponding area until the rain accumulation can infiltrate the soil layers below, usually within 24 hours. A rain garden of 192 square feet could hold 200 gallons of rainwater. Refer to a helpful webinar on all aspects of constructing a rain garden under resources below.

A large rain garden with trees, shrubs, grasses, and perennials.

Reducing the number and size of impermeable surfaces, such as driveways, patios, and other paved areas; using permeable materials, such as pervious concrete, swept sand pavers, and uncemented brick; and installing green infrastructure, such as green roofs or walls, can all assist in addressing excess water. Infiltration trenches, dry wells, and French drains can provide solutions to more serious stormwater issues. Locally, Arlington County and the City of Alexandria are working to increase resilience to extreme weather events, encouraging residents to do their part to protect water quality and decrease the impact of flooding. Learn more about their stormwater management programs under resources below.

Permeable drive strips replace impervious paved driveways

Approaches to Drought

At the other end of the climate spectrum, rising temperatures and periods of stagnant weather are contributing to worrisome drought conditions during the growing season. In order to manage use of precious water resources wisely, it can be helpful to know how much water plants need. Most plants generally require 1 inch of rain or supplemental water per week. Street trees can use 15 to 20 gallons per week from May to October for their first three years, and new shrubs may need extra water for the first two years after planting.

To judge when supplemental watering may be required, it can be helpful to install a rain gauge and monitor rainfall. To make watering more effective when necessary, it is important to water infrequently but deeply. Watering in the early morning not only allows plants to take up more water but prevents the growth of fungus on foliage that remains wet on humid summer nights.

To conserve water and reduce evaporation, David J. Ellis, editor of The American Gardener, recommends following the example of western gardeners by using drip irrigation or soaker hoses rather than sprinklers. He also advises the use of mulch to reduce water loss and insulate plant roots from the extremes of heat and cold. Hand watering where appropriate and the use of irrigation bags for new trees are useful techniques for delivering water directly to the plants that need it most.

Installing rain barrels and cisterns can be a great way of collecting rainwater as a reserve for drought conditions. A 1000-square-foot roof will enable the collection of up to 600 gallons from each inch of rainfall. Typical rain barrels hold 55 gallons, and they can be joined together to increase capacity. The capacity of cisterns varies from 300 to over 1,500 gallons. See the various public education presentations listed under resources for more details.

A 300-gallon cistern at the Glencarlyn demonstration garden

It is wise to arrange plants in the garden by their watering needs, locating plants that require more frequent watering, such as food crops, near the house or water source. Xeriscape plantings may be placed further away or in dry areas of the yard. The July column of Climate-Conscious Gardening will address choosing plants appropriate to particular garden conditions. In the meantime, review Best Bets fact sheets under the “Plants” menu tab and plan to attend public education classes on “Native Plants for Wet Conditions” scheduled for April 16 and “Best Bets for Dry Conditions” on May 21.

Xeriscaping at the Bartholdi Garden in Washington, DC

Please return on May 5, 2021 to learn how Rethinking Your Lawn can help conserve water and other resources as well as saving time and expense.


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