I will be planting Karl Foerster and Hakconechloa grasses together (the Foerster in the background against a fence). Currently I have a lot of clay in the area that I want to plant both of them (they will be together). What do you recommend I do for the soil prep? Should I remove all the clay and replace with soil? Or can I mix in soil, composted manure and fertilizer with the existing clay? I live in midtown Toronto.
Thank you for contacting Toronto Master Gardeners.
When dealing with clay, the less you disturb the soil the better. Digging clay and adding amendments especially sand usually leads to even more compaction and sometimes ends up like cement. I have attached an article at the end from our library which explains the science behind not disturbing clay.
The best approach would be to work with mulch. When you plant, dig your holes and put the grasses in. Do not add any amendments. Just fill in with the soil you removed. Disturb the site and clay as little as possible. Once planted use wood chips- not the large bark ones they do not break down enough and are water resistant. Put a layer of mulch a few inches thick over the surface. Leave a ring around each plant so the wood is not up against the leaves. The mulch will absorb water and stop further compaction on the surface from water.
If you do continue to mulch , after a few years you will have much improved soil. It takes time but creating the correct environment encourages the microbes and insects to come along and do their thing. In the mean time your grasses should grow. Only water when the ground is dry and try to water slowly so the water can soak in. The mulch will help with this.
It looks like there are many varieties of Calamagrostis ‘Karl Foerster’ and Hakonechloa grasses. Be sure to pick ones that match your site. Take into account the lighting and moisture requirements and winter tolerance when choosing your plants. I have included an article for you on choosing the correct ornamental grasses for your site.