Why Raising a Drain Hole 5 Centimetres Halves a Pot's Air

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Almost certainly yes, and the arithmetic will tell you by how much, because a drain hole is not the top of the water. The water table sits at the outlet, and capillarity holds a fully saturated zone above it whose height belongs to the growing medium rather than to the pot. That height is twice the surface tension divided by the density, gravity and the largest pore radius, which comes out at 1.5 centimetres in bark, 7.4 in coco and 29.7 in a peat based compost.

Which produces the number you want. On a published container 7.3 inches deep filled with coco, a drain hole five centimetres up cuts the pot's average air content by 45 percent, and the hole cannot go higher than 4.38 inches at all, because above that the saturated zone reaches the rim and there is no air anywhere in the pot.

Capping the hole and drilling lower is the right instinct. How much lower is a calculation, and it turns out to be a calculation about what you filled it with.

The height that belongs to the medium

Water is held in a pore against gravity by surface tension, and it drains out of that pore once it is high enough above the water table that the pull of gravity wins. The balance gives a height of twice the surface tension divided by the density, gravity and the pore radius. With water's surface tension at 0.0728 newtons a metre at 20 degrees and a contact angle taken as zero:

Coarse perlite or bark with a largest pore radius around 1 millimetre holds a saturated zone 1.48 centimetres tall.

Coco coir, around 0.2 millimetres, holds 7.42 centimetres.

A peat based seed compost, around 0.05 millimetres, holds 29.68 centimetres.

Read it the other way and it is equally useful. The pore that is still full of water at a given height above the drain is 297 micrometres at five centimetres, 148 at ten, 80 at eighteen and a half, and 49.5 at thirty. Every pore wider than that has already drained; every pore narrower is still holding. That single sentence is the whole of container drainage.

A box whose own numbers close

A self watering tomato planter with a trellis, in a two pack at 56 inches tall publishes heavy duty metal boxes with rust resistant stainless rods, modular stacking trellis sections, a self watering reservoir, and internal dimensions of 10.6 by 10.6 by 7.3 inches with roughly 3.5 gallons of soil capacity.

Multiply the three numbers: 820.3 cubic inches, and 231 cubic inches to the gallon makes that 3.551 gallons. The record's own arithmetic closes, which is worth checking first and is not always the case.

Seven point three inches is 18.54 centimetres. So in that one box, with the drain at the base:

Perlite leaves the bottom 1.48 centimetres saturated, which is 8.0 percent of the depth.

Coco leaves 7.42 centimetres, which is 40.0 percent.

Peat compost leaves 29.68 centimetres, which is more than the box has, so the whole thing is saturated to the rim and stays that way.

Same box, three media, eight percent to a hundred. Nothing about the container changed. More in our review of the self watering tomato planter, whose own caution is the right one for the volume rather than the depth: "3.5 gallon boxes favor compact plants."

The drain question, answered in centimetres

Raising the outlet raises the whole moisture profile with it, because the profile is measured from the water table and the water table is at the outlet. So the saturated zone becomes the outlet height plus the medium's own height, and two consequences follow.

First, the air. Roots respire, and the share of a pot that is air rather than water or solid is the quantity that supplies them. Take coco's drained air filled porosity as a representative 20 percent, which is typical for a well made mix:

With the drain at the base, 40 percent of the depth is saturated and carries no air, so the pot averages 12.0 percent air.

With the drain five centimetres up, 67 percent is saturated and the pot averages 6.6 percent.

Five centimetres of drain height cuts the pot's air by 45 percent, and the horticultural rule of thumb puts the minimum useful air filled porosity somewhere around 10 to 15 percent, which means that one decision moved the pot from acceptable to below the line.

Second, the ceiling. The drain cannot be higher than the depth minus the medium's saturated height, which for 18.54 centimetres of coco is 11.12 centimetres, or 4.38 inches. Above that the saturated zone reaches the surface and the pot has no air anywhere in it, whatever you fill it with and however well the mix drains on a bench.

So the practical answer to the question is a number rather than a judgement. Measure the medium's coarseness as best you can, subtract its saturated height from the container depth, and drill below that. In coco, in a 7.3 inch box, go below four inches and preferably at the base. In a bark or perlite mix the ceiling is 17.1 centimetres and almost any hole works. In a peat mix there is no height that works, and the honest conclusion is that the medium is wrong for the depth rather than the hole being wrong for the medium.

Why a deep pot drains better, and a seed cell never does

Two corollaries fall out for nothing and both are more useful than they look.

The saturated zone is a height, not a fraction. So double the depth and the fraction halves. Thirty seven centimetres of coco gives a saturated fraction of 20 percent instead of 40, and an average air content of 16.0 percent instead of 12.0. That is the real reason a deep pot drains better than a shallow one, and it is not that the water falls further or has further to go. The medium holds the same centimetres either way, and in a taller column those centimetres are a smaller share.

And a seed cell cannot be drained at all. A cell 3.8 centimetres deep filled with a peat compost that holds 29.7 centimetres is saturated through its entire depth, permanently, whatever holes are moulded into its base. Which reframes the whole design of a propagator: the capillary mats, the enlarged drainage holes and the vented domes are not drainage features competing with each other, they are the only levers available on a container that is physically incapable of holding an unsaturated zone. Our page on why a seed's soak time goes as the square of its size works out what that costs in oxygen, where the ratio between diffusion in air and in water is ten thousand, and this page supplies the container half of the same argument.

A two week claim that is about the room

A FIASON 12-inch self watering planter with a water level window publishes a built in reservoir with wicking, a visual water level window, multiple drainage holes with an overflow outlet, UV resistant weatherproof plastic, a detachable base, and a stated interval of up to about two weeks between refills.

A twelve inch pot has a plan area of 729.6 square centimetres. Plants lose water at a rate that is properly expressed as a depth per day over that area, and for a moderately lit indoor position somewhere between one and a half and three millimetres a day is reasonable. So:

At 3 millimetres a day, the pot uses 219 millilitres a day and two weeks needs 3.07 litres.

At 1.5 millimetres a day, it uses 109 and two weeks needs 1.53 litres.

So the two week claim implies a reservoir somewhere between about one and a half and three litres, and the record publishes neither the reservoir nor the rate it assumed. Its own drawbacks say exactly that, which is the right disclosure: "Reservoir volume is not published", and "Two-week claim varies by plant and climate". The honest reading is that two weeks is a statement about a room's light and humidity with a pot attached, and the arithmetic above is how you convert it to your own room. More in our review of the FIASON 12-inch planter.

The wick is never the limit

A pack of Cdd's Path clear self watering pots, six in mixed 5, 6 and 7 inch sizes publishes a slotted multi-hole inner section with water inlet holes and an absorbent cotton wick, a detachable clear reservoir base, and transparent pressure and drop resistant eco-plastic chosen partly because orchid roots benefit from light.

Run the same expression on the wick itself. Cotton at a pore radius around 50 micrometres lifts water 29.7 centimetres, which is taller than any of those three pots. The wick is never what limits a self watering pot. The medium above it is.

Which puts a published caution in an interesting light. That record's drawbacks say it "Works best with a free-draining medium, not dense soil", and that is correct for drainage while pulling the opposite way for wicking: a free draining medium has large pores and therefore a short rise, so in a bark mix at a one millimetre pore radius the water climbs 1.48 centimetres into the medium and stops. The resolution is that in this design the water does not have to climb, because the roots come down. An orchid in bark grows roots into the wet zone at the base and leaves the rest of the pot airy, which is exactly the arrangement the arithmetic recommends and exactly why the record is right about orchids. Our guide to how self watering pots work and which plants suit them says "capillary action does the rest", which is true, and the length it does it over is 1.48 centimetres in bark and 29.7 in peat. See also our review of the Cdd's Path pots.

Two products that are the same dial in disguise

A GreenShip 11.8-inch self watering cylinder planter set with saucers publishes plugged drainage holes with matching saucers, Jun Porcelain Clay with recycled resin, and one large and one small pot. Plugs in runs it as self watering, plugs out gives traditional drainage.

That plug is a drain height switch with exactly two positions. Plugs out puts the water table at the base, which is the lowest it can be, and the saturated zone is then the medium's own height and nothing more. Plugs in removes the outlet entirely, so the water table is wherever the water you poured in reaches and the saturated zone starts from there. There is no setting in between, which is worth knowing before you choose a medium for it, because a cylinder is also the shape that puts the most volume into the bottom centimetres. A pot that narrows toward its base keeps less of its medium in the saturated zone than a cylinder of the same height. Its own drawbacks are candid about the gaps: "Reservoir capacity figures are not given", and "Small pot size is not stated in the listing". Our review of the GreenShip cylinder set has the rest.

And the same dial gets left on by accident on almost every pot in the house. A CODIAO 4-inch geometric 3D printed planter with drainage and a saucer publishes a built in drainage hole, a matching terra cotta drip tray, a 3.5 inch opening with a 4.7 inch widest width, and a spherical belly the maker says holds about 50 percent more soil than standard pots.

A pot standing in a centimetre of saucer water has its water table a centimetre up, which adds a centimetre to the saturated zone. Not because the water wicks back in some mysterious way, but because the drain is submerged and therefore is not a drain. So emptying the saucer is worth exactly the depth of water you pour away, which on a four inch pot is a meaningful share of the whole thing, and it is the only intervention on this page that needs nothing at all. Our review of the CODIAO planter and our guides to how self watering planters work and choosing planters and pots cover the rest of the selection problem.

The honest limit, and what it leaves alone

What is exact here is the capillary balance and every ratio built on it. The saturated height is twice the surface tension over the density, gravity and the pore radius, so it is inversely proportional to pore size: the 20 to 1 ratio between bark and peat is exactly the 20 to 1 ratio between their pore radii. The saturated zone is a height, so doubling the depth halves the saturated fraction, exactly. The 820.3 cubic inches against 3.551 gallons is arithmetic on three published dimensions. The 729.6 square centimetres follows from a published diameter. And the ceiling on the drain height, depth minus the saturated zone, is a definition rather than an estimate.

What is representative rather than published is the input that sets every absolute number, and it is a big one. No listing in this catalogue publishes a growing medium, let alone a pore size distribution or a water release curve, so the 1 millimetre, 0.2 millimetre and 0.05 millimetre pore radii are typical values for three named classes of material and nothing more. Real media are mixtures with a spread of pore sizes, which means the transition from saturated to drained is a gradient over a few centimetres rather than the sharp line this page draws, and the 40 percent figure for coco should be read as a middle of a band. The contact angle is taken as zero, which is generous: peat in particular becomes hydrophobic when it has dried out, which raises the effective angle and shortens the rise, and is why a dried out peat pot sheds water rather than absorbing it. The 20 percent drained air filled porosity for coco, and the 10 to 15 percent minimum, are horticultural conventions. The transpiration range of 1.5 to 3 millimetres a day is a range for an indoor position and could be triple that in a greenhouse.

Three things this page deliberately does not compute. It does not predict how long any pot goes between refills, because that needs a transpiration rate, which is a property of the room, the season and the plant rather than of the container, and the FIASON record is right to say so. It does not compute how long roots survive in a saturated zone, which needs an oxygen consumption rate and is named as the real limit on the practical advice: the arithmetic establishes that the zone has no air in it and cannot say how many days of that a given plant tolerates. And it does not work out the exact saturated fraction for a tapered or bellied pot, because that needs a height for the pot and neither of the two shaped planters here publishes one; the direction is certain, a pot narrowing toward its base is better than a cylinder, and the size of the effect is not computable from what is on the listings.

Three numbers to carry. The saturated zone above a drain is 1.5 centimetres in bark, 7.4 in coco and 29.7 in peat, and it belongs to the medium rather than to the pot. The drain cannot sit higher than the container depth minus that figure, which in a 7.3 inch box of coco is 4.38 inches. And five centimetres of drain height, or five centimetres of water left standing in a saucer, cuts the air in the pot by 45 percent.