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Rain Garden Design

Rain Gardens: Site Prep

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This is the first article in a series that will detail the importance of rain gardens, as well as how to maximize rain garden performance on your site including site preparation, installation, plant selection, signs of a healthy rain garden, and maintenance.

Rain gardens have become a hot topic in landscaping recently. They are known for their beauty meets function approach to landscape design. Generally filled with a rich selection of plants, these gardens are not only aesthetically pleasing, but also serve a multitude of purposes. A properly planned, installed, and maintained rain garden will be a planted depression that allows rainwater runoff from nearby impervious areas the opportunity to be absorbed. These gardens provide benefits such as:

  • Filtration of runoff pollution
  • Reduction of flooding and water damage
  • Improvement of water quality
  • Recharging of local groundwater
  • Increase in curb appeal

Undoubtedly rain gardens show a great deal of promise as a green solution to common stormwater management and development problems. However, oftentimes rain gardens do not live up to expectations because of improper preparation and maintenance. In order to maximize performance of your rain garden, proper site preparation is key.

It is estimated that rain gardens can cut down pollution by up to 30%, while also providing a natural solution to stormwater management issues.

When preparing your site, first and foremost it is important to consider environmental factors such as the slope of your site. It is recommend that the slope be less than 15% for residential sites and less than 5% for bioretention sites along curbing. It is also important to reflect on sun and shade locations as this will help determine which types of plants will thrive in your rain garden. Please try to keep your rain garden out of close proximity of trees though in order to avoid cutting roots. Soil type is another factor that will determine the performance of your rain garden. Be sure you do not construct on soils with a high water table. Keep bedrock depth a minimum four feet for bioretention. Also, it is important to note that your bioretention basin will likely require infiltration testing every 2-5 years. If it has required engineered soils, these will need to be periodically replaced. Keep these considerations in mind as you evaluate upfront and long term costs. Hardiness zone and water distribution on the property are also important considerations.

Infrastructure can also help or hurt your rain garden’s performance. Locating utilities prior to construction is obviously extremely important. However, it is also important to consider other construction issues such as long term costs to the municipality and existing plants inhabiting the site. As with environmental factors, it is helpful to try to work with the already built environment, rather than against it when possible.

Lastly, one should consider social factors and implications when preparing a site for rain garden installation. Social factors will include people, views, location and proximity to other things. For example, if not properly planned, your rain garden may be capturing runoff from other, undesired sites making it harder for your rain garden to operate effectively. Consider the type of space the rain garden will inhabit. Is it on a residential site? Commercial? Municipal? This may impact whether a true rain garden or a bioretention basin will be best for your site.

There are many factors to be considered when preparing your rain garden site in order to achieve maximum performance. Be sure to work with the existing natural, built, and social environments surrounding your proposed site as much as possible to make site preparation and installation of your rain garden as easy as possible.

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