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How you fill your wicking bed is critical. Wicking beds are a relatively new concept, working on the principle of sub-irrigation. Watering from below. Once you have installed a water reservoir in the bottom of the bed, what is the best choice of fill? Soil? Potting mix? Other additives? The growing medium used is key to a wicking bed working effectively. This article explains which materials to fill a wicking with, and more importantly where to put them.
It is best to fill wicking beds with a highly porous potting mixture with a large proportion of organic matter. Normal garden soil should be avoided if it contains silt. Additional anti-compaction and anaerobic control particulate is also beneficial.
Read on to get the lowdown on how to create the perfect wicking mix.
Can I Use Garden Soil in a Wicking Bed?
When you go to the trouble of carefully building a wicking bed, you realise that a significant proportion of the cost can be the filling mixture. It can be tempting to simply fill it with garden garden soil. But is that the right thing to do?
Garden soil should generally not be used in a wicking bed. Potentially it can make up part of the mix, but typical soils contain silt which can clog up the reservoir over time and reduce the capacity of the bed. Soil is also prone to compaction which is the enemy of wicking. Good aeration is necessary to facilitate wicking and prevent anaerobic decomposition.
If you do need to use soil, at the very least mix it at least 50:50 with some potting mix and other materials to increase aeration. But we find it's best to avoid it altogether. The sand content can also work against you in assisting compaction.
So What Materials Should Be used To Fill A Wicking Bed?
The components of a wicking bed mix need to address several considerations, listed here broadly in order of importance :
Facilitating wicking
Water retention
Preventing compaction
Aeration
Discouraging anaerobic decomposition
Slowing evaporation
Keeping the reservoir from silting up
Key materials that work well together in wicking beds are organic potting mix, organic composts and manures, pine bark mulch, vermiculite , perlite, scoria, bio-char, and good quality mulches. We wrote an article extensively about mulch - maybe check it out for more information.
The 5 Layers of a Wicking Bed
Urban Veggie Crew have installed dozens of wicking planters over the past few years. Over this time we have found that a layered approach works best. The considerations listed above apply at different layers. For example, limiting evaporation only applies to the top; limiting anaerobic decomposition only near the bottom. So by varying the mixture in those layers we get the best results.
Reservoir Layer
This is the bottom layer of the bed, and in our view should be kept clear of organic matter with the possible exception of charcoal. It's sole purpose is to store water, making it available to higher layers.
We use specialised filled wicking cells with a geotextile above which gives a clear barrier between the reservoir layer and the soil above..
The feet of the wicking cells should not contain any organic matter, as it will eventually decompose and either become non-useful to the wicking process, or cause anaerobic decomposition making the water smell. Charcoal additives can assist with this to a certain extent, keeping things sweet, but better to not allow any organics in this layer in the first place really.
We fill the feet with a mixture of scoria and fine perlite particles. Both are great at wicking, but the additional contact surface of fine perlite makes it many times more effective for wicking than scoria alone.
Wicking Transfer Layer
This is a thin layer just above the wicking cells. It's three purposes are:
to spread out the water wicked up from each wicking column below and give good even water coverage to the base of the layer above
to allow excess water to drip back down into the reservoir preventing over-saturation
to give a good area of exposure to the air gap above the water
This layer as only around 15mm and comprises:
geo-textile placed above the wicking reservoir in good contact with the perlite stacks
a few charcoal pieces spread about to limit anaerobic decomposition
a 50/50 mixture of potting mix and coir
A generous sprinkling of fine perlite
Root Layer
This layer receives an even spread of water from the transfer layer below and moves it up the bed. However, where shallow rooted vegetables are grown, it can remain largely undisturbed by roots . So it's important to prevent compaction here.
Rather than just treating this as a bulking up layer, we choose to use it as an opportunity to provide nutrition for future growing seasons.
So we place here a mixture of pine bark mulch to prevent compaction, with manure to provide nutrition to facilitate the decomposition of the mulch. This prevents the mulch from robbing nitrogen from higher levels as it decomposes.
The symbiosis of the pine bark mulch with the manure works well- one prevents compaction, whilst the other is prone to it. One uses nitrogen, the other provides it.
In terms of additional fertilisation, this layer is where we put some trace elements from a seaweed based pelletised organic fertiliser, and more importantly phosphorus for good root and tuber development.
To assist with wicking through this layer, it is also crucial to add plenty of potting mix and a generous amount of perlite.
Planting Layer
This is the layer of a wicking bed that seeds are planted into. It therefore needs relative fine particulate, plenty of nitrogen and good water retention.
Here we use lots of manures and composts for nitrogen, and a little perlite for that final bit of wicking. But more importantly here we want to encourage water retention. So we also use coir together with vermiculite, which holds on to any water that reaches it, making it available for root transfer. The coir we use here is a fine grained mix which also assists with seed contact, although it is quite light on nutrition in itself.
Manures provide plenty of nitrogen, with more being released from the lower layers in future seasons. But in terms of additional fertiliser, we recommend adding potassium here for fruit growth plus some additional seaweed based pellets for more trace elements .
Mulch Layer
This is a dual purpose layer: it drastically reduces the rate of evaporation from the bed lowering water use, and it provides nutrition to the soil over time. Where worms are used in the bed (and we strongly recommend them), they will have a constant supply of food from the mulch.
We therefore recommend an organic mulch rather than using stones or other inorganic materials. Lucerne and Pea straw are great, but sugar cane mulch works just as well.
It's important to part the mulch down to the growing layer when planting seeds, and pull the mulch back around once the seedlings are established.
A third reason to mulch a wicking bed is to reduce fungal disease on plants like cucumbers, tomatoes, squash and pumpkins due to splash back from soil when it rains.
There's a lot more info about mulch in our "Should you mulch a wicking bed?" article - we recommend checking it out if you want to know more.
Table of Wicking Materials by Layer
Material | Reservoir Layer | Transfer Layer | Root Layer | Planting Layer | Mulch Layer |
Scoria | Y | N | N | N | N |
Perlite | Y | Y | Y | Y | N |
Charcoal | Y | Y | N | N | N |
Coir | N | Y | Y | Y | Y |
Potting Mix | N | Y | Y | Y | N |
Pine bark | N | N | Y | N | N |
Manure | N | N | Y | Y | N |
Vermiculite | N | N | Y | Y | N |
Compost | N | N | N | Y | Y |
Lucerne | N | N | N | N | Y |
Sugar Cane | N | N | N | N | Y |
Peastraw | N | N | N | N | Y |
Should I use Coir in a Wicking Bed?
Coir is ground up coconut by-product and available either as ground up particulate or as dried blocks of a more stringy medium that requires hydrating to break up and use.
Coir is excellent for use in wicking beds as it has great water retention and wicking properties. It's best used in the growing layer for good water retention high up in the bed, and also just above the reservoir to assist water transfer.
Should I use Perlite in a Wicking Planter?
Perlite is a hard, crumbly volcanic rock that is superheated until it explodes like popcorn, leaving a safe, lightweight material with great properties.
Perlite is an excellent wicking medium and an essential part of most wicking mixtures. It's stability means it is very useful near the bottom of a wicking bed in the reservoir as it will not decompose. However it greatly assists wicking when also used higher up the bed.
Should I use Vermiculite in a Wicking Bed?
Vermiculite is a soft, spongy volcanic rock which looks rather like a packing material.
Vermiculite has excellent water retention so is perfect for use higher up in a wicking bed to help retain the wicked water for root use. It's wicking properties are not as good as those of Perlite, but it is great at preventing compaction and making water available to plants.
How Should I Mulch My Wicking Bed?
There are many materials that can be used for mulching, from inorganic stones and pebbles to woodchip, straw, leaf mould or event compost.
Wicking beds that are mulched have slower evaporation and therefore retain water for longer. This means more water available higher up the bed to young plants, and less drying of soil. Organic mulches are recommended to feed worms and microbial life and improve the soil. They also help replace nutrition in your bed and reduce fungal disease.
There's more on this in our mulching article, and more on watering here
How High Should I Fill My Wicking Bed?
Tall raised beds are growing in popularity, but can be expensive to fill. But they really save your back when gardening. But how tall can a wicking bed be?
A wicking bed should only be filled around 12-15 inches (30-40cm) above the water line. This is because water only wicks up around that far in most wicking mixtures. Whilst a wicking bed can be any height, it is important that the top of the reservoir be only that height from the bottom of the mulch. Taller beds therefore required subfloors or have their lower section filled under a liner.
We have built many taller beds and typically use a subfloor. See our Projects page for examples.
