Young native oak trees are much more tolerant and adaptable to environmental changes, while mature, established oaks are intolerant of change. This means that any major change in a mature oak’s environment may weaken the tree or kill it. To reduce the chance of this happening, leave the tree’s RPZ, or root protection zone, undisturbed. The RPZ is defined as one and a half times the area from trunk to drip line. A healthy root system for a mature oak begins with the formation of a tap root when the tree is a seedling. This allows the tree to have assessable moisture. As the tree grows, lateral roots extend horizontally (sometimes up to 90 ft.) past the dripline. This root system also forms fine roots typically within the top three feet of the soil, and these roots absorb moisture and nutrients. To support the natural dry weather conditions in the summer, the tree also grows deep vertical roots, usually within ten feet of the trunk, which helps support the mature tree. Therefore, mature oaks need protection from excessive water, inadequate drainage, over fertilizing, filling, paving, over pruning, and trenching within the root zone.
Excessive water and inadequate drainage can smother roots and encourages crown and root rot fungi. Conditions should be dry during summer months, therefore only drought-tolerant plants that require no summer watering should be planted around mature oaks. Even then, plants should be planted no closer than six feet from the base of the tree. Groundcovers such as cobbles, gravel, and wood chips can also be used because they don’t interfere with getting the right amount of moisture and oxygen. Usually, mature oaks don’t require any irrigation, except in cases where there is an impervious surface within the RPZ. If this is the case, occasional watering may be helpful – as an extension of the normal wet season. Otherwise, oaks should be watered only outside the RPZ. Moist, warm soil at the base of the tree encourages crown and root rot. If a tree needs to be watered, water only once or twice during the summer with a slow, all-day soaking. Shallow watering promotes shallow root growth and also encourages crown and root rot. Rot and root drowning may also be caused by placing a swimming pool, which acts as a dam, downhill from a mature tree.
Mature oak trees usually need no supplemental fertilization, because they get nutrients from the leaf litter that breaks down around the tree. If fertilizing is to be done, it should be done in late winter or early spring. Trees that have undergone recent severe pruning should not be fertilized. Sometimes when leaves yellow, it is not a sign of nutrient lack, but rather a sign of crown or root rot.
Trees that have fill or paving placed with the RPZ are more likely to have problems with soil compaction, which prevents water from soaking into the soil and inhibits gases from being exchanged between roots, soil, and atmosphere. Fill can also trap moisture in the soil and cause crown and root rot. Paving also usually requires excavation to create a stable base for the paving material. Both excavation and the compaction adversely affect the root system. If any paving is needed, decking on piers is the least intrusive to the oak’s root system. It’s also best to leave the natural soil grade within the RPZ and, if necessary, use retaining walls to keep soil out of the RPZ.
Another aspect of oak care is pruning. Excessive pruning affects an oak tree in a number of ways, from exposing interior branches to sun damage, encouraging new growth that is more easily attacked by mildew, and possibly causing decline in the health of the tree. Pruning should be done during winter dormant periods and only weak, diseased, or dangerous branches should be removed. Any wounds should not be sealed, as recent research shows sealants do more harm than good. It is recommended that pruning should only be performed by a certified arborist.
Oak trees also need protection from trenching within the RPZ. Trenching can sever important tree roots when underground utilities are installed. Especially detrimental is when multiple trenches are cut through the roots. If utilities are installed within the RPZ, they should be dug by hand or they should be bored at least three feet below the surface.
Oak tree care also includes being aware of diseases and insects, which can impact the health of the tree. The tree can usually be saved if the disease is caught early. Crown rot is caused by a microscopic fungus and is promoted by saturated soils and poor soil aeration. Symptoms of crown rot are decline in tree vigor, twig die-back and wilting, lesions that ooze a dark-colored fluid, and abnormally yellow leaves. Oak root fungus, or Armillaria root rot, is almost always found in California oak trees, but doesn’t usually affect the tree unless the tree is over watered or weakened in some way. Trees affected by root rot can have branch die-back or yellowing and thinning of foliage. To avoid these symptoms, avoid summer irrigation. Mistletoe is a parasitic plant that can harm oak trees, usually be causing structural weaknesses and branch breakage. Major infestations are difficult to control and it’s best to consult an arborist for advice on mistletoe or any unexplained decline in an oak tree. Insects can also harm oak trees; especially pit scales, oak moths, and other leaf eaters. These insects weaken the tree and make it susceptible to disease. Consult an arborist when insect infestation causes a change in leaf color, twig die-back, substantial leaf loss, or sticky or sooty foliage and branches.
If all of these issues are addressed in an oak tree’s care, the oak should be healthy and last a considerable amount of time.