It is a sad thing when a mature tree dies and a good time to review gardening practices that will help keep our trees healthy. I did not see the Engelmann oak that fell last week in South Pasadena and cannot comment on the cause. I do know that while I was employed at Rancho Santa Ana Botanic Garden, several large, mature, native trees fell. Often the cause was not obvious nor something that could have been remedied. Sometimes it was due to an extreme weather event; other losses may have been caused by environmental conditions, including air pollution, and weather and climate stresses. Sometimes trees just fell with no apparent disease or cause.
Although we should consider whether anything could have been done to save this particular tree in South Pasadena, I think it is not productive to be too quick to blame. Trees are living organisms and sometimes they just die.
Having said this, here are practices that promote stronger, healthier, more long-lived trees, and should be followed in our city.
For new trees:
- When planting, care should be taken to select healthy new trees. Healthy young trees do not have a “lollipopped” shape but rather have juvenile branches growing along the main stem. Smaller container-sized plants are usually better than larger specimens. Each specimen should be inspected before planting to ensure that is disease-free and has a healthy root system that is not coiled or pot bound.
|Small healthy tree with juvenile branches intact, properly staked. Grass should be cleared from the base of the tree.|
- The nursery stake that is tied to the main stem of newly transplanted trees (as is the case on Fair Oaks Drive) should be removed as soon as the trees are planted.
|Nursery stake left on newly planted tree can cause damage to the bark, poor root develop and arching trunk. It should be removed as soon as the tree is planted.|
- Trees should be properly staked, but only when absolutely necessary. These stakes should be removed as soon as possible to allow for the development of a strong and healthy root system, and to prevent damage to the bark.
|This tree in Santa Monica should have been liberated from its stake years ago. It is unlikely anything can be done to save this tree.|
|The soil is piled too high on this newly planted tree. The flare (where the trunk flares out to roots) should never be covered. This will easily kill this young tree unless corrected.|
For mature trees, especially native oaks:
- Mature locally-native oaks do best with little to no summer water. Locally-native plants are adapted to our weather conditions and usually do not require supplemental water; however, Engelmann oaks may benefit from occasional deep watering, especially in their early years, and those growing in very lean, well-drained soil.
- Trees that require supplemental water should be watered deeply, thoroughly and infrequently. Surface water encourages surface roots; deep water promotes deep roots. Furthermore, wet conditions during hot weather promote the growth of disease-causing bacteria and fungi in the soil. Our native oaks are especially susceptible to root and crown rot due to warm, moist soil.
- Keep the canopy and crown (area at the base of the tree) clear of plants and debris. Mature native trees, especially oaks, will be healthiest without turf grass beneath the canopy (as is the case in Garfield Park). Allow natural leaf litter to collect beneath mature trees.
|This magnificent tree, nicknamed Majestic Oak, is hundreds of years old. Horticulturists at Rancho Santa Ana Botanic Garden care for it by keeping the base clear of plants and allowing leaf litter to accumulate naturally beneath it. The tree and the nearby low-growing native shrubs receive no supplemental summer water.|
- Topping or severe pruning is detrimental to trees. Mature trees should only be pruned under the supervision of a licensed arborist. Mature trees require little pruning of the living canopy, never exceeding 25% – and even this much can severely stress a tree.
- Any construction or grade change near a mature tree puts the tree at great risk. All efforts should be made to keep this kind of disturbance well beyond the canopy of valued, heritage trees.
|These liquidambers in So Pas require severe, ongoing pruning to keep the power lines clear. Trees should never be pruned this way and should not be planted where they can become a hazard.|
Although we may not be able to determine what led to the demise of the Engelmann oak on Ashborne, we can know that many of the practices that can be observed throughout our city put old and young trees at risk. If our city truly wants to be a “Tree City” it should do more to protect this most valuable asset.
For more information on keeping trees healthy consult:
Care of California’s Native Oaks
International Society of Arboriculture (ISO)
New Tree Planting
US Forest Service, Tree Owner’s Manual
|Our Engelmann oaks are the pride of So Pasadena. This highly endangered species grew naturally in our town. Young ones should be planted and mature trees cared for and treasured.|