Rain gardens
What is a Rain Garden?
A rain garden is a landscaped depression in the land, with soil designed to help rainwater runoff from a roof, driveway or other impervious surfaces to soak into the ground and filtered.
Bioretention Cells, Bioswales, and some Hedgerows are sometimes referred to as "Rain Gardens", because they are basically doing the same thing in different situations. For example, a "Bioswale" is basically a kind of rain garden where water is slowed and filtered, but much of the water is directed to another location.
Bioretention is a more complex rain garden with drainage systems and amended soils.
Hedgerows are not technically rain gardens but can be used to slow surface water as it heads down slope, as an edge to a Bioswale, or as check dam
Curb Cut Extension
Curb Cut Raingarden
Curb Cut Inlet
A Bioretention cell (strip or trench) is more complex rain garden with engineered drainage systems in a slightly recessed landscaped area constructed with a specialized soil mixture, an aggregate base, an underdrain, and site-appropriate plants.
Bioretention
A bioswale is a slightly recessed landscaped area constructed downstream of a runoff source. At the beginning of a rain event, a bioswale absorbs and filters water runoff. Once the soil-plant mixture below the channel becomes saturated, the swale acts as a conveyance structure to a bioretention cell, wetland, or infiltration area.
There is a range of designs for these systems. Some swales are designed to filter pollutants and promote infiltration and others are designed with a geo-textile layer that stores the runoff for slow release into depressed open areas or an infiltration zone.
Bioswale
Rain gardens are designed to be self-sufficient. Some weeding and watering will be needed in the first two years, and perhaps some thinning in later years as the plants mature, but a well-planned raingarden can be maintained with little effort after the plants are established.
Include native plants!
Native plants eliminate the need for fertilizers and pesticides, and require little or no supplemental water. These plants evolved and adapted to the local climate and growing conditions.
Native plants are important to wildlife including bees, butterflies, and birds adapted to using native plants as a source of food and shelter. Adding even just a few native plants to your landscape can go a long way towards supporting wildlife.
Rain gardens function only as well as they are designed. Planning is needed as well as knowledge of soil and local weather conditions. Take the time to plan effectively.
​
Note: Don’t locate a rain garden within 10 feet of a building foundation, near the edge of a steep slope or bluff, in low spots that do not drain well, where groundwater is within one foot of the bottom of the finished rain garden, over a septic drain field or tank, over shallow utilities (call before you dig), or in areas that would require disturbing healthy native soils and vegetation. For more in depth information, see the online books below.
There are many ways plants can be effectively used in the landscape, but plant selection and proper planting and care are vital.
If planting near the road, consider sight lines and setbacks
Anatomy of a Rain garden
RAIN GARDEN DESIGN TIPS
Make it part of the landscape. It should work together with and be visually and functionally integrated into the rest of the landscape.
Choose a shape. Consider all the rules of composition, screening, and circulation—not just the rule that says to put a rain garden in a low spot 10 feet from the house.
Consider style. A rain garden can be as formal or as wild as you like, but pay attention to how it looks with your home’s facade.
Integrate with other gardens. Consider making a depression within a perennial bed or shrub border (especially if space is tight and you don’t have room for a larger rain garden that stands alone).
Create repetition. Put in more than one rain garden for repetition and continuity. If it works with your overall design, create a little rain garden for each downspout. Or add other water features around your yard such as a fountain, birdbath, or waterfall to repeat the water theme, which is another way to lend cohesion to your landscape.
Consider aesthetics and function. Make your rain garden more attractive and user-friendly. Use decorative stones as edging, create an adjacent seating area, build an attractive pathway, or add other hardscape and accessories to make your rain garden more visually appealing.
Select plants. Include native plant species that provide food and habitat for wildlife and insect pollinators. Rain garden plants should be able to tolerate moisture as well as intermittent dry spells. Include sedges, grasses, and rushes with deep root systems that will help water seep into the soil. Select a mix of plants with different foliage, texture, and flowers that bloom at different times for season-long interest. Add marginal plants that are more drought-tolerant around the perimeter. Plant shrubs in groups of 3 to 5 plants and small plants in "drifts" for greater visual impact.
Plants for shady zone 1
Slough sedge (Carex obnupta)
Small-fruited bulrush (Scirpus microcarpus)
May Lily (Maianthemum dilatatum)
Pacific waterleaf (Hydrophyllum tenuipes)
Ferns Lady fern (Athyrium filix-femina)
Deer fern (Blechnum spicant)
Goat's beard (Aruncus dioicus)
Red-twig dogwood (Cornus sericea)
Black twinberry (Lonicera involucrata)​
Plants for sunny zone 1
Dagger-leaf rush (Juncus ensifolius), Taper-Tipped rush (Juncus acuminatus)
Cascade penstemon (Penstemon serrulatus)
Henderson’s checker-mallow (Sidalcea hendersonii)
Rocky Mountain Iris (Iris missouriensis)
Red-twig dogwood (Cornus sericea)
Pacific ninebark (Physocarpus capitatus)
Black twinberry (Lonicera involucrata)
Pacific crabapple (Malus fusca)
Plants in zone 1 need to be able to tolerate wet conditions and seasonal flooding.
Plants for shady zone 2
May Lily (Maianthemum dilatatum)
Oregon wood sorrel (Oxalis oregana)
Sword fern (Polystichum munitum)
Lady fern (Athyrium filix-femina)
Low Oregon Grape (Mahonia repens)
Snowberry (Symphoricarpos albus)
Salal (Gaultheria shallon)
Western Pussy Willow (Salix scouleriana)
Cascara (Frangula purshiana)
Plants for sunny zone 2
Daylily (Hemerocallis spp.)
Giant camas (Camassia leichtlinii)
Henderson's Checker Mallow (Sidalcea hendersonii)
Douglas Iris(Iris douglasiana)
Red-twig dogwood (Cornus sericea)
Snowberry (Symphoricarpos albus)
Western Pussy Willow (Salix scouleriana)
Tall Oregon grape (Mahonia aquifolium)
Pacific crabapple (Malus fusca)
Plants in zone 2 need to be able to tolerate moist to occasionally flooding conditions.
Plants for shady zone 3
Inside-out flower (Vancouveria hexandra)
Western bleeding heart (Dicentra formosa)
Sword fern (Polystichum munitum)
Evergreen huckleberry (Vaccinium ovatum)
Low Oregon grape (Mahonia nervosa)
Rhododendron macrophyllum
Vine maple (Acer circinatum),
Cascara (Frangula purshiana)
Indian plum (Oemleria cerasiformis)
Plants for sunny zone 3
Black-eyed Susan (Rudbeckia hirta)
Cooley's Hedge-nettle (Stachys cooleyae)
Oceanspray (Holodiscus discolor),
Red-flowering currant (Ribes sanguineum)
Snowberry (Symphoricarpos albus)
Oregon Boxleaf (Paxistima myrsinites)
Tall Oregon grape (Mahonia aquifolium)
Serviceberry (Amelanchier alnifolia), Oceanspray (Holodiscus discolor)
Mock Orange, Philadelphus lewisii
Plants in zone 3 need to be able to tolerate moist to dry conditions.
New plantings require extra care during the first 1-3 years.
Make sure new plants receive a deep watering 1 or 2 times per week for the first several months and then at least once per week for the first year.
Good maintenance while the garden is becoming established is important.
Apply a good layer of mulch to maintain soil moisture and reduce weeds.
Once Rain gardens are established, they need little maintenance to perform well and look good.
Rain Garden Maintenance
-
After heavy storms, check the inflow and overflow areas to make sure they are still intact and can continue to carry water into and out of the rain garden.
​
-
At least twice a year, check around the inlet and overflow areas for debris build-up such as leaves, sticks, and other items.
Keep the Flow
Water flowing into the rain garden can carry with it various types of debris that can clog the soil mix and slow drainage.
Fast flowing water can also slowly eat away at your soil layer, washing it away and damaging your garden.
-
Maintain a cover of decorative rock around the inlet and overflow area to protect the soil.
​
-
Look for areas where water may not be soaking into the ground. This may be due to fine sediment or compaction of the soil.
​
-
Remove sediment that may be building up and rake the soil surface. If you suspect compaction, break up and loosen the soil when it is not saturated.
Maintain soil coverage​
-
Mulch provides multiple benefits for rain gardens by helping to:
Keep the soil moist.
Replenish organic material in the soil.
Prevent erosion.
Discourage weeds.
​
-
Every year check the mulch layer and, if needed, apply enough to maintain a layer of shredded or chipped wood mulch that is about 3 inches deep all throughout your rain garden—on the bottom, the sides, and around the perimeter.
Maintain a healthy cover of plants
-
Replace any dead plants to fill in holes.
​
Like mulch, full coverage of plants provides multiple benefits for rain gardens by helping to:
-
Keep the soil moist.
-
Replenish organic material in the soil.
-
Prevent erosion.
-
Discourage weeds
Edging (such as pavers, stones, etc.) can facilitate access for maintenance and provide separation from lawn and other landscaped areas.
To maintain access to the middle of the garden for weeding and other tasks. A few strategically placed flat rocks or pavers can allow access without compacting the soil or leaving room for weeds.
Weeding
Rain gardens will still soak up and filter rain water even if they are full of weeds. However, the plants in the rain garden may not grow as well with all the competition. Soils in rain gardens have good structure, so weeds should be easy to pull by hand, especially in the spring when the soil is moist and the weeds are small.
Watering
Plants will need to be watered every few days until established (about 4 weeks). For the first year, most plants need deep watering during the dry summer season to establish healthy root systems. After two or three years the native plants in your rain garden will need little or no watering, except for during times of drought.
Fertilizing
Do not apply fertilizers
to your rain garden. The rain garden soil mix provides plenty of nutrients and the native plants in your garden are well suited to local growing conditions, so extra fertilizing is not needed.
​
​
​
​
​
​
​
