I live in a condominium in downtown Toronto. The landscape company installed triple mix into 2 cubic ft. containers. We are planning on growing perennials like Japanese maple, lily, etc. And annual flowers and vegetables. I think the soil is the wrong type and should be a potting soil. Can you please tell me what soil to use triple mix or potting soil in the containers and why?
Thank you for contacting Toronto Master Gardeners.
You are correct that soil used in gardens- like triple mix- are not usually used in container gardening.
When garden soil is put in containers it does not allow proper drainage. The normal capillary action for drainage in garden beds does not exist in the same way in a container. This in turn creates compacted soil and will not allow the soil to properly hold air and water in such a way that it is available to the plants. This will either mean your plants may not thrive or they may die.
There are many types of potting soil available. Potting soil is made up of many different products that stop the medium from compacting and encourage good drainage. It is a good idea to decide on your plants before choosing your mix. The list of potential plants above includes plants with different needs. Sometimes plants may prefer acidic soil so will require an acidic mix. The ingredients in different brands have varying abilities to hold water and have different ranges of nutrients available to the plants. Once your plants have been in the pots for a few months you will also need to consider which fertilizers may be needed. Again, different plants have different requirements. Your nursery should be able to help you with this.
When filling the planters do not add any other materials at the bottom of the container. People often mistakenly put rocks and other materials to help improve drainage. Because of osmosis, this ends up having the opposite effect and hinders drainage leaving a very wet layer than can rot around your roots.
I have included a couple of links below for further reading. Good Luck with your future container garden.
https://extension.uga.edu/publications/detail.cfm?number=C787
https://www.clemson.edu/extension/hgic/plants/vegetables/gardening/hgic1251.html