Wick Systems
Part of Irrigation
Wick irrigation — also called capillary irrigation or sub-irrigation — delivers water from a reservoir below or beside the root zone upward into the growing medium through capillary action, the same force that draws water up a towel placed at the edge of a bowl. Wicking beds use this principle at garden bed scale, providing a constant supply of moisture exactly at root level without surface wetting, evaporation, or overwatering. For water-scarce environments or gardeners who cannot irrigate daily, wicking beds can cut water use by 50–80% compared to surface watering while producing reliable, high-quality yields.
The Physics of Capillary Action
Water molecules adhere to fine particles and tube surfaces (adhesion) and to each other (cohesion). In fine-pored growing media, these forces pull water upward against gravity. The maximum height water can wick upward depends on pore size:
| Material | Maximum Wick Height |
|---|---|
| Fine sand | 30–60 cm |
| Sandy loam | 40–80 cm |
| Good potting mix | 25–40 cm |
| Coarse gravel | 2–5 cm (too coarse to wick) |
| Sphagnum moss | 30–50 cm |
| Coconut coir | 20–40 cm |
This physical limit defines the maximum height of the growing medium above the water reservoir. A wicking bed with 30 cm of potting mix above the water will reliably wick to the surface during the growing season. More than 40 cm of growing medium and the top layer risks drying out.
Wicking Bed Design
A wicking bed consists of:
- A waterproof container or lined bed
- A water reservoir at the base (filled with coarse aggregate or empty space)
- A divider or upward-wicking transition layer separating reservoir from growing medium
- Growing medium above the transition
- An inlet pipe for refilling the reservoir
- An overflow outlet at the maximum reservoir level
Standard Dimensions
| Component | Dimension |
|---|---|
| Total bed depth | 40–55 cm |
| Reservoir depth | 15–20 cm |
| Growing medium depth | 25–35 cm |
| Transition layer | 5–8 cm coarse aggregate or mesh |
| Inlet pipe diameter | 25–40 mm |
| Overflow outlet height | At top of reservoir zone |
Keep Growing Medium Under 35 cm
If the growing medium layer exceeds 35 cm, the capillary action cannot reliably wet the top 5–10 cm consistently. In dry weather the top layer dries and seeds fail to germinate. Keep growing medium between 25–35 cm for reliable wicking to the surface.
Reservoir Construction Options
Option 1: Aggregate-Filled Reservoir
Fill the base of the bed with coarse aggregate (20–40 mm clean gravel) to the intended reservoir depth. The aggregate holds water in the void spaces between stones. Water surface sits at the aggregate surface level. Growing medium is placed above.
Pros: Simple, durable, no materials to rot Cons: Heavy; the aggregate itself displaces water volume
Water volume stored in aggregate-filled reservoir: approximately 35–40% of aggregate volume (the air void fraction).
A 2 m × 0.6 m × 0.18 m aggregate layer: volume = 0.216 m³; water stored = 0.216 × 0.37 = 80 litres
Option 2: Open Water Reservoir with Mesh Divider
Build the reservoir as an open water zone (no aggregate) divided from the growing medium by a mesh, perforated shelf, or woven fabric stretched across the bed. The growing medium sits on the mesh and wicks water from the open water below.
Pros: Maximum water storage per unit volume; easy to see water level through a sight tube Cons: Requires a structurally sound divider that will not sag under the weight of wet soil
Mesh divider construction: Stretch 10 mm hardware cloth (wire mesh) across the bed at the reservoir height. Lay a single layer of non-woven geotextile on top of the mesh to prevent fine soil migrating through. Place growing medium on the geotextile.
Water stored in open reservoir: 100% of volume. A 2 m × 0.6 m × 0.18 m reservoir = 216 litres.
Option 3: Fabric Wicks into Separated Reservoir
The growing medium and reservoir are in separate containers connected by fabric wicks — strips of material extending down from the growing medium into the water reservoir. Suitable for containers, pots, and small raised beds.
Wick material: Old cotton T-shirt strips, thick felt, cotton rope, or commercial capillary matting. Synthetic fabrics that have been “pre-wetted” also work.
Wick sizing: One fabric wick per 10 cm × 10 cm of bed base area is sufficient in most growing media.
Self-Watering Containers
Small-scale wicking applies directly to container gardening. A self-watering container has a lower reservoir and an upper growing section, connected by wicks or a perforated base.
Recycled Container Method
Materials: Two containers of the same diameter (e.g. two 20-litre buckets), fabric strips, drill, gravel.
- Drill 10–12 holes in the base and lower walls of the inner container (the one that will hold soil)
- Fill the inner container with growing mix, packing firmly
- Thread 3–4 fabric wick strips through holes in the base, extending 8–10 cm below
- Place a layer of coarse gravel in the bottom of the outer container
- Rest the inner container inside the outer, sitting on the gravel layer so the wicks reach the water zone
- Cut an overflow hole in the outer container at the maximum intended water level
- Insert a fill tube — a short pipe through the outer container side above the water level — for refilling
Performance
| Container size | Growing medium volume | Reservoir volume | Refill frequency (hot weather) |
|---|---|---|---|
| 10-litre pair | 7 L | 3 L | Every 2–3 days |
| 20-litre pair | 15 L | 7 L | Every 3–5 days |
| 40-litre tub | 25 L | 15 L | Every 5–7 days |
Growing Medium Composition
Not all growing media wick reliably. Media must be fine enough to support capillary action but not so fine they become waterlogged. Ideal wicking bed mix:
| Component | Proportion | Function |
|---|---|---|
| Compost | 50% | Nutrients, fine pore structure |
| Sand (fine-medium) | 25% | Drainage, capillary support |
| Perlite or vermiculite | 15% | Aeration, pore uniformity |
| Coconut coir or peat | 10% | Water retention, capillary support |
Avoid using pure garden soil — it compacts, reduces capillary function, and can introduce waterlogging in the reservoir zone.
Test your mix before filling: pack it into a 30 cm tall container, place the base in 3 cm of water, and observe how high water rises in 12 hours. Good wicking mixes show visible moisture migration to at least 20 cm height.
Pre-Wet the Growing Medium
Wicking works best once the growing medium is pre-moistened. When setting up a new wicking bed, fill from the top with water first to wet the medium thoroughly. Then begin filling via the inlet pipe. A completely dry growing medium may take 24–48 hours to establish the capillary connection to the reservoir.
Inlet Pipe Design
The inlet pipe allows refilling the reservoir without disturbing the growing surface.
- Use 25–40 mm PVC pipe with a cap or mesh screen at the top
- Position the pipe at one end of the bed, running from surface level down to the reservoir zone
- Angle the bottom of the pipe horizontally inside the reservoir to spread incoming water
- Mark reservoir capacity on a stick inserted alongside the inlet pipe — check weekly and refill when reservoir drops to 20% remaining
Overflow Design
An overflow pipe or hole prevents the reservoir from overfilling and waterlogging the growing medium.
- Drill or cut an overflow hole in the bed wall exactly at the maximum reservoir level
- In rainy climates, route overflow to a drain or adjacent garden bed
- In dry climates, cap the overflow and rely on the level stick to avoid overfilling
Suitable Crops for Wicking Beds
Wicking beds suit crops that prefer consistent moisture but not waterlogging:
| Crop Category | Examples | Performance |
|---|---|---|
| Leafy vegetables | Lettuce, spinach, chard, kale | Excellent |
| Herbs | Basil, parsley, chives, mint | Excellent |
| Fruiting vegetables | Tomato, capsicum, eggplant | Good |
| Root vegetables | Carrot, beetroot, turnip | Moderate (deep roots) |
| Brassicas | Cabbage, broccoli, cauliflower | Good |
| Cucurbits | Zucchini, cucumber | Moderate |
Avoid in wicking beds: plants requiring dry root conditions (lavender, rosemary, most Mediterranean herbs in high rainfall areas) and deep-rooted crops in beds with shallow growing medium.
Salt Accumulation Management
The one significant drawback of wicking beds is salt accumulation. Because water is drawn up from below and evaporates from the soil surface, dissolved salts from the water and fertiliser concentrate at the top of the growing medium over time.
Symptoms: white crust on soil surface; plant leaf edge burn; reduced germination at the surface.
Prevention and correction:
- Water from the top occasionally (once per month) with several times the reservoir volume to flush salts back down
- Use low-salt fertilisers (compost tea, worm castings) rather than synthetic salts
- Scrape and discard the top 2–3 cm of growing medium after each crop and replace with fresh compost
Wick Systems Summary
Wicking beds supply water continuously to plant roots from a reservoir below, using capillary action through fine-pored growing medium. The reservoir depth (15–20 cm) and growing medium depth (25–35 cm) are critical — deeper growing medium exceeds the wicking height and creates dry upper layers. Aggregate-filled reservoirs are simple and durable; open water reservoirs with mesh dividers hold more water per unit volume. Self-watering containers using two nested buckets allow small-scale application without construction. Use a compost-heavy, fine-textured growing medium and pre-wet before relying on wicking. Manage salt accumulation with monthly top-flushing. Water savings of 50–80% over surface irrigation, with minimal daily attention required, make wicking beds one of the most productive per-area food production methods available in water-scarce situations.