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This tip could save a houseplant’s life

Gardening columnist Don Kinzler writes about achieving the proper headspace, which is the space between the potting soil and the rim of the pot.

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The proper amount of soil in a pot will help keep houseplants healthy.
David Samson/The Forum
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Did you hear about the fire at the flower shop? It was a florist fire.

Speaking of florists, flower shops aren’t the only businesses that sell houseplants anymore. Grocery stores, national hardware chains and garden centers all sell houseplants, which are trending popular as new generations catch houseplant fever.

Houseplant owners share a common goal – we want our plants to thrive, and a few years ago, I discovered something that can mean the difference between life and death for a houseplant.

After years of observing our own houseplants plus receiving photos of other people’s ailing plants, I noticed a common trend in plants that weren’t doing well, and I rarely see this observation discussed in houseplant how-to information.

I’ve noticed that houseplants with problems often have a deep headspace. What’s that, you say?

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The headspace is the space between the potting soil and the rim of the pot. Potting soil normally isn’t flush with the pot’s rim, because a reservoir is needed when watering, or water would spill over the edge until it soaks in. A certain amount of headspace is needed for practical watering.

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The pot filled higher will have better drainage, due to gravity and physics.
David Samson/The Forum

I noticed, though, that suffering plants are often situated in their pots with a considerable distance from the soil surface to the rim of the pot. In other words, there’s a deep headspace. I’ve concluded that a deeper-than-optimum space can lead to houseplant failure, while the proper space can promote houseplant health.

Why would the amount of headspace be crucial to a plant’s health? Let’s investigate.

It’s well known that a major cause of houseplant failure is overwatering. Overwatering doesn’t mean applying too much at one time, because excess would drain away and can be discarded. Instead, overwatering means the soil stays consistently too soggy, and the roots suffer from lack of oxygen.

To avoid overwatering, good drainage is vital, ensured by a pot with a drain hole, high quality potting mix, and proper watering frequency. But the appropriate headspace is just as important, maybe more so.

A plant with a too-deep headspace doesn’t have a well-filled pot. A greater soil height drains better than a lesser soil height because gravity pulls water down through higher soil with a greater force.

It’s easier to understand the concept if we think about buildings. Gravity will have a greater pull if I hop off a 30-story building, than if I hop off a two-foot stepstool. Likewise, gravity pulls water more effectively down through the soil of a well-filled pot, than a lesser-filled pot with a deep headspace.

A houseplant with a too-deep headspace often doesn’t drain well, leading to overwatering, potential root rot, yellowing leaves and related troubles. Gravity moves water down through a well-filled pot with a better force, pulling in oxygen, creating a healthy, aerated soil.

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What is the optimum amount of headspace in a houseplant pot? About one-half inch from the pot’s rim to the soil surface provides a reservoir for watering, while giving a high soil profile for good gravity water pull.

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When correcting a too deep headspace, it's usually best to add soil to the bottom and only a little at the top.
David Samson/The Forum

What if a plant currently has a headspace that’s too deep? It might be tempting to just add more potting mix at the top of the pot, which can work if just a little is needed, but raising the soil level up onto the plant itself can cause stems, leaves or branches to rot. Instead, it’s better to remove the plant from its pot and add soil to the bottom of the pot until the plant is situated with a headspace of one-half inch.

When potting or repotting a plant, the soil mix often settles with the first watering. If you fill the pot to the rim with soil mix, then water, the mix will often settle to the recommended one-half inch space.

Houseplant soil also settles with time. If only a small amount of potting mix is needed to replenish the appropriate headspace, about a half-inch, it can be added to the surface.

It might seem simple, but the proper headspace can save a plant’s life.

More gardening columns from Don Kinzler

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.
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