Rain gardens and rain barrels have always made sense. But for owners of homes and businesses in Portland, they are going to mean dollars as well.

Details are not yet set, but the preliminary proposal would have homeowners pay from $6 to $24 a month for a rainwater fee, depending on the size of a their rooftops, driveways and other solid surfaces. They can lower those fees by adding rain barrels and rain gardens.

The first question people might have is this: Why is rainwater a problem? It has always rained, and rivers and streams have always taken care of that rain.

The problem comes from the man-made creations that get rained on as opposed to the fields and forests of nature.

When you install rooftops and macadam surfaces for roads, parking lots and driveways, the rain cannot soak into the ground on those areas. That means it has to flow somewhere – whether to a stream or a storm drain. And when it does, it takes with it a lot of things that people leave behind, including spilled oil and gasoline, excess pesticides and fertilizer that have been applied to lawns, and other such unpleasant items as cigarette butts and pet poop. All of that stuff then ends up in the ocean – although for inland communities it might hit a lake first.

Rain gardens and rain barrels make use of that rainwater and filter the pollution before it harms the environment.

Rain barrels are simpler, so I will discuss them first.

Nancy and I have three 55-gallon rain barrels on our property, all purchased from the Portland Water District. They are offered only in the spring and last spring cost $60, but a price has yet to be set for next spring.

I like them because I am cheap, and because of the rain barrels we seldom have to pay for Sebago Lake water to hydrate our plants. We grow a lot of patio plants in pots, and they have to be watered more frequently than plants in the ground. We also water all transplants, whether in the vegetable garden or the flower gardens. When we go a couple of weeks without rain, I will use the rain barrels to water the vegetables.

Over the past summer, we used a sprinkler with Sebago Lake water only once, on the vegetable garden and a newly planted perennial border, when the rain barrels were empty.

The Friends of Casco Bay like rain barrels because the water captured in them would otherwise end up as run-off, because the soil is saturated when it is raining hard. When I use that water later, it will soak into the soil and be used by the plants.

Rain gardens also store water for use later, but skip the barrels.

I attended a program on rain gardens at the University of New Hampshire last month, and James Houle, program manager of the Stormwater Center at UNH, said rain gardens designed to handle runoff from parking lots are carefully engineered water disposal systems.

Rain gardens have to be lower than the parking lot, layers of crushed stone of successively larger sizes are put into the bottom of pit where the garden is being created. Then soil, a mix of sand and clay without compost, is put in for the plants. On top of that they put in rocks of about 6 to 10 inches in the longest direction.

By happenstance, New Hampshire was having major downpours while we were looking at two rain gardens in Durham, and the rain gardens were dumping water through an overflow drain. Houle said the rain garden at UNH we looked at was 5 percent of the size of the parking lot area it drained, and it could easily handle an inch of rain a day.

He said that rain gardens can be designed to take up even less than 5 percent of the space of the area being drained.

When the depression of a rain garden is filled, it will drain in a few hours, Houle said, so the plants used in rain gardens have to handle dry periods as well as periods of heavy rain.

Some of the water in the rain garden is taken up by the plants, some of it will drain into aquifers below the soil, and some of it will evaporate. The plants also will use up some of the pollution that flows into the rain garden.

Houle said it is important to have a grate of some sort to prevent larger items from flowing into the rain gardens, and the rocks on the top of the engineered soil have to be taken out and cleaned every couple of years.

The city of Portland installed a rain garden – with the $65,000 to $75,000 cost paid by Stantec, the engineering firm that designed it – at the northerly end of the Back Cove parking lot. The rain gardens I looked at in New Hampshire had more plants than the one in Portland, but they serve the same purpose.

Putting a rain garden in your home would have to be a lot less complicated. Create a depression in your yard, preferably in a place where water from your driveway or rooftop will flow, and put in plants – rudbeckia (black-eyed Susan) and eupatorium (Joe Pye Weed) were the dominant plants in the New Hampshire gardens, but they might have been the only ones blooming this late in the season. Houle said early rain gardens had some trees at the edge of them, but it turns out that perennials work better.

A good backyard rain garden looks like just any other flower garden, except that it is planted in a dip in the lawn. And more flowers are always a good thing.

Tom Atwell has been writing the Maine Gardener column since 2004. He is a freelance writer gardening in Cape Elizabeth and can be contacted at 767-2297 or at:

tomatwell@me.com