Redesigning the Garden: Drainage
As fall progresses, I’ve thought about how the garden could be different.
Our neighbours have almost finished waterproofing their home, which is semi-attached to our home. They saw this renovation opportunity, when their neighbour, who rents out his house two houses away, had basement flooding, and he decided to waterproof his home, digging up the common pathway.
Since all this excavation has taken place, and the deafening sound of the construction has subsided, I’ve been reflecting on how their plans will affect us, since drainage was an issue, we were told, when I politely shared our plans, which feel through for a smaller project.
How should the soil around a home, particularly the garden, even in our our small west end Toronto lots of 500 square feet(excluding the garage) be designed for optimal drainage?
I was surprised to hear that their soil was washed out during the large rainstorms, but our garden just absorbed the water, within a day.
I suppose the soil should drain away from the house foundations, but how do I ensure the water isn’t draining into our garden?
I ask this since I’ve noticed that everyone “amends” their soil annually, without realizing that amending means improving and replacing, not just piling up the soil until the soil on each of our adjacent neighbours’ lots have become higher than ours, showing they haven’t grasped the difference. It’s taken a lifetime to realize that if we all kept piling up dirt, the gardens would be higher than the house and garden foundation(our garage is at the rear of the lot, and the garden is between the house and garage) and the water would pressure the building foundations.
Let’s see if we can untangle some of the questions in your query.
First, about drainage and topography. Yes, ideally the ground should slant away from the house foundations, although this may be impossible in some sites (on a hill, in a ravine, &c.). The standard for hardscaping (such as stone patios or walks) is that the pitch of slope should drop about 1/4″ for every running foot, perpendicular to the wall.
That is because hardscaping is usually impermeable – a hard surface, with no way for water to penetrate. Permeable paving and permeable paving methods, however, are increasingly available, changing this to some degree.
With soil, as you’ve seen in your own garden, water can penetrate fairly quickly. The soil structure can determine how quickly this happens. Sandy soil, with large mineral particles, has wide spaces between particles that allow water to percolate through quite rapidly. Clay soil, with tiny mineral particles and tiny air spaces, absorbs water more slowly. Picture a jar of marbles (sand) vs a jar of flour (clay) – water runs through the first instantly, while it might simply sit on top of the latter.
In addition, the texture of the soil surface can have an effect on water penetration. A rough, fluffy surface slows down water run-off, giving it time to sink in. That’s one of the many reasons that mulch is a beneficial amendment. On the other hand, a smooth, compacted surface – especially one that’s very dry – almost seems to repel water. Again, the sand or especially the clay content of the soil can contribute to rainwater management in this way.
But soil is not composed only of minerals. A large proportion will be organic matter, including compost, manure and peat as well as dead plant roots and soil-dwelling creatures. When first added, the particles of organic matter will be large and loose, taking up more space. The action of microorganisms, fungi, worms, water and weather breaks up the organic matter into finer and finer particles. Some will be used in their nutrient parts by the plants — eaten, in short. Some will even be washed away through the soil. In this way, the amendments reduce in depth over time. But because of this loss, it is necessary to amend soil if you want a healthy garden.
Finally, and I hope I’m touching all bases for you, it is possible to modify the topography of your landscape to create catchbasins, sometimes called rain gardens, that slow down or capture water run-off, giving it time to penetrate rather than run off, either into the street or towards your foundations. For information on rain gardens in Toronto, you might find this link helpful:
I hope this answers your questions.