Much Ado About Mulching
Why mulching is important, it is an essential component of gardening but it’s often misunderstood. Let’s approach it from the standpoint of the benefits of mulching and some of the challenges associated with it.
One of the biggest reasons we mulch is to reduce weeds in the bed. However, just because we cover the soil with a mulch material, doesn’t mean we eradicate weeds. Mulching can suppress certain weeds, but not all of them. The annual weeds that germinate at shallow depths are readily suppressed by mulching. The deeper germinating annuals and simple perennials which might germinate quite deep, will still push up through the mulch. Rhizomatus perennials (those which propagate along the root under the soil) can push through any organic material. The type of mulch used can also bring a new assortment of weed pests into the garden which we want to avoid, or be prepared to manage them.
When using organic mulches to cover your garden, they will eventually add organic matter to the soil. However, on their own, they will not create a healthy soil out of an unhealthy soil. Weathering, temperature, aeration, and seasonal changes; along with the digestive processes of insects,microbes,fungi,and other influencers all come together to complete the mineralisation process. Most high carbon mulches like chips or shredded bark do not contain a very high percentage of the major nutrients for plant growth. We want to encourage aerobic (oxygen rich) bacteria in the soil environment; and discourage anaerobic (oxygen poor) bacteria. In order to do this, we need to achieve good air exchange in the soil beneath the mulch. Aerobes respond to being fed with nutrient rich compost and/or fertilisers, and respond in kind to increase these elements over the long term. To avoid creating an anaerobic (oxygen poor) soil environment, mulch layers that cake up at the soil surface should be blended into the soil, or avoided altogether. Hay, straw, and leaves can create this undesirable environment These types of materials need to be shredded or blended in with the soil. If you dig through the mulch to the soil surface, and it is hard, or feels ‘tight’, take the time to pull off the surface mulch or incorporate it with some compost and/or fertiliser, then lay down new fabric, and mulch over the fabric with new mulch.
Why mulching is important in late Autumn – applying a mulch at this time of year is extremely beneficial for retaining water and protecting the roots of some of the more relatively, tender plants. The common perception is this is carried out to keep the soil warm but in fact, by heaping mulch up over the root area and around the stems you will help to protect them from harsh frosts, retain moisture and most importantly, help to protect from the cold, drying winds of winter.
Compost vs Mulch?
Mulches have primarily three functions. Aesthetics, Moisture Preservation, and Weed Suppression. Compost, whether it comes from your kitchen and yard waste; or from decomposed animal manures, provides a more complete soil conditioning package because of the more readily available nutrients it brings to your soil. Well managed compost will also bring valuable aerobic bacteria which proliferates in the soil and creates a more balanced, healthy soil that can cycle decomposing mulches more readily. Animal manure-based composts will inevitably have weeds in them, even when processed aerobically (turned often). Incorporating composts into the soil itself will always be more beneficial if you can do it. If you can’t, you should at least incorporate it into the old mulch, water it in, then add fabric and your new mulch over the top.
When to Mulch?
Mulches can actually be applied any time, year round. Given the mild winter weather we’ve experienced in the UK this year, applying them now is still a great idea because you can still get into the garden. Having the soil covered and prepared, as summer annual weeds begin to germinate in the early spring is a big plus. Applying your compost now is also beneficial because during the early spring when the soil begins to warm, the compost (and its beneficial microbes) begin contributing to the nutrient pool earlier than when we might typically start garden activities, and the plants need the nutrition to get started.
What are the Best Mulches?
Composted Green Waste as a Mulch:
This type of compost is comprised of garden clippings and prunings which are collected by local councils, and when managed properly to create and maintain heat in the pile, can be a great source of compost. Source separated food wastes, when part of the equation, provide a great ‘fuel’ to the green waste, adding nitrogen, which helps heat the pile and break down the high carbon content of the yard waste it is mixed with. The important consideration is that this type of material is processed properly by turning regularly to heat the pile continuously. This high temperature environment kills many of the weeds normally associated with yard waste. When these piles are just left to sit and break down (static pile), the anaerobic (bad) bacteria thrive and the aerobic (good) bacteria are not as plentiful. When sourcing these types of composts, be aware of how they are made.
Composted Farm Manure:
When farm manures are turned on a regular basis, the result is far more beneficial. Through turning, the pile can heat to a point where pathogens and weed seeds are significantly reduced. True compost is defined as having three components:
1. They are of ambient temperature (meaning, the same as the air around the pile)
2. They are odourless (they smell like ‘earth’) and,
3. You cannot tell where they originated from (manure and other components are not evident in the pile)
Even when animal manure based composts are properly turned and aerated, the harder weed seeds that were present in the crop before feeding it to animals can survive the digestive process of the animal and even the best processing system.
Shredded Bark Mulches and Wood Chips:
These are the most popular types of mulch but source them reliably. They are usually clean and reasonably easy to handle. Shredded mulch will break down more rapidly than chips, so may be able to contribute to the soil if renovated every couple years as noted above. Again, watch out for mulches that came from undifferentiated sources, for example from clippings from along roadways, where brambles and vines were growing. Many of them can propagate from the cut stems, and will love the garden soil they’re applied to.
Mulching with Inorganic Materials:
Some examples of these mulches would be river rock or stones (there are others). Though these can be beautiful in the right garden space, remember they are difficult to remove for renovation; and they contribute nothing in terms of soil value.
Ahead of using them, the soil needs to be well conditioned; and after using them, you’ll be primarily counting on chemical fertilisers to maintain them. Your chance for adding Carbon is gone. They also do very little for retaining moisture in the soil, and might even be a drawback because they will heat and actually pull moisture from the soil in some cases.
Some other Considerations:
Nitrogen and Mulches:
A key ratio in organics is the Carbon:Nitrogen ratio. Whether in composts or soils, the process of breaking down high carbon (woody) mulches will tie up nitrogen in the process of mineralisation. Adding nitrogen by using fertilisers; composted animal waste or food waste compost may be needed to assist this process. Refer to the crop, and if it’s a heavy nitrogen feeder, add some available nutrients. If it is not, then focus on adding compost to build soil health.
pH and Mulches:
One common perception is that using pine needles and other similar sources will lower the pH of a soil. This often is misconstrued to mean it is a permanent effect, but it is not. Pine needles are initially low in pH and yes, will lower the pH at the surface of the soil immediately under the needles, but in time, as they break down, the soil will revert to near neutral pH if the soil was near neutral to begin with. Most organic material, once composted and broken down, will revert to near neutral pH. Therefore, if you’re growing azaleas, camellias, or roses, for example, you should look to apply acidifying materials to maintain pH at their lower, more optimum level (sulphur, ammonium sulphate, aluminium sulphate, lemon juice, vinegar, etc) over the long run.
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