How Do Self Watering Planters Work and Why They Wick Water Up
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How do self watering planters work? Most designs hold a water reservoir beneath the soil and use a wick, or a soil-filled channel, to draw that water up to the roots. The process depends on capillary action, the same force that lets water climb through a paper towel. This article explains the mechanism step by step, the parts that make it work, and the limits that university extension sources describe.
Quick answer
- A self watering planter has a water reservoir at the bottom and soil above it.
- A wick or soil-filled basket connects the reservoir to the root zone.
- Capillary action pulls water upward from the reservoir into the soil.
- An overflow or drain hole limits how much water the reservoir holds, which prevents overwatering.
The basic structure
A self watering planter has two zones. The lower zone is a reservoir that stores water, often in a separate tray or the bottom of the container. The upper zone holds the soil and the plant. Between the two sits a path for water to travel, which is usually a wick, a porous material or a basket filled with soil.
The Illinois Extension blog describes the core idea in plain terms: “The idea behind a self-watering container is that there is a reservoir of water at the bottom of the container ready to keep the soil moist as needed.” The reservoir is what allows the plant to draw water over days rather than needing daily refills.
How capillary action moves the water
Capillary action happens because water sticks to the surfaces of small spaces and climbs through them. In a wick or soil channel, the narrow gaps between fibers or particles act like tiny tubes. Water is pulled into those gaps and moves upward as the soil above it dries. The University of Kentucky publication AEN-157 puts the mechanism this way: “plants are watered from the bottom up through the process of capillary action.”
The effect is that the soil stays moist from below rather than from the surface. The publication also notes that this changes the surface itself: “Since wicking containers water from the bottom up, the surface soil is drier, which reduces weed competition.” Water is pulled up as the plant uses it, so the system adjusts to demand instead of relying on a fixed watering schedule.
The soil basket and the wick
Many designs use a soil-filled basket that sits in the reservoir. Illinois Extension describes it directly: “This soil serves as a wick to pull the water up into the rest of the container.” The soil in the basket stays in contact with the water, and capillary action carries it into the larger soil mass above.
Other designs use a fabric strip, a cord or a porous material as the wick. The key requirement is continuous contact between the wick and the water, and between the wick and the soil. If the wick dries out or loses contact, water stops moving upward and the plant can dry out even though the reservoir still has water.
Why the drain or overflow hole matters
A reservoir that fills too far can keep the soil waterlogged and starve roots of air. Extension designs handle this with a hole that sets the water level. The Illinois blog says so directly: “There is also an overflow hole in place to make sure the reservoir doesn’t get too full.” The Kentucky publication makes a related point: “Overwatering of plants is controlled by the level of the drain hole.”
In practice, the hole sets the maximum water level, so the gardener only needs to refill the reservoir up to that line. That is the main reason self watering planters tend to be more forgiving of irregular watering than an ordinary pot.
What affects how long the reservoir lasts
Reservoir size, plant size, temperature and light all change how fast water moves. A large reservoir supports a plant for longer between refills, and a hot, sunny location increases evaporation and uptake. The sources describe the principle but do not give a universal refill schedule, so gardeners should check the reservoir by hand and adjust.
The Kentucky publication also describes a benefit in water efficiency: watering from below reduces evaporation from the soil surface. That effect depends on the design and the climate, and it does not mean the planter needs no attention at all.
Checking a self watering planter by hand
Because the reservoir is out of sight, the simplest check is to look at the water level through the fill opening or by lifting the container. A reservoir that is nearly empty for several days in a row suggests the plant is using water quickly, which may call for a larger reservoir or a more shaded spot. A reservoir that stays full while the soil stays wet may mean the drain or overflow hole is blocked or the soil is too heavy.
Soil moisture can also be checked by pushing a finger into the top layer. Because water is supplied from below, the top may look drier than the root zone, so checking a few centimeters down gives a more accurate reading than the surface alone.
How we chose
The sources in this article were chosen because they could be opened and read at the time of checking, and because they come from university extension services. Only statements found in the fetched text are quoted. Commercial and blog pages that appeared in search results were not used as sources, and claims about precise percentage reductions in plant problems were left out because they could not be confirmed.
What we could not verify
Several figures that appear in commercial material about self watering planters were not confirmed by the extension sources. These include specific refill intervals in days, percentage gains in water efficiency and the claim that wicking reduces root disease. Those points are not stated in the checked text and are not quoted here.
What the sources do not say
The sources do not give a fixed refill schedule, because refill timing depends on reservoir size, plant size, climate and whether the planter is indoors or outdoors. They do not give a measured height that water can climb in every soil type, and they do not compare self watering planters with standard containers under controlled conditions. Those questions need field data that the sources used here do not provide.
Frequently asked questions
What is capillary action in a self watering planter?
Capillary action is the movement of water through narrow spaces, driven by the way water adheres to surfaces. In a self watering planter, it pulls water from the reservoir up through the wick or soil basket into the root zone.
Why does the soil stay drier on top?
Water enters from below and is drawn upward as the plant uses it. The Kentucky publication notes that this keeps the surface soil drier, which reduces weed competition.
How do I keep a self watering planter from overflowing?
Fill the reservoir only up to the level set by the overflow or drain hole. The Illinois Extension blog and the Kentucky publication both describe the hole as the control that prevents the reservoir from getting too full.
Can I make my own self watering planter?
Yes. The Illinois Extension blog describes a homemade design using a basket filled with soil that sits in a reservoir. The same principle applies to other homemade versions that use a wick or soil channel.
Do self watering planters work for every plant?
The sources describe wicking containers for vegetables, fruits, herbs and flowers, but they do not rank plant types by suitability. Plants that need very dry roots may do poorly if the reservoir stays full, so the drain hole and soil choice matter.
For more on container growing, see how to grow herbs in pots and how to grow lettuce in pots.
Sources
- University of Kentucky Extension, AEN-157, “Self-Watering Wicking Container,” https://publications.ca.uky.edu/files/AEN157.pdf (opened 2026-10-27).
- University of Illinois Extension, “Water less with self-watering containers,” https://extension.illinois.edu/blogs/know-how-know-more/2017-06-27-water-less-self-watering-containers (opened 2026-10-27).
