Mulching - Why and How

Many food gardeners have strong opinions about mulching and garden writers report heated discussions on the subject.  Not here.  This blog post will hopefully be an interesting but uncontroversial read.  It might even change the way you use mulch in your food garden.

Why mulch?

• The number one aim of mulching is to keep the root zone of plants moist and to keep the temperature of the soil down, so plants survive and thrive. Because moisture is retained rather than lost through evaporation, less watering is needed and that saves time, water and money.

• If applied in a thick enough layer mulch is a great way to suppress weeds.

• Good mulches perform a third role: they gradually break down and improve soils.

Four approaches to mulching

Having visited many Tasmanian food gardens over the years I see four practices in relation to mulching:

Approach 1: no mulch

Some food gardeners don’t use mulch at all.  People feel there is no need, or that it is too much work, too expensive, too untidy (the stuff blows everywhere), it introduces too many weeds, or is attracting too many slugs and snails. If this is you, I invite you to keep reading. I hope you will change your mind because the top five centimetres of your soil, an important area for most plants, may have become a zone where there are no microbes and other soil organisms to help plants take up nutrients.

Approach 2: no mulch in winter

Some people put mulch on their garden when the weather warms up in spring, and remove it and put it on the compost heap, when it gets cool in autumn. The thinking behind this is as follows:

• In Tasmanian winters little or no moisture evaporates from soils
• Uncovered soil heats up quicker in spring, so plants take off quicker
• When removing the mulch take away hiding places for overwintering slugs and snails

I reckon that half a century ago this would have been a perfectly fine approach. 

Unfortunately our climate in winter has become consistently drier and warmer over time. 

In recent years I have frequently seen situations in which soils dried out in winter. For me, no mulch in winter no longer works.

Approach 3: mulch all year round

Some people have mulch on their garden all year round and make sure no soil is ever exposed to the sun. They add mulch whenever and wherever needed, to keep their soils covered. 

One determined advocate of this approach is Letetia Ware. Her thinking is that the healthier your soil, the more nutrient-dense your crops.  Soil is healthy if the micro-organisms in that soil are healthy. Therefore never allow your soils to dry out because micro-organisms will die and you may end up with ‘dead soil’ in which plants will find it much harder to find the food they need. 

For more on the central role of micro-organisms in soil health and fertility see for instance Twelve Simple Food-gardening Practices at http://foodgardengroup.blogspot.com.au/2014/09/twelve-simple-food-gardening-practices.html )

Approach 4: very thick mulch all year round

Some people cover their soils all year round with a 20-centimetre-thick layer of hay and top it up to that level whenever needed. Several of our members use this method and are very enthusiastic about it.  The plan is to discuss Deep Hay Mulching in a separate future post on this blog.

How to mulch

• Make sure your soil is nice and moist and then put mulch on it. Ideally you mulch straight after rain or after watering has made the root zone of plants moist. Check moisture levels with your finger. For most vegetables at least the top five centimetres of soil needs to be moist.

• Always be generous with mulch: as long as you make sure that branches and leaves of plants are exposed to the air you can’t put on too thick a layer if within that layer there are small cavities for ventilation that allow moisture from above through to the soil. A nice thick layer of mulch of at least 7 centimetres will deliver.

• Problems with mulch going everywhere because of wind or blackbirds or your dog or whatever, can be countered by making mulch wet immediately after putting it on the soil, or making it wet before spreading it, and/or covering it with mesh (first photo below). In my garden animals are no longer interested in areas covered with this mesh and it also keeps mulch in place on windy days.  I also partly dig gutter-guard into the edge of raised beds, so mulch no longer spills onto paths (second photo below).

With mesh on mulch animals are no longer interested 

Gutter guard to keep mulch from spilling onto paths

• Consider putting mulch on top of irrigation lines. This is done a lot on the mainland where avoiding evaporation is even more critical.  It also means that you don't have to worry that overhead irrigation or rain may not be able to get through a thick layer of mulch.

How not to mulch

• Don’t mulch dry soil as this helps to preserve that dry situation.

• Don’t mulch with materials (eg. fresh grass clippings) that will become a dense layer and form a barrier to moisture from above. Fresh grass clippings will aid composting if loosened up and dispersed in and over a compost heap, so use them there, not as mulch.

• Mulching with materials that do not break down is not recommended for food gardens. Most of the self-sown plants in my garden are in my pebble paths and they love it there, so you could argue that pebbles are a great mulch. However, pebbles add nothing beneficial to the soil underneath and become a nuisance when mixed with soils. Weed mat suppresses weeds, but does not really prevent evaporation. It is not considered to be an effective mulch.

• Don’t apply mulch thinly. It may look fine, but it may not actually make much difference. Do you want a ballpark figure? A layer of mulch of less than five centimetres thick, if it is an organic material, will not do the job.

• Compost or manure can do a good job as mulch, but manure will be much more beneficial if it is composted first, and compost will be much more beneficial if it is put in soil, rather than on top of soil, so my advice is ‘don’t mulch with compost or manure’.

• Some mulches naturally contain toxic substances that should be avoided in food gardens. For example, Eucalyptus and She-Oak contain and deliberately release into the environment substances that act as germination or growth inhibitors for other plants. Do not use them fresh on your food garden.  These mulches are safe to use after they have been composted (see Aged Woodchips further down in this post). For more info see for instance How Plants Suppress Other Plants at http://edis.ifas.ufl.edu/hs186. 

• Some mulches contain residues of toxic substances added by producers. Treated pine bark is a good example of a mulch that should be avoided in fruit and vegetable areas.  The only way to make sure that hay and straw do not contain long-lasting broad-leaf herbicides is to buy certified organic products, or to buy directly from a farmer that you know does not use them. You can be certain that lucerne hay or pea straw don’t contain long-lasting broad-leaf herbicides because they are broad-leaf plants themselves and these herbicides would kill them. Mushroom compost, often used as a mulch, will contain chemicals if it comes from a non-organic producer. I recommend that you compost mushroom compost and don't use it as a mulch.  Paper, cardboard, plastic and old carpet do a great job as mulch, but all contain chemicals that may leach into your soils, so are best avoided in food gardens.

Mulch as a soil improver

The purpose of mulch is to keep soil moist and suppress weeds. All mulches do this if applied sufficiently thickly. However, an additional purpose of mulch can be to change soil conditions. 

Improving soil for blueberries

Blueberries do best in acidic soils with a pH of between 4 and 5. In comparison, on average, vegetable garden soil is best if its pH is 7. Once in a while I collect pine needles in a pine plantation. In mid winter I mulch my blue berry patch with a thick layer of pine needles. Over the next year they will help with soil moisture, keep the weeds down AND gradually break down and bring down the pH of the soil in this bed.

Blueberries with a pine needle mulch

Improving soil for asparagus

Asparagus prefer a pH of between 7 and 8, which can be hard to achieve with Tasmanian soils. After a storm in mid winter I collect a few bags of sea grass and sea weed from a shore somewhere (this is allowed if collection is not regularly and not in commercial quantities) and put this on my asparagus patch. This type of mulch will help to retain soil moisture, suppress weeds AND gradually increase pH to ideally between 7 and 8.

Asparagus plants mulched with seaweed and seagrass

Improving soil for fruit trees

Most garden soils are dominated by bacteria and that is fine for most annual vegetables.

However, fruit trees do best in soils that are dominated by fungi.  

A good way to achieve this over time is to mulch fruit trees with shredded prunings of bushes and trees in your garden (low in nitrogen and high in carbon).
I allow fresh garden pruning (except anything that has thorns) to go brown for a week or so, then I shred them, and put them in thick layers under fruit trees.  This mulch does not cost anything, it does not blow away, and it encourages fungi.

Shredded branches and leaves under my cumquat tree

If you don't use mulch, you forego the benefits of mulch as a tool to adapt soil conditions.

Recommended mulches for food gardens

Let’s have a look at mulches that are available in Tasmania (either you collect them or buy them) and which are good to use in various parts of food gardens.  The list below is in alphabetical order.

Aged woodchips

• Aged wood chips are untreated pine or hardwood chips that you put on a heap in a shady spot of the garden, keep moist, occasionally fork over to help aeration, and allow to partly decompose.  In the process toxins they contained when they were fresh are broken down.  For pine chips allow decomposition for at least a year.  For Eucalyptus chips the period is two years.
• Wood chips aged in this manner are excellent as a mulch under fruit trees.  There might well now be fungi in them.  They will certainly encourage fungi, and that will change soil conditions to the benefit of fruit trees.
• One should never use fresh wood chips or treated wood chips anywhere in food gardens.
• Aged wood chips are not really the right mulch for vegetable beds.

Green manures

• Green manures can be an effective way of mulching and improving the nutritional content of soils.
• Green manure is the confusing name of a process.  You sow oats or lupin or a combination of deep rooting plants (don't need to be edible) in a vegetable bed, allow them to germinate and develop, and then, just before these grasses or plants go to seed and look lush and green, you dig them in, or cut them off at ground level.  If you allow the plants to stay on the ground you are using the plants as a mulch.
• Green manure will only cost you the seed you buy so it is cheap compared to nearly all other mulches.
• Green manure is rich in nitrogen and therefore encourages bacteria.  It is great for vegetable beds.  It should not be used under fruit trees where fungi-dominated soil is what is needed.
• Inspirations Garden Centre is a Tasmanian supplier of a range of green manure seeds that suit the Tasmanian climate.

Hay

• Hay and straw are very different things.  Hay is dried grass, whereas straws are the stalks of wheat, rye, oats or barley.
• Good hay improves soil a lot more than straw does.  Straw is grain stalks after the grain has grown seed.  The quality of a bale of hay depends for instance on how healthy the paddock was it came from and whether the weeds among the grasses are nutritious ones. Hay is best cut just before grass goes to seed as it contains the most nutrition at that time (this explains why straw is not much more than carbon).  Some grass will be cut at the right moment.  Other paddocks will be cut when grasses and weeds have already gone to seed (and of course they don't all go to seed at exactly the same time).  Some hay will be of excellent quality.  Other hay will be mediocre and full of grass and weed seeds.  The best bales will be bought directly from a farmer you trust.
• Hay will always contain some weed and grass seeds.  Bad hay can introduce new weeds in your garden.  Good hay does not contain many weed seeds and is worth the few seeds for the soil improvement it brings.
• The price of a bale of hay will depend on supply and demand, but in a non-drought year is substantially lower than the price of a bale of sugar cane mulch.  

Lucerne Hay

• Lucerne is a legume like peas.  Lucerne hay is not a by-product, but a dried plant that is fed to  livestock and horses.  
• The price of bales depends on supply demand and can be high if there is a drought. It is definitely more expensive than straw.
• Lucerne producers will sell bales that have become wet and begun to rot at much lower prices. These bales can not be fed to livestock, but are ideal as mulch.
• You can often break up a bale into thin layers that are easy to put on soil.  They don't blow away or are pushed aside by birds.  Just make sure the layer allows water through.
• There can be some weed seeds in lucerne, but this is seldom a problem
• Adds organic matter to the soil plus a good range of nutrients
• Some lucerne hay suppliers like people to think that, because lucerne are a legume, it adds nitrogen to soil. This is not the case.  Legumes fix nitrogen in nodules attached to their roots. There is little or no nitrogen in their stalks.
• Lucerne hay has a wonderful smell.  I found that slugs and snails come from far and away to have a feast on your lucerne hay.   They may prefer the hay over your seedlings, but don't take my word for it.
• Lucerne hay will never contain long-lasting broad-leaf herbicides because it is broad-leaved itself.  Farmers will not use these herbicides on lucerne.

Pea straw

• Pea straw is the left-over stalks after harvesting peas. It is sold in bales.
• The price of a bale varies depending on supply and demand.  Pea straw is more expensive than normal straw.
• You can often break up a bale into thin layers that are easy to spread out and that don't blow away or are pushed aside by birds.  Just make sure that the layer allows water through.
• There can be some weeds in pea straw, but this is seldom a problem
• It adds organic matter to the soil and it will also feed your soil
• Some pea straw suppliers like people to think that, because peas are legumes, pea straw adds nitrogen to soil. This is not the case.  Legumes fix nitrogen in nodules attached to their roots. There is little or no nitrogen in their stalks.
• Pea straw will never contain long-lasting broad-leaf herbicides because it is broad-leaved itself.  Farmers will not use these herbicides on peas.

Pine needles

• I have never seen pine needles for sale in nurseries or hardware stores.
• It is very useful as a mulch for plants that need soil with a low pH.
• Just collect them yourself anywhere under any type of pine tree (don't collect she-oak 'needles' - see last dot point in 'How not to mulch')
• Collect brown pine needles that have been on the ground for a while, not green pine needles still on trees.
• Don't use pine needles on vegetable beds as pine needles are too acidic. 

Sea grass and sea weed

• Sea grass and sea weed land on shores after storms
• You are allowed to collect them if you only do this occasionally in non-commercial quantities.
• They are very good for soils and help increase the pH (see 'improving soil for asparagus' above).
• I would not use it straight on vegetable beds, but add it to my compost heap

Shredded garden prunings

• Free mulch if you are prepared to shred with a shredder or a lawn mower.  Allow prunings to dry out a bit and go brown before shredding them and using them as a mulch.  That makes shredding easier and it means even less nitrogen in the prunings, which is the aim because this type of mulch is meant for fruit trees.  Low nitrogen and high carbon will encourage fungi that make ideal soil for fruit trees.
• It saves you having to cart the garden prunings off to the tip and you are recycling your own material for use in your own garden.

Straw

• Straw is the left-over stalks after wheat, rye, oats or barley have been harvested
• It is sold by the bale and often readily available
• It is cheaper than sugar cane mulch
• Individual pieces of straw are much longer than pieces of sugar cane mulch
• Will contain some grain seeds that will sprout (see photo below), but this is not a big problem as they are easily pulled out
• Adds organic matter to the soil, but does not really feed soils

Straw with some sprouting seeds that are easily pulled out

• And yes (how did you guess?) straw is good as a mulch for strawberries.

Sugar cane mulch

• By buying sugar cane mulch you provide an incentive to Queensland sugar cane farmers to convert sugar cane waste into a source of income rather than burning it and causing massive environmental pollution.
• On the other hand, the 'carbon footprint' for transport to Tasmania is high compared to mulch from local sources.
• Sugar cane mulch is compressed into bales that are readily available at hardware stores
• When putting it on soil loosen up the compressed material so water can get through
• I have used sugar cane mulch for years now and found it to be consistently weed-free.
• Many brands of sugar cane mulch are now (certified) organic.
• Compared to straw for instance, individual pieces are short and thin and therefore easy to use as mulch in pots and other situations where plants are close together or where there is not much room (see photo below).
• It is not cheap compared to other commercially available bales of mulch
• It adds organic matter to the soil, but does not really feed soil
• May blow away if not bedded down in windy spots (remedies for this are discussed above)

Sugar cane mulch on soil in a small pot

Happy mulching,

Max Bee