When winter arrives, many gardeners turn their attention to houseplants. Whether your passion is delicate orchids or workman-like philodendrons, a well-blended potting mix is one of the most important secrets to healthy performance. Here's an introduction to some of the most common ingredients in a good container mix.
This is the most common ingredient in potting mixes. Peat moss is made up of partially decomposed plants harvested from peat bogs. Peat absorbs water (some types hold 10 times their weight), yet it provides air space so roots don't suffocate. Many "soil-less" mixes are composed primarily of milled sphagnum or hypnum peat moss.
Some controversy exists over harvesting peat. Opponents say peat is drawn from a diminishing resource since natural peat bogs can take thousands of years to form. Peat proponents say that sphagnum and hypnum peat moss reserves are enormous and that harvesting and restoring the bogs, which is often required, make the harvest sustainable.
Coir is made from coconut fibers--a waste product in the coconut processing industry. It has similar qualities to peat moss--holding water well, yet providing drainage--and it comes from a readily renewable resource.
Some potting mixes, especially those for epiphytes (plants that don't need soil, such as some orchids and ferns), contain composted bark. It is excellent at increasing drainage, but holds few nutrients.
The white, puffy material in potting mixes that looks like Styrofoam balls is generally perlite, although some mixes actually use Styrofoam. Whatever the makeup, these white balls provide air space in a potting medium. Perlite is volcanic rock that has been heated and then "popped" like popcorn. It contains numerous pores that help water drain through a potting mix quickly.
Vermiculite, which generally has a shiny, golden appearance, comes from the mineral mica. Like perlite, the mica has been heated until it expands, which is why vermiculite helps "lighten" a soil. However, like peat moss, vermiculite also helps hold water in a potting medium. Vermiculite comes in different grades. One grade is used as insulation; others are for horticultural purposes.
Horticultural sand is composed of tiny pieces of rock. Just like sandy soil outside, sand improves drainage. If you want to add sand to a medium, use horticultural grade sand and not beach sand to prevent contamination. Plants that do well in a potting medium that contains a large amount of sand include cacti and succulents.
Since container-grown plants rely on regular watering to survive, some mixes contain polymers to hold moisture. Polymers, which may look like tiny plastic marbles, act like sponges. They absorb and hold water when the medium is moist, but release it back into the soil when dry. This helps maintain a consistent level of water for plant roots. Several different brands and forms of polymers are available at local nurseries.