When we water our plants, most just pours through. Some is immediately absorbed by the plant and medium, and some (“bridging water”) is held in the gaps between media particles by surface tension. It is that fraction, that if too extensive (too fine, or old, decomposed media) that leads to root suffocation, death and rot.

Many people put pot shards or foam packing peanuts in the bottom of a flower pot, thinking it will keep the potting medium more airy. While there is no doubt that having the entire pot diameter available for draining is a good thing, reducing the height of the column of potting medium actually increases the amount of bridging water held in the medium. You can prove it with an ordinary kitchen sponge.

Hold a sponge under water and repeatedly squeeze it until saturated. Keeping it horizontal and without squeezing it, lift it out of the water and let it completely drain. Once it has stopped dripping, turn it on its edge. More water will drain out.

I have had a long-running disagreement with another grower over this, so recently ran an experiment in which I compared a 6” tall, 3” diameter pot 1) completely filled with potting medium (I used Grodan rock wool mini cubes for uniformity), and 2) half filled with foam packing peanuts and half medium. If the amount of water held was independent of media column height, the “half” pot would have held half the amount of water that the “full” pot did. In fact, the “half” pot held about 30% more water than that.

I have published the experiment here, if you’d like the details.