Wide Mulch Rings for Big Trees

Spreading a layer of mulch presides high on the list of important tasks when planting a new tree. In most cases, the mulch ring normally encompasses up to a 4 foot diameter area around the trunk. If you have followed Urban Forestry for a while through this newsletter or on social media, you know that we have a thing for mulch. The benefits incurred at a low cost and with very little effort are difficult to match, and maintaining that mulch ring around a tree is really one of the best things that a tree owner can proactively do for their tree’s health.

With that said, a lot of the time a mulch ring stays the same size, while the tree continues to grow. Don’t get me wrong – any mulch is good. Let’s stop for a moment and consider what mulch actually does and how it benefits the tree.
  • Mulch essentially mimics the forest floor and the growing conditions trees experience in their natural habitat. A forest has what we know as a “duff” or “litter” layer, made up of fallen debris such as leaves, branches, decaying wood, and other organic material. In urban areas, we strip trees of this highly beneficial medium and substitute it with turf grass.
  • Mulch helps to insulate the soil, which means less moisture is lost through evaporation, and the temperature is regulated. Trees with mulch tend to endure less drought stress than trees without it.
  • The woody material in mulch breaks down over time and delivers minerals back into the soil. These elements are then available for uptake by tree roots and give trees what they need for their physiological process.
  • Increased microbial activity and other soil organisms beneath mulch help to improve soil structure, which makes it easier for absorbing roots to grow. Studies have shown that trees within mulched areas have significantly more absorbing root growth than areas with turf.
  • Mulch reduces competition with weeds and prevents damage caused by lawn maintenance equipment.
  • Mulch looks nice, and when paired with understory plants, a nice woodland landscape bed can result from choosing mulch over grass in areas beneath trees.
Mulch on Kelly.jpg
Now, you may have heard about these benefits before, as Urban Forestry repeats them often. The last two benefits could justify a consistently narrow mulch ring, but considering this list of benefits leads us to ponder where the roots are generally growing and whether the narrow ring isn’t targeted quite right. Let’s consider what we know about trees’ root systems and how they grow:
  • The roots of a tree can extend far beyond the tips of the branches – sometimes 2-3 times that distance! At the same time, roots will proliferate where access to resources is best.
  • As roots continue growing away from the tree, older root growth becomes woody and functions mostly for anchorage, storage and transport.
  • The most active area for absorbing roots is where most of the new roots are being produced – generally near the drip line. However, older, woody roots do produce some absorbing roots as well.
If trees have absorbing roots out to the edge of their canopies and beyond, why do we spread the same width of mulch around trees, regardless of their size? When considering how mulch impacts a tree’s health and the growth habits of tree roots, logically, a mulch ring should expand as a tree’s canopy expands. You might be thinking, “That’s crazy! If I expand my mulch ring with the growth of my trees, I will have a yard full of just mulch!” This is a very understandable concern. We are not here to tell you that it’s all or nothing with your mulching efforts. An ideal recommendation is a mulch application across the area within the drip line; however, any amount of mulch you’re willing to commit to around your tree will help. Mulched areas expanding closer to the drip line will simply offer more benefit for a tree.

If you decide to go the wide mulch ring route, spread at a thickness of about 3-4”. Avoid piling mulch against the trunk, which can encourage rot. Try planting some understory shrubs or perennials, if you like. With periodic maintenance, you’re well on your way to healthier trees and a nice woodland garden.

View a fact sheet on Proper Mulching Techniques
Check out other topics from the Fall 2016 edition of Edmond Tree Mail
View last season's Tree Issue