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Rain Garden Building A DIY Workbook Created by: ---PAGE BREAK--- Size and Location The size and location of a rain garden are determined by how much water the garden will collect from impermeable surfaces and how quickly the soil drains. Large areas of impermeable surfaces, clay-based soil, and a slow infiltration rate will all require a large rain garden or multiple gardens. Assess Current Conditions The more impermeable surfaces you have the more water your rain garden will be collecting and the bigger it will need to be. Review your location and note the following: ● What impermeable surfaces do you have and how large are they? ● Which direction do the surfaces drain towards? ● Do you have any areas that are wet regularly or have standing water in the spring? ● Do you have an area for excess water to overflow to? Notes: Pick your location Locate your rain garden in an area where water from impermeable surfaces can naturally run into it and where it will improve the look of your home. Based on the characteristics of your site you might also consider building more than one rain garden, such as one on each side of your house or at two different gutter downspouts. Pick an Area: ● Where water naturally drains to ● To improve the look of your home ● That has an overflow path for excess water Do Not Locate: ● Within 10 feet of a foundation ● On steep slopes or uphill of runoff ● Near septic tanks, drainfields, or utilities ---PAGE BREAK--- Location Features Draw the layout of your site noting where water runs off your roof, gutter downspouts, sidewalks, driveway, and lawn. Also add any areas that need to be avoided such as septic systems or utilities. ---PAGE BREAK--- Know Your Soils Determine Soil Type Water moves easily through sandy soils but clay soils can prevent water from infiltrating back into the ground. If you have mostly clay-based soils you will need a larger rain garden and will likely have standing water in your garden for longer periods. • Collect a handful of soil from about 6 inches below the surface at your chosen rain garden location • Hold the soil in your hand, dampen the soil with water, and squeeze it to make a clump • If the soil crumbles and has a gritty texture it is mostly sand, if it holds together and has a slimy or sticky texture it is mostly clay Notes: ---PAGE BREAK--- Average Rate: Add Rate 1 + Rate 2 + Rate 3 = Rate total divided by 3 = Infiltration Rate The infiltration rate is a measure of how quickly water collected in your rain garden will drain into the soil below. To function properly a rain garden must drain at a minimum rate of 0.1 inch per hour and any rate below 0.25 inches per hour could have standing water periodically. Follow these steps to determine your infiltration rate: • Dig a test hole approximately 1 foot deep and 1 foot wide • Place a measuring stick or ruler in the hole • Fill the hole with at least 6 inches of water and record how long it takes to drain • Calculate the infiltration rate by dividing the total inches by the total hours • For example: 6 inches of water that drains in 12 hours has a rate of .5 inches per hour or (6/12=0.5) • Repeat this process three times then average the results by adding and dividing by 3 DIY Infiltration Rate Calculation: Test 1: inches hours Rate 1 = (in/hours) Test 2: inches hours Rate 2 = (in/ hours) Test 3: inches hours Rate 3 = (in/hours) ---PAGE BREAK--- Pick your depth A depth of 6-12 inches is recommended for rain gardens but you can consider your visual preferences too. Make sure to plan for an overflow location for excess water during heavy rains or snowmelt and keep in mind that a shallow rain garden is more likely to regularly overflow water and a deep rain garden is more likely to have standing water for longer. Pick your shape The shape of your garden will be determined by the features of your site and your personal preferences. Bean-shaped or teardrop-shaped gardens have a more natural look than a perfectly circular shape. Longer and skinnier shapes can be used on slope contours to limit the amount of excavation needed. A few shape examples are below. ---PAGE BREAK--- Plant Selection The plants in a rain garden must be selected for their water preferences. In the center of the garden (Zone A) choose species that are adapted for wet areas and can tolerate occasional standing water. Along the sloped banks of the garden (Zone B) choose species that like moisture but can also tolerate some dry conditions. On the outside rim of the rain garden (Zone C) select plants that prefer dry conditions. Below are a few recommendations. For a complete, comprehensive list, check out the Rain Garden Plant List. Zone A Skullcap Twinberry Honeysuckle Thick-headed Sedge Zone B Red Columbine Wild Chives Red-osier Dogwood Zone C Showy Aster Kinnikinnick Paper Birch ---PAGE BREAK--- Final Design Considerations Below is an example cross-section view of a functional rain garden. The area that collects stormwater runoff from impermeable surfaces is on the left and the overflow area for excess water is on the right. The center of the rain garden is lined with rocks which can help with infiltration and the plants are located in each zone according to their preference for water. If you make sure to account for each aspect when planning your rain garden, it will function well - reducing pollution in our local waterways, conserving water, and recharging ground water supplies! Notes: ---PAGE BREAK--- Rain Garden Design Draw the shape and layout you’d like for your rain garden keeping in mind your location and soil characteristics. Mark the location of the water runoff into the garden and overflow out of the garden. Define zones A, B, and C and indicate what species you’d like to plant in what locations. A collaborative project created by the Flathead Conservation District and the City of Kalispell