Plants for Bioretention: Tips for Design and Installation

By Justin White, K&D Landscaping Inc

Over the last ten years, I have worked on many bioretention projects. Some were LID (low impact development) and served the purpose of improving an existing property with stormwater catchment elements. Other projects were new construction where the bioretention component played a major role in reducing or eliminating stormwater runoff. Below are a few tips I have picked up over the course of my career. 

bioretention 01Juncus plugs planted closely together in a bioretention area instead of gallon sized plants. This reduces the need for additional mulch.Bioretention swales or ponds usually consist of a 24” layer of sand/soil medium. The engineer may also specify a 24” aggregate course to be installed below the bioretention medium to provide added volume during heavy rain events. Another vital component is an overflow drain that will allow excess water to be piped into an additional retention area. Once the engineering portion of the project is complete, then comes the landscaping. The design and installation directly contribute to the effectiveness of the system.

Planting: One of the common mistakes I have seen starts with the wrong plant material being used. There is no one size or plant fits all; within our area we have many microclimates that can dictate which will do well and which will fail. Here are a few things to consider when selecting plant material for bioretention areas.

Soil type: The bio-retention soil is always going to be a sandy loam soil, but what is common with sand is that it increases the PH of the soil. This can be mitigated with proper soil preparation practices, utilizing amendments and fertilizers to balance the soil's PH structure. The challenge here is balancing the engineering of the soil with the plant requirements.

Drought Tolerant: The plants need to have the ability to go without regular water and handle being inundated with water. During the winter, these plants will live in a wet and soggy climate and during the summer, a sandy/desert-type environment.

Plant Hardiness: Most Bioretention areas are in parking lots with heavy pedestrian traffic. The plants are often trampled upon and need to be able to withstand without breaking.

Plant Size: If you can install plug plants and let them mature, they will be more adapted to the site environment as opposed to bringing in large plants that will take longer to acclimate.

bioretention 02Gravel mulch consisting of 3/8” or 1/2” Cal Gold rock with larger rock used to dissipate water energy.Irrigation: Planting is all about timing! By planting in the fall or early winter, you can take advantage of natural rainfall and increase the plant success rate. Unfortunately, many projects end up with plant material installed during the heat of summer where an automatic irrigation system can mean the difference between pass or fail. While we hope to cut the irrigation down to zero after 2-years, it is crucial in getting the plant material established. On projects that have omitted the irrigation system and in its place specified hand-watering, we see that plant material struggles to mature and many of the plants die during the first year. The hand-watering schedule typically ends up using more water than an automatic irrigation system, due to inefficient application. I recommend installing a sub-surface drip irrigation system that will apply water to every plant on the new project site, or installing MP rotator type overhead spray. Back this up with a smart controller that can adjust watering amounts automatically and you will be able to establish plants efficiently.

Mulch: In my experience, the mulch is something that is often overlooked. I feel that the mulch material specified is just as important as the irrigation or plant material. The bioretention area receiving the mulch will fill up with water at some point during winter and if the mulch is buoyant, it will float and drift with the stormwater. Wood mulch tends to be displaced after the first significant rain event, causing damage to the plants or getting carried away into walkways and parking areas. The mulch type that I have seen the best success with is a 3/8” or 1/2” Cal Gold rock; it does not float or move around with normal rainfall and helps retain moisture in the soil.

Maintenance: Bioretention areas are not like all other planting areas on a landscape and deserve a specific maintenance routine. Organics and soil nutrients are often leached out during the winter months, so having a strong spring fertilization program is important. Weeds are also very common in these areas because they stay moist during winter and spring, having an herbicide or organic weed control program will save you labor on the back end. It is important to maintain the integrity of the bioretention areas by repairing erosion and removing sediment build up during the winter months. Not doing so may compromise the effectiveness of the system and result in larger problems. Gophers can easily destroy a bioretention area because of has limited soil compaction, so including pest control in your weekly maintenance routine will ensure that they never get the upper hand..

In the coming years, bioretention areas are only going to become more important as we work to improve our groundwater and eliminate stormwater runoff. The bioretention element of a project is the intersection of Civil Engineering and Landscape Architecture, they can coexist in one space serving a common purpose. By working together as an industry and community to design and install these critical elements to the best of our ability we can make a difference. I look forward continuing work on biotention projects to help improve and restore water resources our community.