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Putting together a well built wicking bed takes time and effort. You have your bed built, filled and irrigated, but should you apply mulch to it ? If so what type of mulch should you use? Are there any disadvantages to mulching a sub irrigated planter? We delve into the pros and cons of mulching your wicking bed...
Mulching wicking beds to a depth of 10cm (4 inches) is recommended to reduce evaporation. The best types of mulch are organic but do not take excess nitrogen to decompose. and therefore also feed the soil. Continuous improvement of soil structure is beneficial to wicking.
Advantages of Mulching Wicking Beds
Mulching Reduces Wicking Bed Evaporation
The primary advantage of mulching is to reduce water evaporation. Experiments show that 5cm of mulch provides around a 50 percent in moisture retention over the first three weeks after saturation. A 10cm layer provides even more improvement, with diminishing returns thereafter. The chart below shows how thicker layers show diminishing returns.
Keeping the top of a wicking bed dry gives a good moisture gradient in the bed, helping maximise the wicking height.
Mulched Raised Beds Need Less Weeding
Mulching helps prevent weeds from germinating within the bed in two ways:
by creating a dry environment in which seeds dropped on the surface struggle to germinate
by acting as a weed suppressant layer - providing a deep covering over any pre-existing weeds within the bed
By parting the mulch to plant your seeds, then returning it around the plant when established, you limit the exposure of the soil to the sun. Thus, only weed seed that happen to be right where you planted your crops have a chance of germination, and then only at the beginning of your growing cycle.
Mulch assists Temperature Control in Raised Beds
Young seedlings can be particularly sensitive to the rapid temperature changes you often get between day and night. Mulch acts as a insulation layer, just as loft insulation keeps your house temperature more even. This can prevent stunting of sensitive plants being germinated at one end of their temperature window. It can therefore also extend your growing season slightly.
Aesthetics
A mulch layer can look great, and add nice consistent look to a garden bed, particularly one with plants that are not yet established.
Fewer weeds, less pests eating your crops and a nice even look adds up to a great aesthetic!
Mulch Reduces Soil Erosion From Water Runoff
Soil erosion is rarely a problem in a raised garden bed, but mulching does reduced water runoff - it provides a layer to soak water into before it starts running, and a sacrificial layer to prevent soil and nutrients from following water channels.
Mulching a Wicking Bed can Suppress Disease
Given a wicking bed continuously waters from below, if it has not been filled to an adequate height, the surface can remain damp. This can cause issues with plants susceptible to fungal diseases. Tomatoes, zucchini, cucumbers, squash and pumpkins are all plants that prefer a lower humidity level for this reason.
A generous layer of mulch helps put a physical barrier between the leaves of the plants and the fungal spores a healthy living soil contains. It also helps prevent splash-up from the soil onto the lower leaves. We have found mulched beds to be much better in this respect.
Higher Yields
Plants like potatoes and tomatoes thrive in a wicking bed. But add in mulch and they do even better. Potatoes love to put out higher levels of tubers, which is why soil is typically piled up around them. Keeping the light off those upper tubers stops them going green, and is greatly assisted by mulch. Potatoes can even grow right there within the mulch layer.
Tomatoes and cucumbers love to put roots out from their stem wherever it is light deprived; more roots means more nutrition entering the plant, which means higher yields!
Mulching Improves Soil Structure and Nutrition
Using organic mulch is a great way to feed your soil. As an organic mulch breaks down and decomposes, it releases nutrition into the soil. Nitrogen is released from straw, clippings and wood based mulches, plus whichever trace elements those products consumed as they were themselves growing.
You need to consider that some tougher mulch structures such as hardwood chip will actually soak up considerable nitrogen in order to break down in the first place, before giving it back once decomposed, so we tend to steer clear of those types of mulches when growing nitrogen hungry vegetables.
The soil structure also benefits as worms feed on the mulch, and mix it's nutrition throughout the soil. Adding organic matter is always of benefit.
Even inorganic mulches can assist with nutrient retention by keeping the sun off the soil and allowing fungal structures to thrive. And reducing water runoff and soil erosion prevents nutrient runoff also.
Mulching Reduces Waste
Often, the materials used for mulch would have ended up as waste. Mulches like sugar cane straw are industry biproducts. Repurposing of organic waste around the home such as leaf mould and grass clippings can often save on kerbside collection. Shredded paper and carboard, whilst recyclable, are best reused locally from an energy perspective.
Disadvantages of Mulching a Wicking Bed
Like everything there are pros and cons to using a mulch. There are not many disadvantages, and most come down to correct usage.
Pests
Wicking beds are great at deterring many pests that like a wet environment given the surface stays dry. However occasionally, mulch can give some unwelcome pests a place to hide. This is not usually too much of a problem, but if you are troubled by any of them, it's worth taking a look at the table below which shows which types of mulch can be hospitable to pests:
| Woodchip | Pinebark | Straw | Clippings | Coir |
Termites | Y | N | N | N | N |
Ants | Y | N | N | N | N |
Mice | N | N | Y | Y | Y |
Cockroaches | N | Y | N | N | N |
Spiders | Y | Y | N | N | N |
Table 1 - types of mulch and their hospitality to pests
Also, mulch can potentially introduce unwanted pests that are living in the mulch that's transferred. The last thing you want to do is introduce snails and slugs to your wicking bed inadvertently! It's important to dry out mulch you source from elsewhere before putting it on the bed, and check through to see what you are putting on your bed.
Disease
Many plants that thrive in a wicking bed love to have mulch right up their stem. A good example is tomatoes - the little hairs on the stem turn into roots when underground, and the plant has evolved to take advantage of high soil line around its stem, and will even put down roots in the lower mulch layer if it finds nutrition there.
However some plants prefer not to be mulched right around the stem, and can be susceptible to disease. This applies to fruit trees (which can rarely be grown in wicking beds), but also to other plants with woodier stems such as chilli and capsicum (peppers).
Contamination
Introducing an organic mulch can also introduce any nasties that were impacting the mulch crop as it was growing, or any chemical treatment it was subjected to.
For example spraying a lawn with a "feed and weed" solution, then mowing it the next day may make those clippings questionable to be used in a mulch. Do you really want those chemicals leaching into your veggie soil?
Also, seeds or weed cuttings can be introduced - whether that be seeds of the mulching plant (e.g. grass seed), or weeds that were growing amongst it.
Plant Suffocation
When planting young seedlings, or seeds straight into the ground, it's vitally important to move the mulch away from the planting area until the plant is a sufficient height that all it's productive leaves clear the mulch layer.
Seeds will rarely propagate through a mulch - one of the advantages when it comes to weed suppression - but suppression of your own seeds is not what we are trying to achieve!
We typically pull the mulch back in rows when we plant, leaving it heaped up between the rows. When the seedlings are well clear we pull it back around them, and even add more.
Some plants that prefer a rockier environment do not respond that well to mulching; this doesn't mean the whole bed can't be mulched though - you can simply expose their root area a little more by pulling back the mulch.
Conclusion - Should You Mulch Your Wicking Bed?
All in all the advantages of mulching far outweigh the cons in our view. And using an organic mulch has such benefits to the soil health, we would choose it over an inorganic mulch every time.
Mulching a wicking bed with an organic medium reduces evaporation and water runoff, provides higher yields, improves soil condition, suppresses weeds, provides great temperature control and looks great. We recommend a 5-10cm mulch layer on wicking beds except when first planting out.
How Thick Should Mulch Be on a Wicking Bed?
Our experimentation indicated that 5cm of mulch provides around a 50 percent improvement in moisture retention over the a three week period. A 10cm layer provided a bit more retention, with diminishing returns thereafter. We therefore recommend 5-10cm of mulch on a wicking bed.
How Often should the Mulch be Replaced on a Wicking Bed?
Wicking beds mulched with straw, lucerne or sugarcane typically need topping up with mulch each time you plant out. Raised garden beds that are mulched with woodchip or other mulches with a slower decomposition time may go a year or more between top-ups.
