When planting in containers, it’s essential to use a ‘potting mix’ instead of using in-ground soil. Potting mixes do not contain any actual soil. Instead, they use a blend of lightweight growing media, each serving a different purpose, which we’ll discuss below.
Before we dive into potting mix, let’s first discuss in-ground soil. In-ground soil is a blend of clay, silt, sand, and organic matter. It is not suitable for containers, as it hardens when dry and expands when wet, causing stress on container walls and plant roots. It may also contain insects and microbes that are not suitable for containers that may be brought indoors.
A growing medium designed for containers is potting mix, which has no clay, silt, or sand. It is usually made up of some combination of:
- Perlite or vermiculite – lightweight minerals that reduce the density and contain numerous pores which hold onto oxygen and nutrient particles for use by plants
- Peat moss or coir – retains water for gradual use by plants
- Compost – gradually releases nutrients for use by plants
- Composted (aged) bark – creates air pockets by adding texture and slowly breaks down to provide nutrients to plants
Whether you’re starting new seedlings, transferring seedlings into larger containers (called potting up), or transplanting full-sized plants indoors for the winter, be sure to use a proper potting mix. Here are the different types:
- Seed starter – a very sterile potting mix that provides the basic needs of seeds and young seedlings, but because it contains few nutrients, fertilizer must be added once the seedling begins to grow. The initial sterility reduces the likelihood of fungal disease (such as ‘damping off’), which can kill seedlings. A typical seed starter is comprised of peat moss and perlite.
- Indoor potting mix – a relatively sterile growing media comprised of peat moss or coir, perlite, sterilized compost, and sometimes bark. It provides everything plants need to grow, although you generally need to add fertilizer a few times during the growing season to replenish what the plants use up. Finally, it provides excellent drainage since overwatering is a frequent cause of stress to plants in an indoor growing environment.
- Outdoor potting mix – often called triple mix, it is comprised of compost, peat moss, and perlite (or vermiculite). It can occasionally contain up to 20% actual soil if additional density is required for taller plants or plants with tap roots. It will usually have more microbial activity, which improves the nutrient cycles in the soil, but it still requires regular fertilizer top-ups.
By choosing the most suitable container growing media for your situation and plant needs, you’ll be setting yourself up for success!