Self-Watering Planters: Wick Height and Reservoir Size Guide

Understanding Wicking Systems

Self-watering planters use a simple but effective principle: water travels upward through fabric wicks from a reservoir below, delivering moisture directly to plant roots as the soil dries. This passive system reduces watering frequency by 3–5 times compared to traditional containers. The key to success lies in balancing three critical components: wick length, reservoir capacity, and soil depth.

Many people use wicking systems for container gardening, raised beds, and window boxes. The mechanism works because capillary action pulls water up through the wick material—typically cotton rope, shoelaces, or specialized fabric strips—into the surrounding potting mix.

Optimal Wick Height Dimensions

Wick Length Basics

Wicks should extend approximately 4–5 inches from the reservoir pipe or container bottom, with only about 1.5 inches protruding above the soil surface. A good potting mix will draw water 6–8 inches from each wick, meaning your soil layer needs sufficient depth to distribute moisture evenly. For window boxes and smaller containers, a 7-inch fill pipe allows you to add 9 cups of water (approximately 0.56 gallons) to the reservoir.

Soil Depth Requirements

Soil depth varies significantly based on what you're growing. Consider these guidelines:

• Planting seeds: 150–200mm (6–8 inches) of soil
• Leafy greens and herbs: 200mm (8 inches) minimum
• Root vegetables (carrots, beets): 350mm (14 inches)
• Shrubs and small trees: Add 300mm (12 inches) to the vertical height of the root ball

When you plant seedlings, roots typically extend downward as they become established, particularly when moisture is available below. By the time your reservoir empties, newly planted roots may have grown down by at least 120mm (4.7 inches). This means the maximum distance from the top of your reservoir to where seedling roots initially sit should be no more than 300mm (12 inches).

Reservoir Depth and Capacity

Standard Reservoir Sizes

Reservoir depth directly impacts how long your system can sustain plants between refills. A 120mm (4.7-inch) deep reservoir generally provides sufficient water for approximately one month before needing refilling. For window box systems, reservoirs typically measure 1.5 inches deep and hold 6–10 cups of water (approximately 0.5 gallons or more).

Larger systems may use 3.5-gallon paint pails nested inside 5-gallon containers, creating a reservoir capacity of approximately 3.4 gallons. The size you choose depends on your container dimensions, the number of plants, and how frequently you can refill.

Total Planter Depth

For raised bed systems, a total depth of 16.5 inches (including reservoir) is considered optimal. This typically means three rows of 6-inch boards with an actual width of 5.5 inches. Going shallower than 12 inches risks oversaturation and root rot, while depths exceeding 24 inches may reduce the soil's ability to wick moisture all the way to the top. If soil becomes too dry at the surface, you may need to irrigate from above occasionally.

Aeration and Water Balance

A critical component often overlooked is the aeration layer—the space between soil and water. In quality wicking beds, this layer measures a minimum of 18mm (0.7 inches) when the reservoir is full. This prevents soil from becoming anaerobic (oxygen-depleted), which can harm roots and create unhealthy growing conditions.

The aeration layer expands as water levels drop and narrows again when you refill. For this reason, it's important to allow the reservoir to almost fully empty before refilling. This cycle enhances soil aeration and ensures your system doesn't become waterlogged. If you're considering an automatic refilling system, use a moisture sensor near the base rather than a timer to maintain this healthy cycle.

Sizing Your System

Container and Plant Considerations

The number of wicks you need depends on three variables: container size, number of containers, and plant type. A 12-inch square container may need 2–3 wicks. Plants requiring less water (like rosemary and thyme) may use two wicks, while thirsty plants (kale, parsley, marigolds) benefit from three.

For window boxes 6–12 inches in width and height, a single 42-inch reservoir system can irrigate the entire box. Larger raised beds may require multiple reservoir sections or a single larger basin depending on total square footage.

Reservoir Sizing Formula

To determine reservoir size, consider:

• Total soil volume in your containers
• Number of containers being watered
• Desired interval between refills (typically 1–4 weeks)
• Plant water requirements (thirsty crops need larger reservoirs)

An 11-quart plastic bucket can water four 12-inch square containers for two weeks. Scale this ratio up or down based on your specific setup and climate conditions.

Practical Setup Examples

Window Box System

A 42-inch window box with a Wickinator-style reservoir (37 inches long, 5.75 inches wide, 1.5 inches deep) holds 6–10 cups of water. Wicks extend 4–5 inches into the soil, with only 1.5 inches protruding above the reservoir. Fill soil to 8–10 inches depth for herbs and leafy greens. This setup reduces watering frequency by 3–5 times.

Raised Bed System

A 450mm (17.7-inch) tall raised bed with 30mm (1.2 inches) of mulch provides 282mm (11 inches) of soil depth—suitable for herbs, leafy greens, root vegetables, and even some shrubs like blueberries, raspberries, and azaleas. The reservoir sits 120mm (4.7 inches) deep at the base, with an aeration layer above it.

DIY Wick System

For a budget-friendly approach, use cotton shoelaces as wicks. Tie a nut or bolt to one end, soak the full length in water, and place the weighted end in a central water reservoir (elevated higher than your containers). Use 2–3 wicks per 12-inch container depending on plant water needs. This system works well for small-scale herb gardens and vegetable seedlings.

Key Terms

Capillary action: The movement of water upward through a material against gravity, driven by adhesive and cohesive forces.

Aeration layer: The space between soil and water reservoir that allows oxygen to reach roots and prevents anaerobic conditions.

Wicking: The process by which water travels through fabric or soil via capillary action.

Sub-irrigated planter (SIP): A container system where water is delivered from below through a reservoir, reducing surface watering needs.

Common Mistakes to Avoid

Many gardeners make the mistake of filling reservoirs too frequently, which prevents the aeration cycle and can lead to root rot. Allow the water level to drop significantly before refilling. Another common error is using wicks that are too short—they won't deliver water effectively to the upper soil layers. Conversely, wicks pulled too high out of the reservoir won't maintain contact with the water.

Soil depth that's too shallow (less than 12 inches) causes oversaturation and limits root growth. Conversely, soil deeper than 24 inches may not wick moisture all the way to the top, requiring supplemental surface watering during dry periods.

Sources and Expertise

For detailed guidance on wicking bed construction, WaterUps – Optimal Height of Wicking Beds provides comprehensive specifications for soil layers, aeration, and reservoir depths. AlboPepper – Self-Watering Raised Bed Design offers practical guidance on total planter depth and soil wicking capacity. University of Kentucky – Wicking Containers: A Self-Watering Garden System provides research-backed information on reservoir capacity and system design.

According to horticultural guidance, "The root systems of most plants will extend down once they become established, particularly if the soil below contains more moisture. By the time the reservoir is almost empty, newly planted seedlings will have extended roots down by at least 120mm, which is the height of the water reservoir." This principle underlies the recommendation that maximum height from reservoir top to seedling root placement should not exceed 300mm.

A quantitative benchmark: A standard 120mm-deep reservoir provides approximately one month of water supply before requiring refilling, making it suitable for most home gardeners who can refill monthly or use it as a backup during travel.