‘Headspace’ is the depth of space between the top of the potting soil and the rim of the pot. Now, not every houseplant with the proper headspace is guaranteed to thrive, but over the years I’ve notice that when plants are sickly, there’s often a correlation between headspace and plant health.
What is the ideal headspace? The preferred space between the pot’s rim and the potting soil surface is one-half inch. To visualize a half-inch: on a human hand the measurement from the tip of the middle finger to the first joint averages one inch (measure it, it’s pretty close on most people,); so one-half inch would be half this finger joint measurement.
Plants can suffer if headspace is deeper than one-half inch:
- If this top headspace reservoir is too deep, plants are more likely to stay soggy from overwatering, which is the number one killer of houseplants.
- A one-half inch headspace filled with water at watering time tends to provide the right amount to saturate the soil.
- With a larger top soil gap, the soil profile is shallower, so when plants are watered, there’s less gravity pull, meaning the soil doesn’t drain as well. With the proper headspace, there is a higher vertical soil profile, causing gravity to pull water through the soil more efficiently.
- If a headspace is too deep, air doesn’t flow across the soil as well, because the soil is recessed farther down inside the pot. Less air flow means greater chance of soil staying soggy and greater chance for moisture-loving diseases to enter.
- Insects are often more troublesome when protected with a deeper headspace.
- Damaging salts tend to build up in soil that is recessed deeper into a pot, versus the proper headspace.
How to correct headspace that is currently too deep:
- Potting soil can be added to the top, decreasing the headspace, IF it doesn’t put the plant at a too-deep planting level. Some plants are sensitive to too-deep planting, and adding soil to the top can put them at a non-desirable level. But adding a little is ok.
- If adding soil to the top would locate the plant too deeply, it’s better to lift the root ball out of the pot, add potting soil to the bottom, and reset the plant at the proper level.
Headspace is often overlooked, but very, very important. Happy Gardening!