Mulching Trees
“Mulch is one of the best and also one of the least expensive treatments for your trees –both sick trees and healthy trees.”
Most trees evolved in forests where the forest floor was covered with a continually renewing layer of leaf litter and decaying twigs and branches. Adding organic mulch to the soil around trees is one of the best and cheapest things we can do for our trees.
Benefits of Mulch
- improve soil structure
- increase soil fertility through the slow, natural breakdown of organic matter into essential plant nutrients
- reduce soil compaction that damages root growth and health
- suppress weeds
- conserve soil moisture
- protects tree trunks from damage from weed string trimmers and lawn mower collisions
- moderate soil temperatures from becoming too hot or too cold
- and yes, enhance the beauty of landscape
Types of mulch
Organic mulch is any material derived from plants or animals. Some organic mulches are better than others with composted ground up yard debris and freshly chipped branches and leaves (tree service debris) being the best. Inorganic mulch does not degrade and includes materials such as stones, gravel, plastic sheeting and recycled tire chips. Inorganic mulch does not enrich the soil and, in my opinion, allows the soil to gradually lose its organic matter, which is essential for healthy trees.
Weed Mats Beneath Mulch
Using a weed mat or other fabric beneath organic mulch defeats many of the benefits of organic mulch by placing a barrier between the soil and the mulch. Do not use weed mat or other fabric beneath mulch. You want the organic matter to break down and enrich the soil with nutrients and organic matter. A fabric prevents this enrichment and prevents beneficial organisms such as earthworms from proliferating in the fabric-covered soil. Fabric-covered soil will eventually become depleted of organic matter because the fabric is barrier to organic mulch and leaf litter that adds to soil organic matter. And soil organic matter enhances the soil biota, the microorganisms, that are so important to plant health. And never, ever put polyethylene plastic over the soil above roots. Plastic will block both air and water and can cause serious problems for a tree if enough of the root zone is covered. See Root Tracing in the navigation bar under Diagnosis and Repair for information on the probable size of a tree’s root zone.
Mulch Depth
Normally a 2 to 4-nch layer of organic mulch is best. After the first year, additional mulch should be added only as needed to fill thin areas. Adding a layer of new mulch each year over the old layer is not necessary and can be detrimental. Sometimes a white layer of fungal mycelium will form between layers of mulch that is too thick. This myceliium layer will actually inhibit water from passing through the mulch into the soil. When this mycelium mat is obaserved, the mulch should be stirred with a stiff rake or pitch fork.
Mulch “Volcanoes”
Placing mulch against a tree trunk is very bad and unfortunately is a very common practice by 
landscape maintenance companies. Mulch placed against a tree trunk is a long-term problem that could eventually cause decay on the lower trunk which has not evolved to tolerate continual wetness and moisture. A short-term problem is the excessive mulch inhibits air exchange between the atomosphere and the tree roots. Keep mulch back from the trunks of trees and shrubs by at least 6 inches.
How large an area should be mulched?
The larger the mulched area is, the better. If you can mulch out to the dripline of the tree crown, that would be ideal. But many homeowners find extensive mulching to be more turf-unfriendly than they can tolerate. But as water use becomes more of an issue and water restrictions become more strict, having a large shaded mulch area in the yard starts to make sense. But you can start a little at a time. Start with a 5 foot mulch ring around a 8-inch diameter tree, for example. Then over time start to increase the mulched area. And to increase the mulched area, you can just place the mulch directly over the turf to smother the turf which will also turn into mulch.
Will mulch deplete oxygen in the soil?
Roots need air to live and grow. Air may not be able to penetrate thick mulch or very fine-textured mulch such as sawdust or matted grass clippings. Research has shown that when mulch is applied in the proper amount, there is no detrimental effect on air exchange between the soil and the atosmosphere.
Will mulch deplete soil nitrogen?
Generally mulch does not decrease soil nitrogen. When organic mulch is applied over the soil surface, there is no evidence in numerous studies that the mulch will significantly deplete soil nitrogen levels.
However, if organic mulch, especially mulch with a high carbon to nitrogen ration such as sawdust, straw or ground wood pallets is mixed into the soil, nitrogen levels can be depleted temporarily while the organic material decomposes. The time could be several weeks to several months or more depending on soil temperatures and the geographic location of the tree. Organic matter decomposes faster in warmer climates. Then the tied-up nitrogen used in the decomposition process is returned to the soil.
Mulch and Pests
Generally, plant disease organisms are not transmitted by spreading mulch from diseased trees around the soil by healthy trees. But there are exceptions.
- Palm trees that have been infected with fatal ganoderma fungus (Ganoderma zonatum) should not be ground up for mulch. The ganoderma organism that kills palms does not affect hardwoods but can be a source of inoculum for other palms where the mulch is spread.
- Another example is mulch made from chipped Scot pine trees that carry the nematode that causes pine wilt. But research has demonstrated the only trees infected by the nematode-infested mulch were those that had the mulch incorporated into the soil or the mulch piled against the trunk of the healthy trees.
- In warmer areas of the country where subterranean termites are a serious problem, many homeowners switch to inorganic mulch such as crushed rock thinking that the termites feed on organic mulch and avoid the inorganic mulch. However, research by University of Florida entomologists shows that the moist soil conditions under mulch attract termites. And crushed rock also creates equally moist soil conditions as organic mulch. Furthermore, organic mulch is not a good source of food for termites because of its fluffy, aerated consistency, conditions that subterreanean termites avoid. The best recommendation is to use organic mulch but keep bare, dry ground about 12 to 18 inches back from the foundation. Also adjust sprinklers so the soil immediately adjacent to the foundation stays dry. Keep landscape shrubs back at least 3 or 4 feet from the foundation. The roof overhang should also help keep the soil dry next to the house.
- Dutch elm disease, and oak wilt are fungus diseases that infect vascular systems and are spread by beetles. Research has not found that using infected mulch spreads the disease probably because mulch is not an attractive site for the beetle vectors.
- Verticillium wilt attacts a wide range of hardwood trees, lives in the soil and infects plants through the roots. Research at the University of Minnesota found no evidence that Verticillium spread from infected trees to other trees through infected mulch.
- A considerable number of insects and arthropods live in mulch. Generally, these critters are neither harmful nor destructive. Sometimes ants or other insects may invade the house and these can be controlled with bait traps and chemical barriers, if you are so inclined. Mulch is a stable habitat for many beneficial organisms that prey on other insects or recycle and breakdown organic matter.
In conclusion research at this point indicates that trasmission of pests through mulch is unlikely under normal landscape conditions. But if in doubt, compost your mulch before using it.