A self-watering planter, also referred to as a sub-irrigated planter (SIP), is a container designed to supply water to a plant’s roots from below rather than from above. This system incorporates a separate water reservoir beneath the soil chamber, eliminating the guesswork associated with traditional top-down watering. Maintaining consistent moisture promotes healthier root growth and minimizes the risk of overwatering or underwatering. This efficiency allows plants to thrive with less intervention, reducing the frequency of watering, sometimes only requiring a refill every one to three weeks.
Understanding the Self-Watering Mechanism
The self-watering mechanism relies on capillary action, or wicking, which is the ability of liquid to flow in narrow spaces against gravity. This action is driven by cohesive forces between water molecules and adhesive forces between the water and materials like soil particles or a wick. A self-watering setup consists of three components: the water reservoir, the growing medium, and a wicking element that connects them.
The reservoir is a sealed chamber at the bottom that stores the water supply. The wicking element draws water upward into the soil above it. As the soil’s moisture content decreases, capillary action pulls more water from the reservoir until equilibrium is reached, ensuring the soil remains evenly moist without becoming waterlogged.
Method A: Creating a Simple Wick Planter
The simplest DIY system uses a porous material as a direct wick between a plant container and an external water reservoir. This method requires two containers—one for the plant and one for the water—and a length of absorbent material, such as nylon cord or polyester felt. The upper container, which holds the plant and soil, must have a drainage hole large enough for the wick.
To construct this setup, soak the wick in water to initiate capillary action. Thread one end through the drainage hole and position the upper container over the reservoir so the wick hangs down into the water. Fill the upper pot with potting mix, embedding the top end of the wick deep within the soil near the plant’s root ball. The soaked wick draws water up, keeping the root zone consistently hydrated.
Method B: Building a Double-Container System
A more robust, larger-scale DIY approach uses a double-container design where the reservoir is built directly into the lower portion of the planting container. This system relies on a perforated false bottom rather than a thin cord. Materials needed include two containers (one slightly smaller to nest inside the other), a perforated cup or plastic barrier to create a wicking chamber, and a fill tube.
The inner container acts as the soil chamber and must be modified to create the wicking area. A perforated plastic cup, with holes drilled into its sides and bottom, is secured to the center base of the inner container. This cup serves as the main wicking point where soil contacts the reservoir water. The inner container is then nested inside the outer container, creating a gap at the bottom for the water reservoir.
A length of PVC or plastic piping is inserted through a separate hole in the inner container, extending up to the rim of the planter. This piping functions as the fill tube, allowing water to be added without disturbing the soil. A small overflow hole must also be drilled into the side of the outer reservoir container, positioned just above the maximum water level. This prevents overfilling and subsequent drowning of the roots. The wicking cup is filled with soil, and then the rest of the soil chamber is filled, completing the system.
Plant Selection and Ongoing Care
Self-watering planters are best suited for plants that prefer consistently moist soil conditions, as the system prevents the soil from drying out completely. Plants that require a distinct period of dryness, such as succulents, cacti, and arid-climate herbs like rosemary, may not thrive and can be prone to root decay. Excellent choices include:
- Tropical houseplants (e.g., peace lilies and ferns).
- High-water-demand herbs (e.g., basil and mint).
- Vegetables (e.g., tomatoes and leafy greens).
The main maintenance task is refilling the reservoir through the fill tube or by lifting the upper pot. Avoid overfilling the reservoir beyond the overflow point, as this can saturate the soil and lead to root rot. Over time, the continuous upward movement of water leaves mineral and fertilizer salts on the soil surface. To counteract this accumulation, occasionally flush the soil by watering heavily from the top until water drains out, washing the excess salts away.