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ENVIRONMENTAL REVIEW OAK TREE CARE AND MAINT. |
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This Oak Tree Care and Maintenance Guide offers basic information and practical guidelines aimed at the preservation and continued health and survival of oak trees in the residential landscape. Increasing pressure for development is changing the oak woodland of Los Angeles County. Heritage oaks which once survived in open rolling hills are now being preserved or replanted and incorporated into the community.How do we protect these trees during the planning and development process, and ensure their survival once they are in the home garden?The Oak Tree Oak Trees in the residential landscape often suffer decline and early death due to conditions that are easily preventable. Damage can often take years to become evident, and by the time the trees show obvious signs of disease it is usually too late to help. Improper watering, especially during the hot summer months, and disturbance to critical root areas are most often the causes. This booklet will provide guidelines on where these critical areas lie and ways to avoid disturbing them, as well as information on long-term care and maintenance of both natural and planted oaks. Lists of additional resources for more information and demonstration areas to visit are also included. The Los Angeles County Oak Tree Ordinance has been established to recognize oak trees as significant historical, aesthetic, and ecological resources. The goal of the ordinance is to create favorable conditions for the preservation and propagation of this unique and threatened plant heritage. By making this part of the development process, healthy oak trees will be preserved and maintained. The Los Angeles County Oak Tree Ordinance applies to all unincorporated areas of the County. Individual cities may have their own ordinances, and their requirements may be different. Permit Requirements: Under the Los Angeles County Ordinance, a person shall not cut, destroy, remove, relocate, inflict damage, or encroach into the protected zone (see text) of any ordinance sized tree of the oak tree genus without first obtaining a permit. Damage includes but is not limited to :
For more information about the County Oak Tree Ordinance, visit the Forestry Division’s or contact: Department of Regional Planning Types of oaks commonly found in Los Angeles County: Many kinds of oak trees are native to Los Angeles County. A few of the more common ones are shown below, but all oak trees are covered by the Oak Tree Ordinance. Older oaks which have thrived under the natural rainfall patterns of dry summers and wet winters often can’t handle the extra water of a garden setting. These trees must be treated with special care if they are to survive. Those oaks that have been planted into the landscape or sprouted naturally tend to be more tolerant of watered landscapes. These vigorous young trees may grow 1½ to 4 feet a year in height under good conditions. Once established these trees would benefit from the same special care outlined in this guide.
The protected zone defines the area most critical to the health and continued survival of an oak tree. Oaks are easily damaged and very sensitive to disturbances that occur to the tree or in the surrounding environment. The root system is extensive but surprisingly shallow, sometimes radiating out as much as 50 feet beyond the spread of the tree leaves, or canopy. The ground area at the outside edge of the canopy, referred to as the drip line, is especially important: the tree obtains most of its surface water and nutrients here, and conducts an important exchange of air and other gases. The protected zone is defined in the Oak Tree Ordinance as follows: “The Protected Zone shall mean that area within the drip line of an oak tree and extending there from to a point at least 5 feet outside the drip line or 15 feet from the trunk, whichever distance is greater.”
Any change in the level of soil around an oak tree can have a negative impact. The most critical area lies within 6’ to 10’ of the trunk: no soil should be added or scraped away. Water should drain away from this area and not be allowed to pond so that soil remains wet at the base. Retaining walls designed to hold back soil above or below an existing tree should avoided if at all possible, especially within the protected zone. These types of structures cause critical areas at the drip line to be buried, or require that major roots be severed. Water trapped at the base of the tree could lead to root rot or other impacts, and to the decline and premature death of a highly valued landscape tree. Construction activities outside the protected zone can have damaging impacts is on existing trees .Underground water sources can be cut off due to falling water tables, or drainage may be disrupted. Trenching Digging of trenches in the root zone should be avoided. Roots may be cut or severely damaged, and the tree can be killed. If trenches must be placed within the protected zone, utilities can be placed in a conduit, which has been bored through the soil, reducing damage to the roots. Insist that as many utilities as allowed be placed in a single trench, instead of the common practice of digging a separate trench for each individual line. Trenching can also be accomplished using hand tools or small hand held power equipment to avoid cutting roots. Any roots exposed during this work should be covered with wet burlap and kept moist until the soil can be replaced. Soil Compaction and Paving The roots depend upon an important exchange of both water and air through the soil within the protected zone. Any kind of activity that compacts the soil in this area blocks this exchange and can have serious long-term negative effects on the tree.If paving material must be used, some recommended surfaces include brick paving with sand joints, or ground coverings such as wood chips (note the advantages of natural materials for providing nutrients under mulching).
WATERING The key is prevention – do not over water. Improper watering is often overlooked as the cause of tree death because it can take years for the damage to show. Once the tree shows obvious signs of decline, it is often too late to correct the problem. The seasonal weather pattern for this region is one of dry summers and winter rain. Oak trees are naturally drought tolerant and adapted to this cycle. If the tree is vigorous and thriving it should not require any additional water. If the natural source of surface or underground water has been altered, some supplemental water may be necessary, but proceed with caution. The goal of any watering schedule for oak trees should be to supplement natural rainfall and it should occur only when the tree would normally receive moisture. This might be in the winter, if rains are unusually late, or in spring if rainfall has been below normal levels. Over watering, especially during the summer months, causes a number of problems which can lead to decline and eventual death of the tree. It creates ideal conditions for attacks of Oak Root Fungus by allowing the fungus to breed all year. In addition, both evergreen and deciduous oaks grow vigorously in the spring and naturally go dormant in the summer. Extra water only encourages new tip growth which is subject to mildew. Oaks need this period of rest. Newly planted oaks may need supplemental watering during their first few summers. After they become established water should be applied according to the previous guidelines.
For oak trees the periodic removal of dead wood during periods of tree dormancy should be the only pruning needed. Any cutting of green wood opens scars that could allow the entry of organisms or disease. Before pruning obtain the advice of a certified arborist or other professional and consult the local city or county where the tree is located to find out what regulations apply. Pruning of both live and dead wood can sometimes require a permit.
MULCHING
Disease and Pests Trees that are stressed, especially because of improper watering practices, are prone to certain diseases and attacks by pests. The most damaging of these diseases is the Oak Root Fungus Armillaria mellea. Occurring naturally in the soil, the fungus thrives under wet conditions and dies back in the summer when soils dry out. This is why summer watering of oaks can be a deadly practice. As noted in the watering guidelines, wet soil in the summer allows the fungus to grow all year. As the population grows, their natural food sources are depleted and they begin feeding on oak tree roots. The fungus does not require an open wound in the tree to gain entry. Indications of the fungus include:
Once the tree begins to show obvious signs of infection treatment is generally ineffective. The best treatment is to avoid the conditions that lead to Oak Root Fungus infections. Pit Scale, Oak Moth, and other pests: any significant changes in leaf color, branch die back, presence of black sooty materials on leaves or other changes should be noted. Seek the advice of a professional forester, arborist, farm advisor or other expert before the application of any pesticides on an oak tree.
The natural leaf litter is by far the best ground cover within the protected zone. If plants must be placed, the following guidelines should be followed: There should be no planting within a minimum 6 to 10 feet of the trunk. Avoid plants that require any supplemental water once established. Choose plants suited for “dry shade.” Those listed in the box below offer some good choices. To see some examples of how these plants have been used under oaks refer to the Additional Resources section on the following page.
Plant Name Description NOTES:
Prevention Bureau, Forestry Division University of California California Native Plant Society The California Oak Foundation Arboretums and Botanic Gardens Los Angeles County South Coast Botanic Garden Los Angeles County Descanso Gardens Rancho Santa Ana Botanic Garden The Lummis Home
Compatible Plants Under and Around Oaks. Bruce W. Hagen… [et al]. The California Oak Foundation.2000. Growing California Native Plants. Marjorie G. Schmidt, Univ. California Press. 1981. Illustrated Guide to the Oaks of the Southern Californian Floristic Province. Fred M. Roberts. FM RobertsPublications. 1996. Living Among the Oaks: A Management Guide for Landowners. University of California Integrated Range Management Program. 1995 Oaks of California. Bruce M. Pavlik…[et al]. Cachuma Press & the California Oak Foundation. 1995. Proceedings of the Fifth Symposium on Oak Woodlands: Oaks in California’s Changing Landscape. Available from the University of California Integrated Hardwood Range Management Program. Regenerating Rangeland Oaks in California. University of California Integrated Range Management
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