Potting soil can make or break houseplant success
In today's "Growing Together" column, Don Kinzler explains why it's important to pick a good mix — and why spending a little more now is worth it.
Houseplants are currently the hottest topic in the world of gardening. Houseplant Facebook groups have large followings with members sharing stories and success tips. And of course, any successful houseplant is only as good as the soil in which it’s growing.
The right potting soil makes the difference between a struggling plant and a healthy, thriving plant. The term “potting soil,” though, is a misnomer because there’s really no soil in the mixes used for houseplants anymore. The very finest outdoor garden soil quickly becomes hard-packed, easily water-logged and oxygen-starved when restricted to the confines of a houseplant pot.
Prior to today’s soilless potting mixes, houseplant growers blended their own, usually using one-third each of garden soil, sand and an organic component such as peat or manure. Because the components usually contained weed seeds and disease organisms, recipes abounded for sterilizing the mixture in the oven or microwave.
Having lived through the soil mixing days, I can say life is better now. Quality potting mixes are readily available at garden centers, and they’re vast improvements over homemade mixes.
Today’s potting mixes are backed by science. In the 1960s, horticulturists from Cornell University studied ways to provide potted plants with a medium that provided support and nutrition, but also the necessary drainage and aeration. Their research resulted in the peat-based potting mixes we use today.
What makes a great potting mix? Such a mix holds just the right amount of moisture while letting the excess drain away easily. It must be porous for root aeration, oxygen exchange and good drainage, but it must also retain the proper amount of moisture and be able to hold nutrients. Because a houseplant’s roots are restricted to a pot and can’t go in search of friendlier soil, a houseplant is totally dependent on the mix we provide.
Today’s potting mixes are made of ingredients including peat moss, composted bark, coconut coir, vermiculite and perlite. Slow-release fertilizer is added to many mixes, because most of these soilless components provide great aeration and create a root-friendly environment, but they have no natural nutrition.
All-purpose potting mixes are fine for nearly all houseplants. Some plants, though, require a mix that’s tweaked to meet their special needs, such as succulents, cactuses and orchids, and can be found at locally owned garden centers.
Is there a difference in potting mixes? Definitely! Houseplants can grow for years in the same mix, so this isn’t the area to scrimp. The cheapest potting mixes often feel heavy in the bag and lack the best ingredients for successful houseplant growth. Top quality mixes are light and well-aerated.
Locally owned garden centers usually stock successful grower-type mixes along with national potting mix brands. Because most high-quality mixes are very dry in the bag, moisten them the day before using by adding water and stirring to distribute. The mix becomes mellow, easier to use and will re-wet more readily than if it’s used dry. Pre-wetting is an important step.
When potting a houseplant, be sure to fill the pot with enough mix, leaving only about a half inch of headspace, which is the depth from the surface of the potting mix to the rim of the pot. When I see ailing houseplants, I’ve often observed a headspace that’s too deep, which contributes to soggy soil, reduces air flow across soil surfaces and provides less soil volume for root growth.
Research has also shown that potted houseplants have better drainage if the pots do not have a layer of rocks or pebbles in the interior bottom, as was frequently added in the past. The pebbles create a layer of change, creating a physical property that actually impedes the drainage they were meant to enhance. For the best drainage, fill the pot top-to-bottom with quality potting mix. Coffee filters, diapers or other screening materials aren’t needed to cover drainage holes when quality potting mix is used.
Potting mix can easily become infested with fungus gnats, the annoying, small, black flies that flit around houseplants. The adult flies lay eggs in the soil, which hatch into larvae which in turn become adults, creating a never-ending cycle. Mosquito Bits is a product labeled for fungus gnat control. The granules are sprinkled onto the soil surface and contain beneficial bacteria that kills fungus gnat larvae, breaking the gnat’s lifecycle.
Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at firstname.lastname@example.org.