Why should you usually not place pebbles, rocks etc at the bottom of a container?

Question

We know that it's a myth that placing rocks, pebbles, broken clay pots etc at the bottom of a container filled with a potting mix improves drainage, and there are various mentions and differing explanations of this on this site scattered amongst other questions.

I'm looking for a definitive answer on why this doesn't help, and whether it's actually detrimental to the plant as some have suggested. Your answer should minimally mention some of the physics of fluids that causes this phenomenon which seems contrary to what one expects.

Answer 1

I'm not going to answer the entire question, but I am going to make a case for the one time it is useful to use curved pieces of broken terracotta pot, since you mention broken clay pots. Many terracotta, metal and ceramic pots have only one, large drainage hole in the bottom, and if that gets blocked, there's a drainage issue, along with the problem of potting soil falling through the hole until roots have grown into it. Placing one or two larger pieces of curved pottery over the hole in such a way as to keep soil and roots and any debris from compacting into the hole, yet still allowing water out, ensures continued good drainage.

Answer 2

Pros of rocks:

Disperses oxygen evenly to bottom of soil. Air prunes roots evenly at the bottom. Prevents a clog (sometimes fungal)

Cons of rocks: Less space for roots

Are rocks necessary? If soil can leak out, yes, otherwise it's about the medium. (e.g. in a bark based medium there's no point, but in a medium meant to stay on the soggy side- maybe.)

Perched water tables: Blown out of proportion. Growing in a semi-transparent container, you can see that the capillary action of peat and coir overcomes the physics of PWTs until well past when the plant needs watering. If PWTs still concern you, though they shouldn't, you can leave a 1" diameter area with no rocks and it will act as a gravity wick.

Personally - mostly I use good terracotta (very porous) with just a single shard to prevent soil leaking.

In a decorative pot (glazed/no air passage) I recommend a handful of lava rocks at the bottom.

Standard trade gallon containers have angled drainage holes on the bottom now for oxygen intake - else they'd still just have the little nubs and holes on the bottom.

In the pictured container, ofc, they're not needed at all.

Answer 3

This site has the best explanation that I've come across so far. As you can see in the image in the question, a hanging planter is able to grow plants with soil that is sitting inside a basket lined with coconut coir. The coir has large spaces even smaller than those formed by pebbles at the bottom of a pot, but the water is not gushing out of the soil and falling to the ground. This is because soil exhibits capillarity which prevents water from falling under the influence of gravity.

Figure 3. Capillary rise in tubes of varied widths. This picture demonstrates the phenomenon of capillary rise. As you can see, the liquid rises to the greatest height in the narrowest tube (at far right), whereas capillary rise is lowest in the widest tube (at far left). Although easily demonstrated by simple experiments using tubes, capillary action occurs in soils. Smaller pores that exist in finely-textured soils have a greater capacity to hold and retain water than coarser soils with larger pores. Source: http://www.wtamu.edu/~crobinson/SoilWater/capillar.html

So, when you have pebbles or other materials with a pore size which either exhibits a pore size greater than the one above it or a pore size so great it does not exhibit capillarity, then water is retained in the soil with the higher capillarity. And this explains why perched water tables form.

Thus the main result of putting large materials at the bottom of a pot is to reduce the amount of space available for potting soils.