Creating Rain Gardens Design Guidelines
- What is a Rain Garden?
- Planning Your Rain Garden
- Location & Overflow
- Sizing a Rain Garden
- Shaping a Rain Garden
- Plant Selection and Design
- Rain Garden Planting Profile
- Plant List References & Guides
What is a Rain Garden?A Rain Garden is a depressed landscape feature that captures stormwater runoff from roofs, driveways or other hardscape surfaces then slows and cleans stormwater runoff and allows it to sink back into the ground.
Rain gardens can be a low-cost, effective, and beautiful way to reduce the amount of stormwater run-off that may wash pollutants from your property into storm drains and the Monterey Bay National Marine Sanctuary. Rain gardens can be filled with native and drought tolerant/wet tolerant plants that provide seasonal color, texture, and provide pollen and nectar for bees, butterflies, and hummingbirds. Below is a list of benefits rain gardens provide.
Benefits of rain gardens include:
- Adds an aesthetic landscape feature with many planting and design possibilities
- Plants used in rain gardens can promote native habitat and food source for hummingbirds, butterflies, and honey bees
- Depending on soil conditions, helps stormwater fully infiltrate into the ground
- Promotes groundwater replenishment in soil conditions that allow for infiltration
- Reduces the amount of pollutants, such as oil and grease from driveways or pesticides and herbicides from lawns, from entering the Monterey Bay National Marine Sanctuary
- Slows stormwater runoff; decreasing peak storm water flows
- Helps clean the stormwater as it is filtered through the soil
- Mulch and compost amendments help with water retention
Planning Your Rain GardenThis document will review planning considerations to help you determine if a rain garden is the right solution for your property. Cost varies per the size, complexity, and context of the rain garden and whether a landscape contractor is used. For larger rain gardens with poor infiltrative soil conditions, underdrain systems, subsurface drains with base rock and/or an overflow, a drain inlet may be needed. This will often require professional design and contractor fees for installation.
Location & OverflowThe location of your rain garden is important to plan out. Determine the sources of stormwater runoff on your property. Sources can include downspout/roof gutter, rain barrel/cistern overflow, or other hardscape surface. Observe how this water is distributed across your home and yard. Your rain garden will need an overflow, some sort of way for water to safely exit the rain garden, so that rainwater that cannot be held or infiltrated, does not cause property flooding. Please use the checklist below to ensure that you comply with the design guidelines in terms of properly locating a rain garden on your property.
- The required size, shape, and depth of the garden depend on how much water is captured. Ponding of water is normal for a rain garden after a rain event, however water should be drained within 48 hours. Ensure your property has enough space available based upon the sizing guidelines found later in this document.
- Rain gardens should be located away from significant underground utilities. The property owner is responsible for determining the location of above and underground utilities before installation of a RainScapes Low Impact Development Technique (LID Technique) to ensure there will be no conflicts with the project. Contact PG&E “Call before you dig!” 811 hotline to locate underground utilities.
- Rain gardens should be located away from septic systems and wells.
- Rain gardens should be located away from public sidewalks and 5 feet from property lines.
- Rain gardens should be located outside of the drip line of large existing trees. Consult a qualified arborist for assistance.
- Owner is responsible for determining any City setback requirements. Contact the Pacific Grove Community Development Department – 831-648-3183.
- Rain gardens should be located 10’ away from building foundations if there is a basement condition.
- Rain gardens must have an outlet/overflow (some sort of way for water to safely exit the rain garden as a result of a large rain event) set at an elevation to achieve the desired maximum ponding depth.
- Overflow from the rain garden must be directed away from structures, foundations, neighboring properties, and other areas that could cause property damage. There must be a clear flow path for overflow water to exit the site and go to the approved stormwater system.
Rain Garden Profile Graphic:
*Graphic from “Rain Garden Handbook for Western Washington”, June 2013, page 30
Sizing a Rain GardenBelow is a chart that gives simplified rain garden sizing information based on how many downspouts your home has. The size of the rain garden, footprint area in square feet, refers to one rain garden located at one downspout. You may install additional rain gardens at the other downspouts if you choose to, however they must follow the design guidelines in this document . Please note that the sizing options vary in terms of ponding depth and soil amendment depth. You can choose which sizing option suits your location best. See the Rain Garden Profile graphic.
* Don’t include downspouts that direct flow into another downspout. For example, where a second story downspout directs water to a first floor downspout, count only as one downspout.
- This is a simplified table which assumes a 2,000 square foot home footprint with a roof runoff equally divided to each downspout. If the roof area, or area contributing to water runoff, is larger or smaller, you can increase or decrease the rain garden size accordingly.
- For each 6 inches of additional amended soil, you can decrease the rain garden footprint size by 5%.
There is another way you may calculate the size of a rain garden. If you know the square footage of the impervious surface that will be contributing to your rain garden area, you can calculate a more accurate rain garden footprint with the sizing guidelines below. You may calculate your rain garden per downspout (as shown above in the blue chart) or per the square foot of impervious surface, shown below. Either will meet the rain garden sizing requirement. Please note which method you chose in the Rain Garden Rebate Form.
|Rain Garden||% Contributing Impervious Area|
|4” Ponding Depth with 2-3” Soil Amendment||40%|
|6” Ponding Depth with 2-3” Soil Amendment||30%|
|4” Ponding Depth with 12” Soil Amendment||25%|
Example: You would like to have a rain garden off of a specific downspout of your roof. The roof area that feeds rain water into the downspout is 600 square feet.
- Option 1: A rain garden with a 4” Ponding Depth and 2-3” Soil Amendment will need a square foot area of 240 square feet. [Equation: 600 x 0.40 = 240 SF]
- Option 2: A rain garden with a 6” Ponding Depth and 2-3” Soil Amendment will need a square foot area of 180 square feet. [Equation: 600 x 0.30 = 180 SF]
- Option 3: A rain garden with a 4” Ponding Depth with 12” Soil Amendment will need a square foot area of 150 square feet. [Equation: 600 x 0.25 = 150 SF]
Shaping a Rain Garden The shape of the rain garden is often determined by existing features such as the need to avoid structures, trees, and utilities or can simply respond to a desired aesthetic. After you have determined the total number of square feet of your rain garden, it can be shaped to fit available space and site conditions. Remember to provide additional space for the overflow, plantings on the surface grade and plantings on the upland conditions.
You will be required to submit a plan which shows the location and size of your rain garden as well as site photographs before installation:
- Take a couple photos of the desired location of your rain garden on your property.
Print out photos as part of this submittal. Please label the photo print out with your name, contact information, and thea ddress of the rain garden.
- On grid paper, outline the desired shape of your rain garden to scale. See the example graphic.
- On the plan, please label:
- Your name, contact information, and address of the rain garden
- Location of the rain garden downspout inflow and its distance from the structure
- Location of the rain garde overflow and where water will be overflowing to
Rain Garden Plan Example:
The example shows an irregular shape based on the sizing calculation of a 100 square foot rain garden. One grid box equals one square foot. When reshaping the rain garden from its initial rectangular shape, the number of square feet should be about equal. This is just one example as rain gardens can be shaped in many different ways.
Plant Selection and Design Your rain garden can be planted as a beautiful landscape feature. Select plants that are appropriate for your location such as Central Coast California native plants or adaptable wet/drought tolerant ornamental plants. Recommended plant list references are located in this section.
When considering your planting design, understand 1) the surface grade and upland conditions and 2) the bottom and basin ponding area, of the rain garden:
- Plants located at the bottom and basin of the rain garden will receive periodic inundation of rainwater. Plants in this zone (Zone A illustrated in the Rain Garden Profile graphic below) will need to withstand ponding, such as the common rush, seep monkeyflower, certain sedges, Douglas iris, yarrow, and coastal strawberry.
- The surface grade and upland conditions of the rain garden are on a more level grade (Zone B), meaning these plants will not be as wet, therefore more drought tolerant plants are recommended such as monkeyflower, native sages, upland grasses, native buckwheats, California wild rose, and California fuchsia.
Please list plants you intend to use in your rain garden in the Rebate Form.
Rain Garden Planting Profile:
Evaluate your site conditions to determine your plant species composition:
- How much sun and shade will your rain garden receive?
- Is the rain garden exposed to high winds or sheltered?
- Is your home located near the coast, within a Coast Live Oak stand, or upland in the Monterey Pine Forest?
- Research larger plants’ mature size (height & width) to ensure that they will not outgrow the intended space.
Drought tolerant plants (and water tolerant for the ponding area) are highly recommended and a variety of species should be planted. Do not use invasive plants. A list of invasive species can be found on the California Invasive Plant Council website.
Plant List References & Guides:
- Pacific Grove Museum of Natural History, Native Plant Garden Plant List
- Monterey Bay Chapter of the Native Plant Society, “Local Plants”
- Central Coast Low Impact Development Initiative, LID Plant Guidance for Bioretention and Plant List
- California Oak Foundation, Compatible Plants Under & Around Oaks
Irrigation or hand watering will be needed during the first two years of plant establishment. Extra water at the time of planting and for a few months after planting will help the roots adapt from a nursery container condition into the new surrounding soils of your rain garden. Once plants are established in the rain garden, the irrigation and watering can be limited to the dry seasons of the year. Native plants to the Central California Coast, if planted in the correct conditions/location will need minimal watering throughout the year in addition to a reduced need for pest management practices.
Please note that plant nurseries carry different plant species at different times of year. Call ahead to find out plant availability, so you can determine any plant substitutions that may be needed.
After you have properly located, sized, and shaped, your rain garden, as well as determined the overflow path, it is time to excavate the existing soil. The site should have well-drained soil and be relatively flat. Achieve the required size, depth, and slopes for positive drainage. Please note that the recommended ponding depth of the rain garden does not include the addition of soil amendment. See Installation, Soil Amendment section.
What to do with the left over excavated soil?
- You may want to consider using the excavated soil as a berm around your rain garden. Firmly compact the soils as the berm is constructed. Don’t forget to maintain your overflow drainage pattern.
- Relocate extra soil to another place in your property.
Soil amendments help the stormwater infiltrate into the ground and support plant health. After you have excavated your rain garden, add 3-4 inches of compost to the native soil and till it in to a depth of about 6-12 inches or add 12 inches of compost to the native soil and till it to a depth of about 18-24 inches, depending on the rain garden sizing option you chose. Please note that this will raise the depth of your rain garden, so be sure to include extra excavation of the rain garden depending on how much soil amendment you choose to incorporate. See Rain Garden Profile Graphic.
Now is the time to set up your irrigation system if you choose to install one. This could be a new system or a retrofit to an existing system. If you are unsure how to incorporate an irrigation system into a design, consult with an irrigation specialist or licensed landscape contractor.
It is recommended to use drip tube and/or drip emitters or a micro-spray irrigation system. Some drip tube can be buried underground per manufacturer’s recommendations. If using drip emitters, stake firmly in place and situate the emitter at the root ball; not at the plant stem as it will cause rot.
Please be aware that installing irrigation tubing at the bottom/basin of the rain garden may upheave and surface after large
rain events. Inspect your rain garden after major rain events.
After your soil amendment has been prepared and landscape rock installed, you can plant your rain garden per your planting design.
Don’t forget to hand-pack the soil around the plant after placing it in the hole as to remove any air pockets. Remove excess soil from around the stem. Hand water each plant thoroughly after planting.
Mulch is a protective layer of material spread on the top soil. Mulch keeps the soil moist, replenishes organic material in the soil, prevents erosion, and discourages weeds. A 2” layer around planting areas is recommended.
For newly installed rain gardens which are mulched, be aware that if a large rain event occurs, bark may float and clog drains or overflow outlets (depending on the design of your rain garden). After such an event, inspect the rain garden and clear the mulch and reapply around the plants. As the plants mature and grow into each other, little to no mulch will be needed, therefore reducing the problem. A 2” layer of pea gravel can be used as an effective mulch layer where erosion is a concern.
If you don’t have deer fencing erected around your property, you may want to consider a deterrent method to keep deer from eating your newly installed plants. When plants come from a nursery they are lush and extremely appealing to deer, even if the plant is known to be “deer resistant”. If you chose deer resistant plants in your rain garden planting design, as they grow larger and more woody, they will become less tempting to deer. However, if deer are hungry and have a difficult time finding food, especially in times of drought, deer are most likely going to nibble on your landscape plants.
Temporary fencing or environmentally friendly deer repellents are recommended at the time of planting and during times of drought.
- Rain gardens require regular landscape maintenance required to meet the specific needs of each plant species in your rain garden.
- After planting, check garden for weeds and pull immediately. Eventually, as the rain garden plants grow, less weeding will be required. Installing new plants at a high density/closely spaced will cover more bare ground and will provide less room for weeds to grow.
- Avoid using synthetic fertilizer or herbicides in your rain garden, these chemicals are pollutants. For preferred alternatives, see the Monterey Regional Stormwater Management Program and Our Water Our World.
- When rain garden plants have established themselves (approximately 1-2 growing seasons), reduce the amount of irrigation.
- As needed, remove sediments that build up over time in the bottom of the rain garden: remove sediment, rake the soil, and replenish mulch or replace with river rock (1-2 year frequency). The rain garden should maintain the depressed landform.
- After major rain events, check that rain garden has a clear entry point(s) for water to move into the landscape area. o check that rain garden has a clear overflow path and is not draining ontoneighboring property, causing erosion, or other issues. o inspect the irrigation system o remove litter or debris o For newly installed rain gardens which are mulched, be aware that after a large rain event, bark may float and clog drains or outlets. Replace mulch as required. Please note that as plant grow in size, less mulch or no mulch may be needed.
- Water should not remain in rain gardens for more than 48 hours. If this occurs, ensure proper drainage of rain garden or reduce the amount of water ponding by raising the grade of the rain garden.
- Ponding is normal in a rain garden, but in the absence of continuous rain the majority of your rain garden should drain over a 48 hour period after it stops raining.
- It is your responsibility to evaluate and confirm with the City of Pacific Grove that your rain garden system is functioning properly.
- Apply deer deterrent as necessary.