Hello, I live in Toronto and have an area next to my house with hard compact clay soil, which does not absorb rainwater. After each rainfall, I have a puddle of water for many days. I would like to remediate this area by removing 18 inches of the clay soil, and replacing it with good topsoil. My question is whether, after I take this action, will the 18 inches of good topsoil be a good spot for me to plant hydrangeas.? My concern is whether the 18 inches of topsoil will generally absorb typical water during Toronto summers, or whether the fact that there is hard compact Clay directly beneath this area, will that still mean that I have a problem with “poorly drained soil”, in which case I should probably not plan to hydrangeas there. Please let know you’re thoughts. Many thanks!
Thank you for contacting the Toronto Master Gardeners
While removing the hard clay soil and adding good top soil seems like a good idea what you are actually creating is a soil pocket. The soil you are planting in is different from the surrounding soil. The plant may initially do well, however as the roots come to the edge of of the soil in the planting pocket, they will resist growing into the surrounding soil. The following is from The Myth of Soil Amendments By Linda Chalker Scott”
“The initial results are positive; roots grow vigorously in this ideal environment as long as irrigation is provided. But what happens when these roots encounter the interface between the planting hole and the native soil? Native soil contains fewer available nutrients, is more finely textured and is less aerated. Roots react much in the same way as they do in containers: they circle the edge of the interface and grow back into that more hospitable environment of the planting hole. The roots do not establish in the native soil, eventually resulting in reduced growth rates and hazard status as crown growth exceeds root ball diameter.”
Instead of fighting whith you could work with the fluctuation water levels by creating a rain garden? Rain gardens are shallow depressions that collect and absorb rainwater. They are made up of plants that are tolerable to both wet and dry conditions, and unlike a pond, they allow water to soak into the ground and are dry between rainfalls. According to the City of Guelph article on Rain Gardens “Water should soak into the ground within 24 hours for ideal plant conditions.Sandy soils absorb water the fastest, while clay soils absorb the slowest. If you have clay soil, your garden will need to be larger.”
The Halton Master Gardeners have a wealth of information on how to create a rain garden, a rain garden calculator to figure out the depth and size of the garden, along with planting designs for various light requiremnets. Rain Gardens, Rain Garden Calculator, Rain Garden Plants for Sun, Rain Garden Plants for Shade, Rain Garden Tree & Shrubs, Creating Your Rain Garden. Rain Garden design: A Workbook for Homeowners is also a great resource for calculating the size and depth of your rain garden and gives step by step information from start to finish.
Good Luck with this project.
June 5, 2021