In the Garden:
Northern California Coastal & Inland Valleys
Even stones piled around the base of a trunk can cause problems.
When Tree Roots Need Excavating
Whenever I visit the southern part of the U.S., I notice (and question) the seemingly common practice of mulching trees with a deep layer of pine needles or leaves piled up to the trunk. We never see it done this way here in sunny California. I figured it must have something to do with cold weather, although the South doesn't suffer from extremes in temperatures like the Northeastern part of the country. I asked Bartlett Tree Experts about this curious bit of arborculture and they assured me that not only is it not recommended, it's downright harmful to the tree.
Trunks and roots are made up of different types of living tissue. Trunks need light and air, roots require moisture and nutrients from the soil. Both work together in harmony to support a living tree. When you bury the trunk, light and air are eliminated, resulting in rot and fungus disease. As a matter of fact, root excavation is a service offered by Bartlett.
Excavating Tree Roots
Bud Reeves, one of the arborists at Bartlett, demonstrated how the root excavation process works. He said that if a tree looks like a telephone pole coming out of the ground, the trunk is buried too deeply. When a tree is planted properly, the top of the buttress roots are exposed to the air. There should be an area of exposed roots about 6 to 12 inches from the base of the trunk all around the tree. Bartlett uses a tool called the Supersonic Air Tool, which is powered by compressed air. It can blast away compressed soil without damaging the roots.
Once the buttress roots have been exposed, it's easy to identify any roots that are crossing over the main support roots. If left alone, these crossing secondary roots will eventually strangle the supporting roots, causing the tree to fail. Crossing roots are formed when roots encounter a stone and when a tree is left too long in the nursery container. It's always important to purchase a landscape tree from a reputable nursery and to inspect the roots by slipping the tree out of the container prior to purchase. Buying a bargain tree might not save you money in the long run if the tree doesn't thrive. When planting a young tree, make sure the top 1 inch of the rootball is above the soil surface to keep the roots at the proper depth as the tree grows.
How a Trunk Gets Buried
When a tree has been growing for a long period of time, leaves and soil debris build up under the canopy. In order for the tree to remain healthy, that layer of debris should be removed from the area immediately surrounding the base of the trunk to expose the supporting buttress roots.
Allow the excavated area to remain open to the air; don't fill it with gravel or mulch. You don't need the Supersonic Air Tool to do this job, a pick and shovel will suffice, as long as you take care not to damaged the roots.
As we head into winter and other gardening tasks have slowed, check your landscape trees to see if they are indeed getting the attention they require for good health!
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