Bad Instructions! A quarter inch of sand in the bottom will work as well to direct excess water to the drain hole. All that gravel diminishes the total soil volume available to the plant. Drainage is determined by the permeability of the soil mix, which is very important.

Make sure the pot has a good holes in it! Sometimes you have to drill them out.

Hi, Phyl. Soil drainage is directly related to soil particle size. The finer the soil particles, the taller the column of saturated soil at the bottom of the container and the more tightly a soil holds water. Sand in the bottom of the container (unless the sand is 1/16"+) would be counter-productive because it would support a tall(er) column of perched water. Large chunks of material are also ineffective at improving drainage because the saturated layer of soil that would normally be at the bottom of the container simply 'perches' on top of the drainage layer. The saturated layer of soil remains, it's just higher in the container.

The physics involved with drainage layers dictates that IF a drainage layer is to be effective, the particles in the bottom of the container must be larger than those in the soil, but no more than 2.1x larger, or the water will perch in the soil above the drainage layer.

If you have concerns that your soil might be too slow (hold too much water), adding a wick through the drain hole IS effective at removing excess water and helping to insure adequate soil aeration where drainage layers fail.

Al

Depends on the size of your pot or planter and what you are planting.
I regularly plant impatiens in huge pots. The impatiens are shallow rooted, so I shorten the pot by putting an upside down plastic flower pot in the bottom and cover the plastic pot's drainage holes with paper, so the soil doesn't; fall into the pot. Then I add about 10 inches of pottong soil and plant my impatiens.
BUT I have some of those same pots in another spot with dwarf Alberta spruce. They have a full pot of soil to grow in.
Linda C

thank you all so much for the well-thought out answers ...

i think what was hidden back in my brain somewhere was the physics of a *column of perched water* ... i wish i understood physics! ... al, now i remember i read, reread, & studied a post of yours from last year over & over again until my brain hurt (grin)... i *knew* it made sense, but i must admit i couldn't totally *get* it ...

in simplistic terms, here's the deal ... i have pots that are much larger (in terms of potting mix) than what the roots of my pelagoriums need ...

hmmm, now i am getting confused on what i am even asking!!!

maybe the suggestion of *taking up space* with crushed plastic pots is the answer ...

al, i am making my first attempt at making my own potting mix as per your instructions ...

phyl, who still has a lot to learn ...

Phyl,

"...maybe the suggestion of *taking up space* with crushed plastic pots is the answer ... "

That seems wrong. Who made that suggestion?

MM

Adding material to the bottom of containers to take up space can lighten containers and/or reduce the amount of soil it takes to fill the container, but don't add anything to the container bottom thinking it will improve drainage. It will not.

The size or shape of your container or what you are planting in it really has nothing to do with the height of a perched water table (PWT). It remains constant in any particular soil. If you have a soil that supports a 3" PWT, there will be 3" of saturated soil in a 12" deep container, or 75% of the soil will remain unsaturated at container capacity (the moisture level of the soil after it has been thoroughly saturated & has just stopped draining) while the same soil in a 3" deep container will remain 100% saturated.

If you want to reduce the height of or eliminate the PWT, you can do it by increasing the size of soil particles or wicking. An overturned container in the bottom of a larger container can reduce the volume of soil occupied by perched water and thus reduce the volume of water in the PWT, but it doesn't change the height of the PWT. Using the same soil and adding a 3" drainage layer of coarse material in a 6" deep container will leave you with 3" of saturated soil resting atop the 3" drainage layer.

From this, you can see that deep containers are easier to grow in, and containers that taper toward the bottom will have a smaller volume of soil for the PWT to occupy; therefore, will have a reduced volume of water in the PWT, even though the actual height remains the same. I hope I didn't confuse you. ;o)

When growing in containers, there is no need to try to accommodate plants that set their roots in the top few inches of the soil if you are growing in an adequately aerated container soil. The in situ plants that prefer rootage to grow in the upper parts of the soil do so because of two things - aeration primarily, but also nutrition. As long as any container soil is well-aerated, even plants that are considered extremely shallow-rooted will show roots quickly colonizing the entire soil mass.

If a soil is well aerated and supports no perched water, you can grow the teeniest plants in huge volumes of soil w/o any concern for root rot or aeration issues. The soils I use and suggest are formulated to last much longer than peat based soils and to support no, or nearly no perched water. Healthy roots = healthy plants.

Al