What is mulch?

Mulch is a material that is used as a protective covering for soil. Professional gardeners prefer organic mulches, which are often made up of wood chips, bark, straw, cocoa hulls, leaves, and compost mixes, as these decompose, thus improving soil structure and increasing soil fertility. Inorganic mulches, such as gravel, stone chips, river rock, and shredded rubber do not decompose, so cannot provide the same benefits to your plants as organic mulches.

Proper mulching

The International Society of Arboriculture has stated “Mulching is one of the most beneficial practices a homeowner can use for better tree health.” Proper mulching will: conserve soil moisture; improve soil structure, fertility, and aeration; moderate soil temperature; and impede weed growth.  

Mulch - Manually

 Mulch is especially important in the urban environment, where soils are often compacted and lack organic matter. It will aid in mimicking the natural forest environment, where the nutrients in the soil are continually being replenished by fallen leaves and branches.

When to Mulch

Mulch may be applied to landscape trees throughout the year, however, in seasonal regions it’s best applied in the middle of spring once soil temperatures have warmed enough for root growth to begin. Spring rains will help to break down the organic materials in mulch, allowing them to better penetrate the soil. Mulching in the fall will benefit your trees by providing an extra layer of insulation throughout the winter. In general, try to choose a time after a rainy period, as the  mulch you apply will help to seal in the moisture.

Buying Mulch

Once you’ve decided what type of mulch you want to purchase, you’ll need to calculate how much to buy.  Multiply the length and width of each space to be mulched to determine its square footage. Once you know the total square footage, you’ll need to figure out how many bags or how many scoops of mulch you will need. Bagged mulch is sold in bags that are measured in cubic feet, but mulch sold by the truckload is measured in cubic yards.

To determine how much mulch you need if you’re buying it by the truckload, multiply the square footage you wish to cover with mulch by the number of inches of mulch you want (see more info about mulch depth below), Then, divide that number by 324 to determine how much mulch you’ll need in cubic yards. The easiest thing to do is check with your local arborist or plant nursery to see how their mulch is sold – whether by the truckload or bag – and let them do the math!

How to Mulch the right way

Tree specialists recommend that you mulch as far around the tree as possible, preferably to the “drip line” – the outermost edge of the tree’s canopy. A 2- to 4-inch layer of organic mulch is best; more than 4 inches may harm the tree’s root system by blocking oxygen. For the same reason, do not mound the mulch at the root flare – where the trunk meets the soil. When applying new mulch over old, be sure to check the depth of the old much. You may need to remove some of this before adding you new mulch.


Gardening - mulching the right way

Over-mulching is to be avoided at all costs.  “Volcano” mulching – where the mulch extends up the trunk, smothering the root flare and root zone –  can lead to decline and possible death of your trees. Over-mulching blocks air from reaching the tree’s roots, leading to oxygen depletion. It can also inhibit water evaporation from the soil, which also blocks oxygen from reaching the roots.

Conversely, when the root flare is covered with mulch it causes the inner bark of the tree to remain constantly wet. This leads to the death of the inner bark, which is vital as it is made up of phloem – the system that transports nutrients around the entire tree. Over-mulching can trap moisture, creating the perfect environment for fungus and bacteria to thrive, leading to plant disease.

Insect pests, such as carpenter ants and termites, also like moisture and places to hide. If insect pests are a particular problem in your region, consider using cedar mulch as its resin is toxic to termites, and can help deter cockroaches, odorous house ants, and other pests. Mulches made from cypress heartwood, melaleuca, eucalyptus, southern tidewater red cypress and California redwood are termite-resistant.

If your trees have been overmulched, carefully check the depth of the mulch and simply rake away any excess. Check the trees’ root flare, and, if necessary, carefully pull any excess mulch 3 to 5 inches away from young trees and 8 to 10 inches away from mature trees. Most trees respond rapidly once the root flare is exposed. Root flare excavations should be performed very carefully, however, as the root flare may be buried rather than just covered with mulch. Such excavations are best left to the experts.

How to apply Mulch

Finally, you’re ready to apply the mulch! First, clean the area around the tree, removing leaves, debris, sticks, weeds, etc. If you haven’t had rain recently, water the area around the tree. Spread the mulch two to four inches thick, and then water to help settle the mulch into place.

Tree mulching