Caring for Mature Native Oak Trees

Native oaks require special attention because their roots share soil space with the Oak Root Fungus which specializes in living off of oak and other woody roots. Under natural California conditions, this fungus (Armillaria) is dormant during the hot, dry summer, and comes to life only with the winter rains.

Our native oaks—Valley Oak, Coast Live Oak, and Blue Oak—do not require and do not tolerate irrigation in the dry months. If irrigation is applied near their trunk during the dry season, the Armillaria fungus will grow due to the combination of warmth and moisture. As the tree matures, continued watering around the trunk maintains the fungus infestation, which in turn will cause the tree to die, or tip over from too few roots to anchor its top weight.

4 key ways to protect your native oaks from disease:

1. Remove competing plants, including ivy and lawn

These plants can hide defects such as areas of decay and can trap moisture around the root crown, creating the perfect environment for fungus. Garbage collects under ivy and it creates a breeding ground for rats. Ideally all competing plants within the drip line will be removed. Laying mulch around the trunk (but leaving the root crown exposed) can help keep ivy and other plants from returning.

IvyTrunkDon’t let your tree look like this! Once established, ivy can be difficult to remove.

How to remove ivy:

The method Canopy recommends is to remove as much of the ivy as you can by hand, including the roots. Ivy that is climbing up a tree should first be cleared away from the base of the trunk.  Then, the ivy should be cut back 2-3 feet around the base of the tree. Pulling down the ivy higher up can damage the tree’s bark. Once detached from the roots, the ivy will die and can then be removed.

You will most likely not kill all of the ivy in this way and some of it will come back. Often you will have an 80% success rate the first time and 20% will return. Keep at it, in the end you will triumph.

If you choose to use a chemical weed killer to speed up the ivy removal process, locate any large main ivy roots. Cut the ivy root cleanly so that there is a fresh wound above the soil. Immediately paint a strong concentrate of Glyphosate, also sold as Roundup, or Kleenup. The Glyphosate will move through the ivy’s system and kill some of the plant without hurting the tree or surrounding plants.

2. Remove built-up soil from around the root crown

Root crown fungus growth slows down when the fungus is exposed to air. By ensuring the root crown is dry and exposed to air, you will prolong the life of your tree. Removing dirt until you can see the root crown is called “root crown excavation.” It may expose the fungus and will remove moisture from the area.

RootflareA properly exposed root crown should look like the one in this photograph.

3. Water properly

The area within 10 feet (or more) of the trunk of a native oak should remain undisturbed and clear of any vegetation and irrigation. Ideally no irrigation should be applied and no lawn installed in the area extending from the base of the trunk out to the tree’s drip line. It’s best to remove existing lawn inside the drip line; this will reduce competition from other plants and help eliminate excess moisture. Do not water or allow water to collect around the root flare. Do not allow sprinklers to spray on the trunk.

4. Have your tree evaluated for root crown infection

Contact an ISA-certified arborist for an inspection. An arborist should also be consulted if you see signs of decay in the wood, or clusters of mushrooms growing from or next to the trunk.