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Gardening glossary

Capillary action

Capillary action is the phenomenon that lets water defy gravity in narrow tubes and porous materials. It happens because water molecules are attracted to many surfaces (adhesion) and to each other (cohesion). In a narrow enough channel, those forces combine to pull a water column upward, against gravity, until it reaches an equilibrium.

In a garden, capillary action is invisibly doing several jobs at once:

- **Water moving from the bottom of a pot upward** when you bottom-water a houseplant. The dry soil pulls water up by capillary action until the substrate is saturated. - **Rewetting hydrophobic compost.** When a peat-based mix has dried out and beads water off the surface, soaking the pot from below uses capillary action to slowly rehydrate it from the inside. - **Water moving from deeper layers to surface roots** during a dry spell. As surface roots take up water, deeper water rises by capillary action to refill the upper layers. - **Xylem transport.** Inside the plant itself, capillary action contributes (alongside the transpiration stream) to pulling water from roots to leaves through microscopic xylem vessels. - **Wick watering systems.** A length of wick or twisted cotton in contact with both a reservoir and the soil moves water continuously upward by capillary action.

What controls capillary rise:

- **Pore size.** Smaller pores pull water higher. Fine silt and clay soils have stronger capillary action than coarse sand or gravel — which is why water rises further in a clay pot's soil than in a sandy potting mix. - **Material wettability.** Hydrophobic surfaces (greasy, waxy, or fully-dried peat) block capillary movement. This is why a bone-dry hanging basket can sit in a tray of water and remain dry inside. - **Surface tension.** Anything that reduces surface tension (some surfactant additives, "wetting agents") improves capillary movement in stubborn soils.

Practical takeaways for home gardeners:

- Rehydrate a dried-out pot by soaking from below for 20–30 minutes; trying to top-water often fails because the water channels straight down the sides. - Avoid a "drainage layer" of gravel in pots — see [the drainage-layer myth entry](/glossary/drainage-layer-myth). It interrupts capillary continuity at the bottom of the pot. - Self-watering pots, wick systems, and capillary mats in greenhouses all rely on this single physical principle.

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Related terms

Wicking

Wicking is the use of a porous strip — usually cotton or synthetic cord — to move water from a reservoir into a pot via capillary action. The plant draws what it needs and stops, eliminating the over- and under-watering swings of conventional watering.

Bottom watering

Bottom watering is the practice of setting a potted plant in a tray of water so the roots draw moisture upward through the drainage holes, hydrating the soil evenly without wetting the foliage or crown.

Self-watering pot

A self-watering pot is a planter with a built-in water reservoir below the soil chamber, separated by a wicking platform or capillary column. The plant draws water from the reservoir as needed, smoothing out the over- and under-watering cycles of conventional pots.

Soak and drain

Soak and drain is a top-watering technique where you water thoroughly until liquid runs freely from the drainage holes, then let the substrate dry to the appropriate depth before watering again — never on a fixed schedule.