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- What is the Critical Root Zone around a tree?
- Tree Protection During Construction
The dripline is the guide measurement used to prevent unnecessary damage to trees during construction works. It is defined as the outermost circumference of the tree’s canopy, from which water drips onto the ground. The ‘dripline area’ is taken to include the soil and roots that lie within that circumference.
It is crucial for the stability and health of a tree that the dripline area is ‘off limits’ from any construction activity. It may be necessary to erect a fence around the dripline or, if possible, further out around groups of trees so as to prevent their roots from being damaged by construction plant, or by works such as excavation, soil compaction or consolidation. Fences should remain in place until all construction work is complete, including the final grading and smoothing of the .
If work inside the dripline is unavoidable, only hand tools should be used. Where possible, tunnels should be excavated beneath the root system as opposed to trenching. Other measures that can be adopted to avoid undue damage include:
- Using porous paving materials such as brick or flagstone rather than concrete or asphalt.
- Not raising the grade of the soil inside the dripline by more than a few inches.
- Not lowering the grade of the soil inside the dripline by more than 2 inches.
- Not piling soil or construction materials inside the dripline, even for a short period of time.
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For efficient drip irrigation, use the Hunter RZWS Root Zone Watering System. Made of plastic, it is perfect for flexibility without sacrificing strength. It has a pressure-compensating design, making it excellent for rugged environments, and a StrataRoot design, which delivers water to roots at all soil depths and eliminating wasteful run-off. This root watering system has a locking cap and is enclosed, which decreases the chances of it being tampered with by a third-party. It is pre-assembled for additional convenience. It is suitable to use with 1/2-inch PVC fittings. This root watering system is used for young plants and trees.
- Pressure compensating bubbler for precise watering
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What is the Critical Root Zone around a tree?
A tree’s Critical Root Zone (CRZ), sometimes also called the Root Protection Zone (RPZ), is defined as a circle on the ground corresponding to the dripline of the tree. Unfortunately the “dripline” of a tree can be irregular and hard to define. An alternative method of determining this dimension is to measure the diameter of the tree trunk in inches at breast height (DBH), multiplied by 12, as so:
Trunk diameter in inches at 4 1/2′ (1.4m) above grade x 12 = radius in feet of the CRZ (essentially, 1′ of CRZ radius per 1″ DBH or roughly 1.2m of CRZ per 10cm of DBH).
Bear in mind that root systems vary by depth and spread based on tree species, age, soil type, etc. The root systems of some oaks, for example, can extend well beyond the canopy dripline. This full root zone may extend 2 to 3 times beyond the CRZ.
Damage to the tree roots can be caused by any disturbance inside this area however bear in mind that nearby trenching, paving, or altering drainage patterns outside the immediate RPZ may also significantly affect the tree.
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Category: General Tree Care
Tags: drainage, roots, tree
← TreeFAQs – Tree Health Answers & Questions – pre 6/2017 format
Tree Protection During Construction
When doing any type of construction work around existing trees, it is important to protect tree roots from harm. Did you know that a trees roots extend out 2-3 times as tall as the tree? Trees need roots in order to stay alive, remain upright, grow larger, reproduce and defend themselves from pests, pathogens and decay. Roots do this by providing trees with stability, access to water, access to nutrients, and cultivating symbiotic relationships with beneficial soil biota. The minimum amount of roots trees need for survival are called the “critical roots.” These critical roots exist in the soil surrounding the tree, called the “Critical Root Zone,” or CRZ.
There are two ways of finding the size of a CRZ:
- A legal CRZ area is determined by using a formula based on a tree’s trunk diameter, as defined in municipal codes. Most municipalities define a legal CRZ as a circle with a 1, 1.25- or 1.5-foot radius for each inch diameter of the trunk.
- To find the size of the CRZ circle when the ratio is 1:1.5, measure the trunk 4.5 feet above the ground, called diameter at breast height (DBH). Then measure outwards from the trunk 1.5 feet for every inch DBH. This will give you the legal CRZ. For example, if the legal CRZ is 1.5 foot for every 1inch DBH, a 20-inch diameter tree would require a 30-foot radius circle of protection around the tree to meet the legal CRZ standard. Generally, municipalities require that the legal CRZ have a minimum of a 6-foot radius regardless of tree diameter, although this varies from place to place.
- A biological CRZ area is determined by an arborist through analyzing tree characteristics, site factors, and anticipated construction impacts. In other words, the biological CRZ is defined as the area needed to preserve the roots necessary for the tree to survive construction. For most trees growing in an open setting, the biological CRZ spans from the trunk to the edge of the canopy, or the “dripline.” For older trees, sensitive species, or trees growing in poor sites, the biological CRZ many actually be much larger than the dripline. Conversely, younger trees, resilient species or trees on good sites may have a biological CRZ smaller than their driplines.
For trees growing in a forest setting or with small crown sizes relative to their trunk size, the dripline may not capture all the roots needed for survival. In this case, the size of the biological CRZ may be found by using a DBH ratio calculation similar to what is done for the legal CRZ. An arborist can help determine what the ratio should be by evaluating tree factors (such as size, age, condition, and species sensitivity) and site characteristics (such as soil quality, water availability, and exposure).
For young, healthy trees growing in good sites, the ratio could be as small as 1:2. So a 10-inch DBH sweetgum would need a circle with a 20-foot radius. For older, sensitive trees growing on poor sites, the ratio could be 1:4 or more. At a 1:4 ratio, a 24-inch DBH white oak would need a 96-foot radius circle of protection.
While both the legal and biological CRZ varies from place to place and tree to tree, it is essential that we make the effort to preserve as many roots as possible. Trees provide many services to your site, and smart investment in protection pays dividends in results. We recommend creating a tree protection plan prior to any construction project, which identifies protection areas for trees remaining on site.
Tree Protection Guidelines: