The City of Madison Engineering Division has been working on multiple ways to incorporate green infrastructure into street and utility projects to improve water quality.

Before development of the Madison area, rain water would fall onto the ground, be slowed down and filtered by plants, and soak into the ground. Fast forward to today, and we find that impervious surfaces such as roofs, roads, and parking lots change the course of this water and add sediment and pollutants; this is called stormwater runoff. This may include fertilizer, pesticides, sediment, motor oil, garbage, and yard waste. Load too many of these pollutants in the lakes and the results are algae blooms, health risks, and habitat degradation.

The aim of this program is to redirect stormwater runoff into rain gardens and filter it into the ground. Streets are like the main arteries that bring stormwater runoff to the lakes. This makes terraces, the strip of land between the street and the sidewalk, an ideal location for redirecting this water and increasing infiltration.To do this, terraces are being redesigned to act like rain gardens. This is a simple process that has already been performed all over Madison. In suitable areas, shallow depressions are dug in the terrace and a cut is created in the curb to allow flow from the street. Terrace rain gardens will be planted with native perennials. These are deep-rooted, pollinator-friendly wildflowers and grasses that are excellent for stormwater infiltration. The cost for homeowners is $100 to cover the cost of the plants as well as initial maintenance to plant and mulch the terraces. Homeowner’s will be able to select from a variety of planting plans.

Frequently Asked Questions

Will my terrace be eligible for a rain garden?

There are a number of factors that affect whether a rain garden can be constructed on your terrace.

The first is that the width of your terrace must be at least 10 feet to function properly and follow code. The maximum slope for a functional rain garden is 3:1. Also, rain gardens need space to have a flat bottom to properly distribute and filter the water. In addition to this, the Americans with Disabilities compliance code requires a one-foot buffer on the terrace from both the street and the sidewalk, which adds another two feet.

Terrace rain gardens also have to been a certain distance away from existing features and boundaries. There needs to be at least 15 feet of clearance from the base of trees. This ensures that tree roots will not be harmed in construction and that the roots will not be exposed. The presence of a gas or water main below your terrace means that construction of a terrace rain garden is not possible. This poses risks during the construction phase and also compromises future accessibility needs. Terrace rain garden boundaries are also required to be 6 feet away from driveways, 3 feet away from property boundaries, and an ample distance away from utility poles and fire hydrants.

How does the curb cut work and what does it look like?

The curb cut is simply a small, two-foot length where the curb lowers to the same level as the street allowing water to flow into your terrace rain garden. Originally, storm water would flow down the street surface, into storm inlets, and through pipes directly into our lakes. With the curb cut, some of that water is taken diverted from the street surface, into your terrace rain garden through the curb cut, and infiltrated down into the ground.

Will the native plants be able to filter whatever runoff comes from the street?

Your terrace will be exposed to runoff in various forms. The goal of the stormwater terrace program is to reduce the amount and intensity of runoff that ends up in our lakes.

Native plants will be able to take up and store nutrient runoff that flows into your terrace rain garden. Reducing nutrient runoff in our lakes is essential in preventing algae blooms, which is caused by an excess of the nutrients phosphorus and nitrogen. Terrace rain gardens will intercept and filter this nutrient runoff.

Terrace rain gardens will also collect sediment runoff such as sand, leaf litter, or any particulate matter that can be carried by water. After a couple of years, you may notice your rain garden becoming shallower. If your rain garden becomes completely full of sediment, it will not be able to take any stormwater from the street. Not only that, extreme sediment load inhibits native plant growth and germination. At this point, maintenance will need to be done to remove the sediment in your rain garden so that it can function properly again. We have instructions on the different ways to do this in our Homeowner’s Guide that we will be providing.

Will my terrace rain garden attract or provide breeding ground for mosquitoes?

No, rain gardens do not increase the number of mosquitoes around your property. Rain gardens and basins are designed to completely infiltrate water collected from storm events within at least 48 hours. Mosquitoes require at least 10-14 days of standing water in order to complete their reproduction cycle. If you notice persistent standing water in your new rain garden for longer than two days, we will be happy to troubleshoot and help solve the issue.

Will there be standing water in my terrace rain garden?

These stormwater terraces are designed to infiltrate water from major storm events within 48 hours.  Mosquito larvae need on average 10-14 days in standing water to develop. Standing water remaining longer than this period of time may suggest the need for maintenance. If you experience persistent standing water for longer than two days, we will be happy to troubleshoot and help solve the issue.

Will adding a curb cut increase the chances of my house or yard flooding?

No, curb cuts will not cause flooding in your front yard or house. Given an extreme rain event, water will fill up your terrace rain garden until it is unable to handle any more in which case it will flow back out onto the street.

If there are no terrace rain gardens up the street from mine, will it cause excess or standing water in mine?

No matter the circumstances of your uphill neighbor’s terrace, your terrace rain garden will not experience prolonged standing water or flooding. If your terrace rain garden becomes full, the excess water will flow to the next easiest path which is to the street. Rain gardens are designed to drain within 48 hours, so prolonged standing water will not be an issue.

How do you control the amount of water flowing into the terrace rain garden?

Water flows into your terrace rain garden from the sidewalk, but the majority of flow will be from the street. Since street stormwater water enters through the curb cut, you can chose to temporarily block this access point with sandbags. This is especially useful when your terrace rain garden is first planted and the native plugs need to be protected from large inundations of water.

Can I remove a tree or prevent a tree from being planted in my terrace?

Trees on the terrace are managed and cared for by the City of Madison Forestry Division. Homeowners are not permitted to remove trees from the terrace or request that Forestry either remove or add trees to the terrace. If you have questions, the Forestry Division can be reached at (608) 266-4816.

What does the $100 cost of the terrace rain garden cover?

The $100 covers over a hundred native plant plugs, the labor of planting the plugs into the terrace, as well as the mulch needed to protect the young plants. Additionally, Engineering staff will be available to respond to any questions or concerns after the rain garden is installed.

Can I plant my own plants in my terrace rain garden?

Homeowners are allowed to plant their own plants in the terrace rain gardens. We would strongly recommend native species as their deep-rootedness is such an asset to infiltration. Not only that, native species provide prime habitat and food for native birds, insects, and other pollinators as they evolved to utilize these species. We also ask that each homeowner consider the height of the vegetation they choose to plant in regards to vision hazards, especially if their rain garden is located on a corner lot.

How will the installment of a terrace rain garden impact my driveway slope and elevation?

If your stormwater terrace is not being implemented at the same time as a street reconstruction project, your driveway will not be impacted. If your stormwater terrace is being constructed at the same time as a street reconstruction project, you can access information about the design from the Project Engineer at

Is it possible to have two terrace rain gardens?

Yes, as long as your terrace is eligible, you can have two rain gardens. Corner lots are often able to accommodate multiple rain gardens.

I already have a planting in my terrace, will the construction disturb it?

Yes, the process of construction will tear up any planting that already exists in the terrace. If you wish to save these plants, you will need to dig them up and either donate them to a friend, or keep them somewhere safe until construction is completed.

How does the payment for my terrace rain garden work?

Terrace rain gardens are paid for as a special assessment line item on your property tax bill.