In a natural forest setting, trees take care of themselves. Each fall the soil is covered with leaves providing an ample supply of mulch and eventual organics and nutrients. Often times urban soils are compacted, lack organics, and have restricted space for tree roots to grow. Trees and lawns did not evolve together, will not thrive together, but can they exist together? Applying organic mulch around trees is often the best way to minimize the battle between these competing forces.
Benefits of Organic Mulch
- Lessens amount of watering needed
- Creates aesthetically pleasing landscapes
- Insulates the soil microclimate from colder winter temperatures and warmer summer temperatures
- Keeps mowers and weed trimmers away from the trunk
- Reduces amount of lawn area and eliminates competition from turf grass
- Breaks down into organics over time
- Helps suppress weeds
- Creates a more natural environment in unnatural urban landscapes
- Stimulates growth; mulched plants grow faster that non-mulched plants
Trees vs. Lawns
Trees planted in a typical urban landscape grow in an unnatural environment. Urban sites often have compacted soil, restricted root space, and are covered in a thick blanket of turf. The majority of a tree’s rooting system grows in the top 2-3 feet of the soil and occupies some of the same area with turf roots. On the other side, trees will create an environment too shady for most lawn grasses. When turf stands are thin, it is easier for weeds to invade. Also, surface roots can make it difficult for turf to be thick and healthy. There is direct competition between your trees and lawn for nutrients and moisture. Replacing grass beneath trees with wood (organic) mulch is the best way to minimize this competition.
How Much Mulch?
Ideally, the entire root zone of a tree should have mulch. Obviously, for most urban landscapes, this is not practical. There is a direct relation between the size of area that is mulched and the benefit to the tree. The depth of mulch is a critical element. A depth of 3-4 inches is the optimal choice for most situations. It is crucial that the mulch is not piled against the trunk of the tree. This often leads to trapped moisture that can accelerate rot and decay. A good generalization is to mulch wide, not deep.
Types of Organic Mulch
There is a wide range of materials that may be used as organic mulches. These materials include wood chips, bark, pine needles, grass clippings, cocoa beans, peat moss, leaves, straw, or sawdust. All of these products can provide considerable benefits for trees and shrubs. However, they all may not provide an aesthetic presence in the landscape. The most popular choices for urban landscapes include shredded cypress, pine bark, or hardwood chips. All are excellent choices but will vary in cost, appearance, and ability to resist rapid breakdown.
Maintenance of Mulch Beds
Tree friendly area created by your mulch can eventually become the focal point of your landscape. Animals, wind, and people often spread weed seeds into mulched beds. These weeds can be managed by mechanical and chemical weeding. The amount of weeding required will depend on many variables including type of mulch, type of underlying fabric (if any), type of plants in the bed, locality of weeds and invasive plants, type of soil, and watering patterns. As old mulch breaks down, new mulch can be added as needed. For most landscapes, new mulch can be added every 2-3 years depending on type of mulch and personal preference on appearance. The addition of a preventive herbicide at this time can be helpful in weed prevention.
With our experience, we find that most weed barriers are ineffective over the long run. Solid plastic should not be used around trees because they disrupt water infiltration and oxygen exchange. Breathable fabric barriers are much better for the trees and shrubs, but will not eliminate weeds over time. Newspapers or biodegradable paper rolls are often a good choice for a short-term weed barrier.
The Mulch Volcano
Excessive mulch around the trunk can be more damaging than the benefits of having no mulch at all. A volcano of mulch will trap unwanted moisture against the trunk which can cause decay at the base of the tree. They can also cause roots to grow above the level of the soil or around the base of the trunk.
How About Rock Mulches?
Even though we do not encourage using rock mulch, there are benefits. During periods of rain and wind, rock mulches tend stay in the mulch beds better than wood mulches. Longevity is a definite plus since they do not break down as fast as wood mulches. Rock mulches are usually more expensive initially, but will last much longer. On the down side, rocks are more difficult to work with, do not provide nutrients back to the soil and can cause excessive heating of tree roots.