Rain Gardens vs. Swales

I am researching rain gardens for a new garden design.  The clients live in Richmond, and have a high water table--inches below the surface of their lawn.  They have asked me to design a garden with a rain garden feature, that would tie into a drain.

I have been pondering how to design a rain garden when water frequently sits close to the surface.  If there is no drain, the rain garden will be a pond for most of the winter months.  If there is a drain, where should it be located to be effective?

Rain Garden

Part of my research has been to define the term rain garden.  In fact, a rain garden is a planted garden that holds water for, ideally, no more than 48 hours.  A rain garden may look no different than the rest of the garden, but the soil could be boggy at rainy times of the year.

Dry River Bed

Construction of bioswales on
Blenheim Street, Vancouver

The photographs my clients sent me of their favourite gardens are in fact dry river beds. Dry river beds are not planted, and they may or may not have a lining.  If a dry river bed has a lining, it is designed to move water.  If a dry river bed does not have a lining, it is designed to absorb water, and may have a French drain beneath, which moves water.  A dry river bed should be located at a lower grade than the rest of the garden.






A swale slows the pace of water down a slope. In Vancouver, there are a series of swales on Blenheim street.  Before these were installed, rainfall was channeled down the hill and into drains strategically placed by the road side. Now, the water is channeled into curb breaks, and collects in the swale with much of the water absorbing into the ground.  If too much water is channeled through the curb break at one time, a catch basin at the end of the swale stops the water from flowing over the sidewalk and back onto the street.  The catch basin is higher than the bottom of the swale.  If the catch basin was placed at the bottom of the swale, water would simply collect in that and be swept into the ocean.  The bottom of the swale should be designed with pervious soils to allow water to be absorbed into the ground.

What's blooming now in Vancouver - April 25 pt 2

Polygonatum

Here is one of my favourite shade plants:  Solomon's Seal or Polygonatum.  I love the delicate weep of the single stalk, covered with tiny scented white bells hanging like earrings from the stalk.  

We had a site with hundreds of Polygonatum lining the top of a steep slope.  Imagine our delight when one morning we spied a hummingbird flitting from one tantalizing white flower to the next!

Polygonatum grow in deep shade, but I like to provide staking so the single stalks do not fall over.  This photo shows my staking device forming a fence around the Solomon's Seal in the direction I know the plants will fall without staking.  If the stakes are placed early enough, the stalks of Solomon's Seal will grow over and hide the stakes.

Solomon's Seal will increase in number each year, which is terrific because it looks best in a large group.  Just make sure you have allowed enough room for a great showing.  Solomon's Seal is also easy to divide and transplant.

Plant Solomon's Seal with Hostas and Ferns, and other woodland plants.  There is also a variegated leaf variety of Solomon's Seal.

What looks good now in Vancouver? pt 2

As I mentioned before, there are not many flowers blooming now in Vancouver, so we have to think of other ways to make the garden interesting.  Here are two trees, suitable for a residential garden, that have interesting bark.  

This first photograph shows an Acer griseum, or Paper Bark Maple.  A. griseum has a beautiful, rich brown, peeling bark.  The warmth of the bark colour contrasts well with our cold grey days.  The way the bark peels gives an extra interesting texture to the garden.

The leaves of the A. griseum are atypical for a maple tree, and they produce a wonderful autumn colour, before dropping to unveil the bark for the winter.

The Stewartia pseudocamellia, or Japanese Stewartia, also has a colourful bark for winter interest.  It, too, is suitable for a residential garden.  The S. pseudocamellia bark peels in patches, resembling camouflage like colouring.

In summer, white flowers appear that resemble the flowers of a Camellia, hence the species name, "pseudocamellia."  

What looks good now in Vancouver? pt 1

Gardens are not only for flowers, but we need to think of complementary shades of green, contrasting textures of foliage, and different shapes of leaves.  Not many plants flower in winter, but many plants are evergreen or have some other interesting quality to feature in a winter garden.

Arum italicum makes a lovely ground cover for winter.  The shiny, elongated heart shaped leaves are highlighted with white veining.  Other Arum species do not have the decorative leaves.  

Come summer, the leaves die down and give way to long stalks holding spires of orange berries.

Be careful when using horse manure in the garden

Recently I attended an informative day of talks, hosted by Bartlett Trees.  One of the speakers discussed the issue of using horse manure in gardens.  He had noticed a lot of plant death and decay at one of his sites, and discovered the gardeners had been composting horse manure from the barns, and adding this compost to the gardens.  The horses being raised were very rare breeds, and injected with a lot of antibiotics.  Antibiotics kill off bacteria, an essential component in healthy gardens.

Because horse manure seems a natural product, many gardeners assume it is organic.  What they do not realize is how many chemicals are injected into the average horse.    

In an organic food course, the instructor recommended going to horse stable neighbourhoods to scoop horse manure off the street for food beds.  When I questioned the chemical possibilities in horse manure, another student googled my query and announced his findings:  horses are only injected with antibiotics once a year, in the spring.  The conclusion was, as long as we avoided getting horse manure within a few weeks of this injection period, the horse manure was organic.

At the time I took this course, I also worked on a horse farm.  I noticed that all of the horse manure was loaded into a truck and driven off the property.  I discussed horse manure chemical components with the owners of the horse farm.  They did not want the horse manure in their gardens because their thoroughbred horses could be injected with antibiotics as often as every six weeks.

These are two examples relating specifically to thoroughbred horses.  I do not know if regular horses receive the same kind of chemical care.  My point is, do not assume horse manure is organic.  Get horse manure from a reliable source.  Ask the horses' owners, if you can, what goes into the horses, because that is what will come out of the horses.

Vancouver rain and soil pH

Vancouver's soil is naturally acidic because of the amount of rain that falls.  

Rain is made of water, which is made of two hydrogen atoms and one oxygen atom (H2O).  When the rain falls to earth, it breaks apart into H+ and HO-.  Now the soil has more free H atoms which makes it more acidic.  

The "H" is pH is for hydrogen.  The "p" has a variety of possible explanations, including power or potential.  

The more free H+ atoms in the soil, the lower the number due to a negative logarithmic equation.  

This part can be confusing, but remember rain = more H+ in soil = high acidity = low pH.

Vancouver's soils always have enough phosphorous

We have brought many soil samples into the lab from our Vancouver area properties.  We have never received a recommendation to add phosphorous to the soil, yet phosphorous is one of the big three in the NPK fertilizer formula:  nitrogen, phosphorous, and potassium.  Why do we have ample phosphorous in Vancouver?

Phosphorous breaks into water insoluble compounds in the soil.  Typical chemical fertilizers require water to break down the fertilizer for the plant to feed from the nutrients.  If the phosphorous is water insoluble, then the plants cannot feed off the chemical phosphorous, designed to be used with water.  

This does not mean there is no phosphorous in the soil.  This means the plants cannot take it up, and then show signs of phosphorous deficiency.  The usual response?  Add more phosphorous.  Add to this Vancouver's excessive rainfall, and the result is soil high in phosphorous.

Organic forms of phosphorous are not bound in water-insoluble compounds, and plants can feed from them.