Taking Care of Our California Native Oaks

Taking Care of Our California Native Oaks

9/5/14 Note: I’ve added an article on care of oaks during drought.

I love our majestic old native oaks here in California. And like many, I get distressed when I see one dying, especially from improper care.

Background: There are nine species of native tree oaks in California – In SoCA most of our live oaks (called ‘live oaks’ because they are green year-round, not deciduous) are Coast Live Oak (Quercus agrifolia) or Valley Oak (Quercus lobata). Scrub Oaks and the rarer Englemann Oaks are common ones in our area.

But back to dying oaks. A neighbor’s 40 foot oak tree die recently. Although they had removed iceplant around the trunk, an arborist surmised it was oak root fungus due to overwatering. The watering of their lawn, situated on a slope above the tree, drained towards it, plus a neighbor’s irrigation water ran down the curb next to it. A double whammy. There’s also a possibility that the Round-up, used to kill the ice plant underneath it, may have added to its woes.

This oak likely died from overwatering – a combo of several factors.

“Never introduce water to an old oak tree.” Bert Wilson, Las Pilitas Nursery

I sit on our city’s tree committee, so I hear about many oaks that are in trouble. Undoubtedly, discussion includes whether they’re watered too much and what’s planted underneath. The committee is devising a handout on proper care of oaks. But in the meantime, I wanted to share some guidelines suggested by oak experts.

1. Don’t use rocks directly around the base of oak trees. Rocks will retain moisture around the trees crown and collar and could potentially kill the tree through drowning or through fungus. Only use mulch around the oak tree base.

2. Mature oaks in the summer need NO water in summer and little the rest of the year. An oak tree’s growth begins in late winter after the seasonal rains. Watering in the summer may cause root fungus. (Gradually reduce the irrigation that is watering oaks; any grass might die but should not be under oaks anyway. Only water oak trees in the winter if it has been an unusually dry year.)

October 2013 addition: There are different opinions on mature oaks that have been receiving irrigation. Bert Wilson from Las Pilitas Nursery says if a mature oak has been watered for most of its life, “keep watering; if watering was a recent introduction, remove it over a period of a year or so. NEVER introduce water to an old tree.”

3. Avoid planting under the oak’s canopy. If you need to plant something, use plants that require similar soil types and water requirements; for example, native plants such as Currants (ribes species)or coral bells. Las Pilitas has a good website page on this.

Las Pilitas’ Wilson recommends people remove non-native plants, including weeds, from under the native oaks. “This makes a dramatic difference.They should not disturb the soil when they remove them, i.e., cut them off at ground level instead of digging them out.”

Oak at Rancho Santa Ana Botanic Garden (Claremont CA)

4. Mature oaks need no pruning. If some is necessary, never prune more then 20% green out of your oak. Also, trimming too much green foliage from the branches can lead to sunburn on the trunk and main branches.

5.  Pruning is best in July or August — the hottest months of the summer during the trees dormant season.

6. Oak trees can get sunburn too. Never remove all of the green foliage from the branches since the trunk and main branches will get sunburned.

7. Avoid azaleas or rhododendrons near your oaks. Besides requiring different soil types they can transmit fatal diseases.

For more info:

California Oak Foundation

– California Oaks Foundation oak care pamphlet

– Great book: Oaks of California, Cachuma Press

Most large oaks in our area are Coast Live Oak (Quercus agrifolia) or Valley Oak (Quercus lobata). Englemann and Scrub Oaks are also common.

Comments

  1. I have six live oak volunteers that grew from acorns I threw out in my yard two summers ago.

    I’ve been guarding them with my life, but now understand I may have been watering them too much recently, based on your info. Thanks.

  2. hey Heather, they’re probably fine, especially since they’re so young. It’s the mature ones that don’t need the water in the hot summer months. I’ll try to find an oak expert to find out how to best wean them off the supplemental water as the years go by – a good question….

    • Leslie Lipton says:

      Hi. Saw the question on weaning oaks off supplemental water.
      I am moving into a house in the foothills outside Fresno. There is a huge beautiful live oak in the yard. A large area under the tree has been drip watered for 20 minutes per day for the last ten years. I am removing the exotic plants from underneath the tree and plan to replace with appropriate California natives. However, how do I wean the oak of the supplemental water? It seems extremely healthy and happy.
      thanks much.

      • if its seems healthy and happy I would probably leave the irrigation as is or decrease it to several times a week. The newly planted natives will need extra water the first couple years anyway. But you know, I’m not an expert on that – maybe you can run your q by a local arborist? (fyi, we watered our newly planted natives very heavily at planting, then every few days the first month or two – depending on the season we planted, then once a week the first year. They don’t need everyday for sure.)

  3. Very good article! We will be linking to this great post on our site.

    Keep up the good writing.

    • Hi all, I wanted to update with some input I received from Bert Wilson, owner of Las Pilitas native plant nursery – I asked him about mature trees that have been irrigated. He said this: “if they have been watered for most of their life, keep watering, if watering was a recent introduction, remove it over a period of a year or so. NEVER introduce water to an old tree.
      BUT, they should remove any non-native plants, including weeds, from under the native oaks. Makes a dramatic difference.They should not disturb the soil when they remove them, i.e., cut them off at ground level instead of digging them out.”

  4. Karen Kaminsky says:

    Help! I am concerned about my oak trees here in Ojai,Ca. In the last ten days, my neighborhood has lost 2 – one might have come down due to watering, the other which lost a lot of cracked off branches, definitely not. It is residing in an open yard that is abandoned, so only got Mama Nature’s water. I have read conflicting reports about watering with soaker hoses at tree canopy edge in a drought. Any thoughts? My trees are big, dry, and old! Thanks.

    • Good qs – I will do a separate blog on this again, and you’re right that there are conflicting thoughts. But in general, it’s ok to water native oaks during drought but timing is important. Here are a couple people’s answers on this issue who I respect. 1) From local arborist. Paul Chaney, here in the Inland Empire (where we have hotter weather than you do) “During unusually dry winters, (drought,) mature oak trees might benefit from (infrequent, deep) supplemental water. (Use a soil sampling tube to check soil moisture 12″ to 18″ deep before deciding to irrigate.” 2) From Bart O’Brien from Rancho Santa Ana Botanic Garden in Claremont, slightly inland from you: “IN GENERAL (there are always exceptions that can be found)
      For PRE-EXISTING native oaks: 1. Do not water in the summer months. 2. During droughts, water the tree in the winter and early spring months (stop by the end of March) – this is the time of year that the tree banks water for the year.
      For PLANTED native oaks:
      1A. If it is an OLDER tree (15 plus years) that has been in the landscape for years, continue whatever irrigation regime that the tree has grown up with as that is what it has adapted to.
      1B. If it is a newly planted young tree, you can get it to adapt to drought or watered conditions. You will have to water a newly planted oak for at least a few summers before it will survive with minimal to no summer water (some of this depends on the size of the plant when it is planted, and it will also depend on the soil conditions).”

      I would consult with an expert in your area too, but from the above, you might want to wait til the fall to water. With our other native shrubs and trees, we do supplement water in the late spring through summer tho we wait til a cool overcast day. Lastly, there is the gold-spotted oak borer that has killed many trees in San Diego but that should not be in your area.

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