Mulch offers several benefits, including weed suppression, moisture retention, temperature moderation and improved aesthetics. Mulch is often overused and is actually not a landscape element, but a soil amendment that replaces organic matter and adds nutrients to the garden. Organic matter needs to be replenished because it is continuously breaking down and your garden will reward you when you use the right mulch and install it properly.
Mulch products come from various sources. The two most common are wood waste, and bark mulch. There are key differences between the options.
Bark mulch – After the sawmills remove the hardwood bark, it is aged for six to twelve months to make sure it won’t harm your plants. This aged hardwood bark mulch is ideal as it won’t burn your plants. It knits together and holds well in place, providing good moisture retention and weed suppression without readily washing away.
Wood waste mulch – This is the most common, and comes from tree logs, ground up pallets and other wood by-products, this dyed mulch is not aged, is slow to decompose and return any organics to the soil, and tends to float and move more readily making it less ideal for weed suppression and moisture retention.
Other forms of mulch – Composted products, such as leaf humus, landscape compost and other branded products like SweetPeet® are also available. Generally, these types of compost are best used to directly amend the soil by incorporating it into the native soil to improve the organic content. Know the source of your organic compost – not all compost products are equal, and some may not have been aged sufficiently causing issues for existing and new plantings.
Most soil is healthiest when it contains at least 5% organic matter, though annual and perennial beds require more. Organic matter reduces soil compaction, which improves air and water flow. Certain mulches are rich in nutrients that benefit the plants. Organic matter needs to be replenished periodically because it decomposes and breaks down. In nature, this process occurs naturally when plant debris is recycled through the decomposition process. That process takes time – time we don’t have in our gardens. In our urban landscapes and gardens, it is necessary to remove leaves, branches and plant debris from our landscapes because they are unsightly and can be good hiding places for pests. By removing the leaves and debris from our landscapes and composting them off site, we continue the cycle when we return the compost, leaf humus and other products to our landscapes. By replenishing organic matter into our beds with materials that have begun the decomposition process already, we speed the cycle and provide many benefits to our urban landscapes.
Soil amendments may be applied and incorporated into the landscape by top-dressing, forking in, or rototilling.
Top dressing – This method is the most commonly used. Mulch is used to fill in areas of the landscape that may not be fully planted, mitigating weeds and aiding in moisture retention. Shredded hardwood bark mulches are great for this, but leaf compost and other composted material can also be used.
Forking in – Organic matter is gently worked into the soil with a pitchfork. This method is best for adding modest amounts of material that need to be incorporated into the soil quickly.
Rototilling – This method is is used for new or empty beds that need a fresh supply of organic material for new plantings where there is little risk of damaging surrounding plants.
Generally, soil amendments should be added whenever the landscape has become depleted. For hardwood or bark mulch, not more than 1.5” to 2” of mulch should be applied at any one time, keeping total accumulation at 2” or less. Excessive mulch on top of the soil can trap moisture in the soil during heavy wet periods leading to root rot; and it can prevent moisture from getting to the roots during drier periods.
Before you mulch
For mulch to be most effective, take time to properly prepare your beds before you invest the labor and cost of materials for the mulch installation. Bed areas should be free of weeds and other undesirable vegetation. Establish a deep bed edge to separate lawn from bed areas. The bed edge will not only prevent mulch from washing into the lawn, but it will also minimize grass from spreading into the bed. Rake over the bed to distribute any old or remaining mulch and smooth the bed out to level uneven areas.
In open areas you can dump small piles and then spread, while in tight areas you may need to place smaller piles by using a pitchfork or shovel. Spread with your hands or a rake. Add mulch so that you don’t have more than about 2.0” to 2.5” of total mulch depth. Do not allow mulch to pile up on the base or trunks of plants; use your finger to swipe around the base (circumference) of the plant to create air space (one or two finger widths). Use the back of a leaf rake to gently distribute and even out the mulch for a final smooth and neat appearance.
There is more to mulching and adding soil amendments than just spreading wood chips over a bed. Mulching is an essential cultural practice for a healthy, beautiful landscape.