Why a Watering Globe's Fourteen Days Is a Pot Size

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There is a number printed on nearly every watering globe and self-watering pot, and it is written as though it belonged to the product. Fourteen days. Two weeks. Up to twenty days. It does not belong to the product. It belongs to the lamp above the plant and the area of the pot, and the reservoir only supplies the top of the fraction.

Which is good news, because it means the duration is computable, and the two numbers you need are usually already on the box. One of them is on the grow light.

We owe this page to a refusal on another one

Our page on why raising a drain hole five centimetres halves a pot's air works out how much water a pot holds above its drain, and then stops, deliberately, with this: it "does not tell you 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."

That was the right refusal and it named the missing quantity precisely. So here is the missing quantity, derived from the one piece of equipment in an indoor setup that publishes its own energy input.

The lamp sets the ceiling, and the arithmetic is four lines

A 100 watt full spectrum smart LED grow light publishes two figures together, which is what makes this possible: a genuine 100 watts, which its record makes a point of, and a stated 2 by 3 foot coverage area. It adds app control, aluminium honeycomb heat dissipation and daisy chaining, and its own drawbacks note there is no stand or mount in the box.

Two feet by three feet is 0.55742 square metres. Our page on why a grow light coverage area needs a height works at a representative 2.2 micromoles of photosynthetic photons per joule, and this page uses the same figure on purpose so the two cannot disagree. So:

100 watts times 2.2 micromoles per joule is 220 micromoles a second, which spread over the stated area is a photon flux density of 394.7 micromoles per square metre per second, a sensible number for a herb tray.

Now turn the photons back into watts, because water does not care about moles. At a 550 nanometre mean, a mole of photons carries 217,540 joules, so 220 micromoles a second is 47.86 watts of radiant power inside the photosynthetic band. A leaf canopy absorbs a representative 85 percent of that, which is 40.68 watts landing on wet tissue.

Evaporating water near room temperature costs 2,442 kilojoules a kilogram. Divide: 40.68 over 2,442,000 is 1.666 hundred-thousandths of a kilogram a second, which is 1.4393 kilograms a day spread over 0.55742 square metres, or 2.582 millimetres a day of water depth.

And here is the thing that makes it worth trusting. The drain hole page uses a horticultural range of 1.5 to 3 millimetres a day, taken from convention rather than computed. 2.582 lands inside it, near the top. Two completely unrelated routes, one from a lamp's wattage and one from growers' practice, arrive at the same answer, which is the best evidence either of them is right.

Millimetres a day into millilitres a day, on a pot that publishes its size

Water depth is the right unit for the physics and the wrong one for a jug, so convert it on something real. A 12 inch self-watering planter with a water level window publishes a 12 inch diameter, a built in reservoir, wicking delivery, multiple drainage holes, an overflow outlet, UV resistant weatherproof plastic, a detachable base, and up to two weeks between refills.

Twelve inches is 304.8 millimetres, so its surface is 0.072966 square metres. Multiply through:

At 1.5 millimetres a day, 109.4 millilitres a day. At the computed 2.582, 188.4. At 3.0, 218.9. So one 12 inch pot wants roughly 110 to 220 millilitres every single day, and the only thing that moves it inside that bracket is the light.

A 350 millilitre globe whose duration claim and size claim describe different pots

A six pack of large 350 millilitre hand blown glass watering globes publishes a 350 millilitre capacity, a 12 inch spike, a 3.5 inch bulb, optional anti-clog sleeves, and flow "regulated for up to fourteen days".

Divide the two published numbers and the claim becomes a rate: 350 millilitres over fourteen days is 25.0 millilitres a day. That is the whole specification, and it is small.

On the 12 inch pot above, 25 millilitres a day works out at 0.343 millimetres a day of water depth, which is 4.4 times below the bottom of the range the drain hole page uses and the energy balance confirms. So invert it and ask what pot the claim does fit. At 1.5 millimetres a day the answer is 0.016667 square metres, a pot 145.7 millimetres or 5.74 inches across. Under the grow light above it is 0.009682 square metres, a pot 111 millimetres or 4.37 inches across.

Now read how the same record sells it. It is described as "sized as a jumbo option", "aimed at bigger pots and longer gaps between top-ups", and "these jumbo globes hold more and reach deeper than small versions, so they favour bigger plants". The size claim and the duration claim describe different pots, and the gap between them is a factor of two in diameter.

To its credit, the record does not hide the mechanism's looseness. Its own drawbacks say "Release rate varies with soil type" and "fourteen days is a maximum, not a promise", and the performance text repeats it. Both true. The arithmetic adds the part that honesty about variation does not cover: on the pot the product is sold for, fourteen days is not at the optimistic end of a range. It is outside it. Full record in our review of the 350 millilitre globes, and the technique side is in how to use plant watering globes without killing your plants.

The globe that publishes no duration at all, which is the better choice

A set of 16 ounce hand blown glass watering globes with soil spikes publishes a 16 ounce capacity, the olla principle its record describes properly, and no number of days whatsoever. Its drawbacks field says "Release rate varies with soil, not precise", and that is the end of it.

So compute it instead. Sixteen US fluid ounces is 473.2 millilitres. On the same 12 inch pot that is 2.51 days under the grow light and 4.32 days at the low end of the range. On a 6 inch pot, where the area is a quarter as large, it is 10.0 and 17.3 days.

Which is the useful shape of the answer, and it is the shape a duration claim destroys. The same globe is a two day device and a two week device depending entirely on the pot it is pushed into, and a record that declines to pick one is being accurate rather than vague. Its description of the mechanism is also right and worth quoting, because it explains why the flow is not constant: "As the soil around the spike dries out, it draws water from the globe, and when the soil is moist the flow slows or stops." Details in our review of the 16 ounce globes.

A duration claim can publish a volume by implication

Back to the 12 inch planter, because it does the opposite of the globes: it publishes a duration and withholds the volume. Its own drawbacks field admits it in as many words: "Exact reservoir volume and pot weight are not listed."

But the claim and the footprint together force the volume. Two weeks on its own published 12 inch diameter requires 1,532 millilitres at 1.5 millimetres a day and 2,638 millilitres at the computed 2.582. So the two week figure is a statement about the reservoir whether the listing says so or not, and what it states is that the reservoir holds at least 1.53 litres.

That is a testable claim rather than a vague one, and it is the useful way to read any self-watering planter: the number of days times the pot's own area times 1.5 millimetres is the smallest reservoir the claim can be true with. If the tank looks smaller than that when it arrives, the duration was written for a plant in a dim corner. The record is in our review of the 12 inch self-watering planter, and how the wick itself behaves is covered in how self-watering planters work.

One product whose numbers actually close, and what its valve really adjusts

A three pack of self watering spikes with an adjustable drip valve publishes a valve adjustable from 0 to 60 seconds per drip, a fill lasting 1 to 20 days, and a fit for bottles with a 1.1 inch, 2.8 centimetre inner neck.

Check the set against itself. Sixty seconds a drip is 1,440 drips a day, and at the standard pharmaceutical approximation of 20 drops to the millilitre that is 72 millilitres a day. Twenty days of that needs 1,440 millilitres of bottle. And a 1.1 inch inner neck is the neck of a 1.5 litre bottle. Those three published figures agree with each other. That is rarer than it should be and it deserves saying plainly.

What the valve range is actually for is the more interesting result. Seventy two millilitres a day is 0.99 millimetres a day on the 12 inch pot, which is below the bottom of the range, and 3.95 millimetres a day on a 6 inch pot, which is above the top of it. So at its slowest setting one spike under-waters a large pot and over-waters a small one. The valve's range is a pot size range, not a plant demand range, and the way to set it is to work out the pot's area first. More in our review of the drip spikes, and the comparison across methods is in how to keep houseplants watered while you are away, which describes the globe as "self-regulating" and is right about the mechanism without putting a rate on it.

The honest limit, and three things not computed

What is exact: the pot areas from published diameters, every millimetre-to-millilitre conversion, the 25.0 and 72 millilitres a day that come from dividing one published figure by another, the pot diameters solved from a rate, and the implied reservoir volumes.

What is representative rather than published, and two items are reused from our own pages on purpose. The 2.2 micromoles per joule photon efficacy and the horticultural 1.5 to 3 millimetres a day range both come from our grow light page unchanged. The 550 nanometre mean photon, the 0.85 canopy absorption and the 20 drops per millilitre are standard values. No globe in this catalogue publishes a flow rate and no planter publishes a reservoir volume, which is why the arithmetic has to run backwards from durations.

And the real softness, stated properly because it runs both ways. The 2.582 millimetres a day is a ceiling on the photosynthetic route only. Real transpiration falls short of it, because stomata close and because leaf area is less than tray area. But the lamp's other 52 watts do not vanish: they warm the air and the medium surface and evaporate water by a second route the calculation ignores. Those two errors point in opposite directions, which is why the honest output of this page is a bracket of 1.5 to 3 millimetres a day rather than a single figure, and why the energy route's job is to confirm the bracket rather than replace it.

Three things deliberately not computed. The actual release rate of any globe, which depends on the soil's suction against the glass bulb's air entry and is the quantity every one of these records correctly declines to publish; treating a 350 millilitre globe as a 25 millilitre a day dispenser is reading its marketing, not its physics, and the point of this page is that the marketing figure is an area claim in disguise. Anything about overwatering, because the risk there is the air-filled porosity of the medium rather than the volume delivered, and that is the subject of the drain hole page and not of this one. And the hydroponic case in the question as asked, where there is no medium at all and the reservoir loses water to the air as well as to the plant; the energy ceiling still applies to the plant's half, but the free surface evaporation needs a surface area no countertop system in this catalogue publishes.

Three numbers to carry. A 100 watt lamp over 0.557 square metres can evaporate at most 2.582 millimetres a day, which sits at the top of the 1.5 to 3 range growers already use. One 12 inch pot therefore wants 110 to 220 millilitres a day, so a 350 millilitre globe is a two day device on it. And any duration printed on a reservoir is really a statement about pot area: divide the volume by the days, then by 1.5 millimetres, and the answer is the largest pot the claim can be true on, which for that globe is 5.74 inches.