Self Watering Window Box Planter: Pack Count, Sill Size and Limits

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A self watering window box planter is a box with a water reservoir below the potting mix, so the plant can draw water up as it needs it. The nine listings in this roundup come from Amazon search results, and they differ mainly by pack count, stated size and whether the title says self watering at all. This guide explains how the reservoir is meant to work, what each listing title does and does not tell you, and two planning calculations for a windowsill. It also marks the points that the listing data cannot settle, such as reservoir capacity and drainage design.

Quick answer

• Count the boxes you need before you pick a pack. Three windows with two boxes each need six boxes, and three of the listings are sold as six-piece sets.
• Check the title for self watering. Eight of the nine titles say self watering. The one that does not, a LaLaGreen planter box, is a plain box with no reservoir stated.
• Measure the sill against the stated size. The titles give sizes from 4 inch to 24 inch, and one title gives no size at all.
• Read the product page for the rest. The titles do not state reservoir capacity, refill interval, box depth or drainage design, so those points need a check before you buy.

How the reservoir is meant to work

The University of Maryland Extension page on self-watering containers describes the basic design. Its page says the growing medium sits on a perforated platform directly above a water reservoir, and that in most cases water is wicked up from the reservoir into the medium. The roots grow through the medium and reach the water below. That is the mechanism behind every listing in this roundup whose title says self watering.

The Illinois Extension blog on self watering containers describes the same idea in plainer terms. A reservoir sits at the bottom of the container and keeps the soil moist as needed, and a basket of soil in the reservoir acts as a wick that pulls water up into the rest of the container. The blog also notes an overflow hole, which stops the reservoir from filling past a set level. Both pages agree on the core point: the reservoir supplies water from below, so you fill the reservoir rather than watering the soil from the top every day.

Neither page gives a reservoir size for a window box, and neither gives a refill schedule that applies to every design. A listing title can tell you that a planter is self watering. It cannot tell you how many cups or quarts the reservoir holds, how often you will need to refill it, or where the fill point is on the box.

Reservoir and wick

The wick is the part that does the work, and the design of the wick is not in any listing title in this roundup. The UMD page describes wicking as the usual way water moves from the reservoir into the medium. The Illinois blog describes a soil-filled basket as a wick in a home-made version. A commercial box may use a different material, and the listing data here does not say what each box uses.

The practical result is that a self watering planter keeps the root zone moist for longer than a plain box. The UMD page says these containers help conserve water and nutrients and can make it possible to ignore the container for a few days. That is a general statement about the design, not a promise for any one listing, and the length of time depends on the plant, the size of the reservoir and the weather.

The same design is a poor match for some plants. The Illinois Extension drainage page says plants that need to dry out, such as thick-leaved cactus and succulents, do not usually justify the extra cost of a self watering container. A windowsill herb or a trailing annual that likes steady moisture is a better fit for this design than a succulent.

Overflow hole instead of bottom drainage

A self watering planter is not built like an ordinary pot. The UMD page says these containers have an overflow hole on one side instead of drainage holes in the bottom. The idea is that water rises from the reservoir into the medium and any excess drains out of the overflow once the reservoir is full. If you fill the reservoir past the overflow line, the water has to go somewhere, so the overflow is part of the design rather than a flaw.

Some readers assume a gravel layer in the bottom will improve drainage. The Illinois Extension drainage page calls that idea a myth. It says a gravel layer under the soil does not improve container drainage, because water gathers in the soil above the gravel until the air space fills up. The same page says drainage of excess water is vital for the health of plant roots. That point applies to a self watering planter as well: the reservoir must not sit in water the roots cannot use.

The Illinois drainage page also warns about saucers. It says to beware of pots with permanently attached saucers where emptying the overflow is difficult or impossible, because water that cannot leave the container can waterlog the roots. Check that any overflow hole is open and that the saucer, if the box has one, can be emptied.

Watering: what the extension pages say

The Iowa State University Extension page on care of plants growing in containers gives a watering routine for ordinary containers. It says to water when the potting mix is dry at a depth of 1 to 2 inches, and to continue applying water until it begins to flow from the drainage holes at the bottom of the container. It also says containers in general need much more frequent watering than garden beds.

That routine is written for containers with drainage holes. A self watering planter with an overflow hole and a reservoir follows a different pattern. You still check the medium, but the reservoir does part of the watering, so you top up the reservoir on a schedule that depends on the box. The Iowa State page also says watering frequency depends on the size and type of container, the composition of the potting mix, the plant species and the weather. Those same factors shape how fast a window box reservoir empties.

For a window box, the practical routine is simple. Check the mix with a finger on a regular schedule, check more often in hot weather, and note how quickly the reservoir drops. Use that record to set your own refill rhythm for that box. The pages we opened do not give a single interval for every reservoir, so this rhythm is your own measurement rather than a published figure.

Potting mix matters too. The Iowa State potting media article, already recorded in our facts file, says many high-quality potting media are roughly 85 percent pores and are lightweight to handle. A heavy garden soil in a box compacts and holds water poorly, which defeats the purpose of a reservoir that is meant to feed a light mix from below.

How we chose

We started with the nine listings returned by the Amazon search results for this keyword and kept all nine in the roundup, because each one answers the same search. We did not drop any listing for being a weak match, and we did not rank the listings by any measure shown on the results page.

We then sorted each listing by what its title states. The sort covers four points: whether the title says self watering, the pack or set count, the stated size, and whether the title names a material or a brand. A listing that the title does not describe as self watering is still shown, and it is marked as a plain planter in the picks below.

For the reservoir, overflow and watering points, we used the extension pages listed in the references, and we quoted or paraphrased only what those pages say. For product facts, we used only the listing titles. We did not open each product page for this article, so the guide does not describe any product detail that the titles leave out.

What the listing titles state

The table below shows only what each title says. Titles are the only product text behind this roundup, so every entry in the table comes straight from a title.

Listing (short name from its title)	Pack or set count in title	Size in title	Brand in title	Self watering in title	Material word in title
Rectangle planter, 6 pack	6	16 x 4.4 inch	not stated	yes	not stated
Self watering planters, 7 pack	7	4, 8 and 17 inch	not stated	yes	plastic
Windowsill herb planter box, set of 3	3	10.5 inch	OurWarm	yes	not stated
Self watering pots, 6 pack	6	10.5 inch	OurWarm	yes	plastic
Elongated planter pots, 2 pack	2	5.5 x 16 inch	not stated	yes	not stated
Rectangular windowsill planter, 6 set	6	not stated	not stated	yes	not stated
Elongated planter pots, 2 pack, second listing	2	5.5 x 16 inch	not stated	yes	not stated
Indoor planter box, 2 pack	2	16 in	LaLaGreen	no	not stated
Window box planter, 24 inch, black	not stated	24 in	Mayne	yes	not stated

Three points stand out. Eight titles say self watering, and only the LaLaGreen planter box does not. Three titles give a count of six, and three give a count of two. Eight titles give a size, and only the rectangular windowsill set does not. Two titles name plastic, and no title names any other material.

The two elongated 2 pack listings have the same size, the same count and the same wording in their titles. The titles do not show a difference between them, so this guide treats them as two listings with one shared size and does not claim a difference that the data does not show.

Three listings, the 16 x 4.4 inch planter and the two 5.5 x 16 inch pairs, give two numbers in the form width by length. The titles do not label which number is the length, so the planning calculations below assume the larger number is the length, and the product page is the way to confirm it.

Planning calculations for a windowsill

Two calculations help before you order. Both use the titles’ numbers, and both are our planning estimates, not measurements of any box.

Boxes needed for the window plan

The count formula is simple:

Boxes needed = windows x boxes per window

Take a plan with three windows and two boxes in each. That gives 3 x 2 = 6 boxes. The table below shows how many packs each stated count covers for that plan.

Stated count per pack	Packs needed for 6 boxes (our planning estimate)	Boxes left over
6	1	0
3	2	0
2	3	0
7	1	1
not stated	cannot be calculated from the title	not known

A six-piece listing covers the plan in one purchase. A two-piece listing covers it in three purchases. The seven-piece listing covers the plan with one box to spare, and that spare box may suit a single herb on a narrow ledge. The listing with no stated count cannot be placed in the table at all, and the title does not tell you how many boxes arrive.

Sill length for one or two boxes

The length formula is:

Total length = box length x number of boxes + gap x (number of boxes – 1)

The gap is whatever space you want between boxes. The example below uses a 36-inch sill as a planning length, which is an example figure and not a listing figure, and sets the gap to zero to show the best case.

• Two 16-inch boxes: 2 x 16 = 32 inches, which leaves 36 – 32 = 4 inches on the sill.
• One 24-inch box: 1 x 24 = 24 inches, which leaves 36 – 24 = 12 inches on the sill.
• Three 10.5-inch boxes: 3 x 10.5 = 31.5 inches, which leaves 36 – 31.5 = 4.5 inches on the sill.

The 16-inch and 10.5-inch figures are taken from the titles, and the length assumption is the one described above. Any gap between boxes, any bracket that holds a box to the sill, and any overhang past the sill edge all reduce the room left. Measure your own sill, subtract the gap you want, and only then compare the total with the stated size.

Choosing by situation

The right listing depends on what the window needs to hold, so start with the plan rather than the pack count.

For a plan with several windows, pick a count that divides the number of boxes you need. A six-piece listing matches a three-window plan with two boxes each, and a two-piece listing matches a plan with six boxes if you buy three packs. Check the title for self watering before you buy, because the one listing that does not say it is a plain box.

For one narrow sill with herbs, a set of three or a set of two keeps the footprint small. The three-piece herb set uses a 10.5-inch size named in its title, which fits a narrow ledge with room to spare on a 36-inch sill. The two-piece elongated pots, at 5.5 by 16 inches, are a shallow option for a sill where height is limited.

For one wide sill, a single long box keeps the planting in one piece. The 24-inch black box is the only listing whose title gives one size with no pack count, and it leaves 12 inches of a 36-inch sill in the calculation above.

For a box you will water by hand, a plain planter may be enough. The LaLaGreen 2 pack in white is a plain planter box by its title, with no reservoir stated, so it suits a windowsill where you will water on a fixed day each week and check the mix yourself.

For a succulent or a cactus, skip the self watering design. The Illinois drainage page says these plants do not usually justify the extra cost of a self watering container, and the reservoir can keep the root zone wetter than these plants want.

Once you have chosen the kind of box, check three points on the product page before you order: the reservoir size and fill point, the overflow hole position and the box depth. Those three points are the ones the titles do not give.

Mistakes to avoid

Buying by pack count alone. A six-piece listing is only useful if six boxes fit the sill and the window plan needs six boxes. Measure first, then count.

Treating the word self watering as a measure of the reservoir. The title tells you the design, not the reservoir size. A reservoir that holds very little will need refilling more often than one that holds more, and the title does not say which you have.

Adding gravel to the bottom for drainage. The Illinois Extension drainage page calls this a myth. It does not improve drainage, and it hides the level where the water sits.

Leaving the overflow blocked. An overflow hole is there to stop the reservoir from overfilling. If it is blocked by soil or debris, the reservoir can waterlog the roots. Check it when you fill the box.

Letting a saucer hold water. The Illinois drainage page says that a permanently attached saucer can make the overflow hard to empty. If the saucer holds water for days, the roots sit in it.

Filling the box with garden soil. Garden soil is heavy and compacts in a box, which blocks the air the roots need. Use a light potting mix made for containers.

What we could not verify

• The product pages were not opened for this article, so reservoir capacity, the fill point and the refill interval are not stated for any listing here.
• The extension pages we opened do not give a refill interval for a self watering window box, so the watering rhythm in this guide is a method to measure, not a published figure.
• The box depth is not stated in any title, and the titles do not say which number is the length for the three width by length listings.
• Drainage design is not stated in any title. We cannot say whether each self watering listing has an overflow hole, or where that hole sits.
• Material is named only as plastic in two titles. Wall thickness, weight when filled and the finish of the other listings are not stated.
• The two 5.5 x 16 inch pairs have the same title text, and the titles do not show whether they differ in any other way.
• The mixed-size seven pack does not say how many boxes come in each of its three sizes.
• Whether any box fits a particular bracket, rail or window frame is not covered by the titles or the pages we opened.
• Winter use, frost and outdoor exposure are not covered by the pages we opened.
• The pages we opened do not cover a specific window, its sun exposure or the light it receives.

Related reading

For a planter built to fit a deck railing rather than a windowsill, see a DIY planter box for a deck railing. For the soil volume and layout side of a raised planter, see how to fill raised beds.

FAQ

Is a self watering window box planter the same as a plain window box?

No. A self watering planter has a reservoir below the potting mix and an overflow hole instead of bottom drainage holes, so it draws water up from below. A plain window box has drainage holes and depends on you to water the soil from the top. In this roundup, eight titles say self watering and one does not.

Do self watering window boxes still need drainage?

They need a way for excess water to leave. The UMD page describes an overflow hole on one side that lets water out once the reservoir is full. The Illinois drainage page says drainage of excess water is vital for the health of plant roots, and that gravel in the bottom does not improve drainage. Check the overflow before you plant.

How often should I water a self watering window box?

The pages we opened do not give one interval for every reservoir. The Iowa State page says to water when the potting mix is dry at a depth of 1 to 2 inches for ordinary containers, and says frequency depends on the container size, the potting mix, the plant and the weather. Record how fast the reservoir drops for your box, and set your refill rhythm from that record.

Can cacti and succulents go in a self watering planter?

A plain box with good drainage suits them better.

How many boxes do I need for three windows?

Multiply the windows by the boxes you want in each one. Three windows with two boxes each need six boxes, which is one six-piece listing in this roundup. A two-piece listing would need three packs for the same plan. This is our planning estimate, and it does not account for boxes you want in a different arrangement.

Bottom line

Choose a self watering window box planter when its title says self watering, its stated size fits the sill with room to spare, and its pack count matches the number of boxes your window plan needs. Choose a plain planter only if you will water by hand, and skip any self watering design for succulents or cactus.

Sources

• University of Maryland Extension, “Self-Watering Containers” (updated March 4, 2024): the overflow hole instead of bottom drainage holes, the perforated platform above the reservoir, and wicking from the reservoir into the medium.
• Illinois Extension, “Container Drainage Options”: the gravel-layer myth, drainage of excess water, the warning about permanently attached saucers, and the note that plants needing dry conditions do not usually warrant a self watering container.
• Illinois Extension, “Self Watering Containers” (Hort in the Home Landscape, 2015): the reservoir at the bottom, the soil basket that acts as a wick, and the overflow hole.
• Iowa State University Extension, “Care of Plants Growing in Containers”: watering to the drainage holes, checking the potting mix at 1 to 2 inches, and the factors that set watering frequency.
• Iowa State University Extension, “Potting Media: Components and Handling”: the roughly 85 percent pore figure for high-quality potting media, cited from the facts file.