Here’s what you need to know about houseplant soil
“I would like to start growing some houseplants and would like some advice on the type of soil to use for the best results.”
— Albert Jefferson, Chicago
Like gardening outside, providing your houseplants with a good growing medium will make a big, positive difference.
It is best to use a lightweight, soil-less growing medium that is commercially available and designed for container gardening. Using garden soil for a houseplant will severely impede both aeration and drainage, no matter how good your soil is.
Growing mixes for containers are modified to ensure proper drainage and aeration. The potting medium should be porous for root aeration and drainage, but also be able to retain water and nutrients.
I find that products described as potting soil are usually too heavy to allow for proper aeration when used in a container, although they are good for amending soil in beds outside. The better-quality artificial mixes contain slow-release fertilizers, which will supply a plant with nutrients for several months.
There are different brands of soil-less growing mediums available that will work well for most houseplants. If growing orchids, cacti or succulents, look for growing mediums specific to those plants, which have very different needs than the average houseplant.
Cacti and succulent mixes will generally have more sand to provide more sharply drained and generally drier growing conditions. Most orchids and bromeliads are epiphytes and attach their roots to tree branches in nature, and their roots will rot if grown in a regular growing medium that holds too much moisture.
Your containers should have a drainage hole or holes in the bottom for excess water to run out. There is no need to put a layer of gravel at the bottom of your containers, as the gravel will not improve drainage.
A screen or clay shard can be placed over the drainage hole to prevent the medium from washing out. Place the container in a saucer to help protect surfaces from water damage. Condensation may form under clay saucers over time and still damage wood due to terra cotta’s porous nature, so pay attention to the condition of the saucer, pot and surface over time.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.