48. Mulch
The first thing I look at when I see garden plants suffering is the soil surface - does it have a protective mulch layer? All too often the leaf blowers of the world have whisked the bark out of the planting beds, leaving bare soil. Mulch is an essential part of any garden with many uses and benefits. I’ve mentioned some of these benefits in other blog entries but now it is time to put the spotlight on mulch.
What is Mulch?
Most people think of mulch as bark chips, but in fact the broader definition of mulch is “any material spread evenly over the surface of the soil to enhance the growth of plants and the appearance of the landscape.” This material can be many things including: gravel, recycled yard waste, cardboard, or one of many wood products. For the majority of garden situations, the term mulch refers to some type of bark chips that are placed on top of the soil in planting beds. These bark chips mimic the natural ground cover in forests in many ways and provide similar benefits.
Benefits of Mulch
Bare soil in a planting bed is simply not good for the health of the plants or the soil food web. A layer of bark mulch over planting area soil, 2 to 4 inches thick, has many benefits:
Soil moisture – having this layer over the soil prevents moisture from evaporating from the soil surface, preserving it for plant roots.
Soil temperature – mulch also blocks direct sunlight from the soil, keeping it cooler and more hospitable to root growth.
Weed control – weed growth is greatly reduced with mulch by keeping sunlight from the soil surface and physically blocking their growth. Mulch also makes any weeds that come through easier to pull out.
Erosion control – the mulch layer also helps to slow down water flowing across the ground and protects the soil from being washed away.
Reduced soil compaction – the spongy nature of mulch also keeps the soil from getting compacted by foot traffic.
Improved soil health – bark chips decompose over time and add organic matter and nutrients to the soil.
Visual appeal – it is also simply a nice looking layer to add to the garden. It helps visually unify the planting beds and give things a cleaner, neater appearance, especially with new plantings that are still small.
Potential Drawbacks of Mulch
In my opinion the benefits far outweigh the drawbacks, but there are a few potential issues with the use of bark mulch:
Nitrogen absorption – some bark mulches are low in nitrogen and if mixed in with the soil can absorb nitrogen, depriving plant roots of this essential element.
Unwanted elements – it is important to know where your mulch comes from and avoid bringing in things like diseases and weeds. Commercially sold mulches are not likely to have any issues, but mulch made from chipping trees does have the potential to transmit diseases like Sudden Oak Death. Always confirm with the source that they are confident that no diseased material was used to make the mulch. (In the San Francisco Bay Area there is a widespread awareness of these issues that minimizes any problems with recycled mulches.)
Fertilizing & compost – mulch does get in the way of applying fertilizers or compost to the soil surface. You don’t want to put the fertilizer on top of the mulch, it will absorb it and pass it on to the soil much more slowly. However it is usually easy to rake back the mulch before applying a fertilizer.
Floating – heavy rain can create temporary ponding and water flow across the ground, and bark chips can float up and travel with the water. This can create piles of mulch in low spots, sometimes blocking drains. This usually means you need to take a little extra effort to rake the mulch back in place after storms. Using dome-top ("atrium") drainage grates with small openings will help keep the bark from creating blockages. Bark mulch should definitely not be applied in drainage swales, various types of rock mulch can be used in its place.
Types of Bark Mulch
Bark mulch comes in several types, with variation in size, color, and source. For general planting beds I prefer a 1” or smaller fir bark mulch as a balance between providing the maximum benefit and nicest look. However the larger sized bark mulches in the 1” to 4” range are possibly better at blocking weeds. They also last longer since the bigger pieces take longer to decompose. There is a trend of adding color dye to commercially sold mulch - in my opinion this is a waste – it looks artificial and regardless of what the suppliers claim, it adds an unnecessary chemical to the mulch that goes into your soil. An excellent alternative to commercially supplied bark mulch is recycled content mulch.
Recycled Content Mulch
There are two general types of wood mulch with recycled content – Green Waste Mulch and Recycled Wood Mulch. Green Waste Mulch (sometimes called Arbor Mulch) comes from tree removal, tree trimming, and other plant clippings that normally go into landfills as a waste product. This material is shredded into appropriate sized chips for use as mulch. It is produced by commercial tree trimming companies such as the Green Waste Recycle Yard in Richmond. City tree trimming crews also provide this sort of mulch for homeowners, here in the city of Albany they regularly deposit piles in public spaces where anyone can load up for home use. Recycled Wood Mulch turns construction wood waste into mulch sized chips for garden use. The trash collection company Waste Management produces this mulch locally “from 100% recycled, clean, untreated lumber”. These mulches are without a doubt the best to use in terms of landscape sustainability and overall environmental health because they take local waste products and turn them into a beneficial material. This keeps a huge amount of material out of landfills and eliminates the negative impacts of trucking over long distances.
Mulch-o-Rama
I’ve focused here on the most common type and use of mulch, bark in a planting bed, but there are many other variations. Sheet mulching is a process I’ve described previously as part of my Native Meadow Experiment. Loose gravel is also a type of mulch I use often, it gives a nice clean look to a planting area, but it does not have the full range of benefits that you get from bark mulches. Recycled rubber mulch has some applications in commercial situations but comes with its own set of pluses and minuses. Fallen leaves and grass clippings also have mulch-like uses and benefits. Don’t let those leaf-blowing gardeners get out of control in your yard, keep your soil protected with a nice thick layer of mulch!
References:
Bay Friendly Gardening Guide and A Bay Friendly Landscaper’s Guide to Mulch http://rescapeca.org/resources/publications-list/
State of California – Cal Recycle information on mulch benefits: http://www.calrecycle.ca.gov/Organics/CompostMulch/
Green Waste Recycle Yard, Richmond http://www.greenwasterecycleyard.com/mulch.htm
Waste Management Mulch Products http://wmearthcare.com/products/mulch/