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, too little and what’s planted underneath. 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: With our lengthy drought, please see this updated article “Taking care of our oaks during drought – There are different opinions on mature oaks that have been receiving irrigation. The late 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 speciesor 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 – and need their foliage to support their great bulk.  If some is necessary, avoid pruning more than 10% of the green from your oak. (Long-standing industry standards put the ideal annual foliage removal for large mature trees at 5%).

5.  Pruning is best in July or August into fall— during the trees dormant season. A local arborist prefers late summer or early fall because foliage removal during extreme heat heaps one stress on top of another.

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

– 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.

About Linda Richards

My goal is to educate about the science of nature in layperson speak, through my writing, science and education background. I grew up in the Chicago area, loved living in Minneapolis before gravitating to the West, which is now home.


  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.

    • Bert Marcum says:

      Hi Heather,
      We just had our 60+ year old live oak pruned (the very beginning of summer). It is spectacular, a living sculpture. If you are the Redlands Heather, drive by sometime. It is on the West side of Elm the second lot South of East Crescent.
      Ours is watered by surface sprinkling of vinca and ivy close by. Also, grey water discharge not too far away. Only 2-3 loads in washing machine per week.

      • Linda Richards says:

        thanks Bert, a good example of how oaks have grown up on differing watering regimens. Seems a big takeaway is if one is doing well, continue doing what you’re doing!

  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.

  5. Dave Detwiler says:

    I have lived in a rural community for 12 years in Southern California and love the 40 ft oak tree that sits between my neighbors house and mine on their property. Earlier last year the neighbors trimmed larger branches near the house and the tree seems to have an an uneven canopy. Last September the neighbors decided to rescue a horse and house it in a pen 15′ square with the trunk of the oak in the center. The horse lived under that oak tree for about six weeks until animal control came. Now, starting in mid February, the tree seems to be losing an extremely large amount of leaves while trying to grow new growth. Is this normal?

    • I’m not an expert so best to talk to an arborist. Some thoughts: Oaks vary in how much they shed (Englemann’s replace their leaves, our neighbor’s mature oak – not sure what type, is shedding quite a bit right now but there are plenty of green leaves.) Activities around an oak can damage the roots, and horses’ manure would be an amendment that oaks should not have – so have they removed all the manure – if not they should…. After that some good watering since we haven’t had all that much rainfall – maybe you can water it on your side?

  6. Kathy Manion says:

    I am re-landscaping my yard, (reducing lawn area), but to establish a level grade, it may be necessary to cut some surface roots that are 1-2 inches in diameter. These roots are located just out side the canopy about 35 feet from the trunk. Will this hurt the tree?

    • sorry for the late reply, I don’t really know – an arborist would be the best one to answer but being outside the canopy is probably not as serious. I would water it well afterwards – but again, a tree specialist would be better to check with…..

  7. Our oak trees are full. We needed to trim the branches over the roofs but otherwise thought that the best time of full trimming and shaping in Northern California was in the winter. Please advise the best reference since we have been given conflicting information from different true trimming businesses. Thank you.

    • you’re right in there are conflicting opinions. Our local ISA certified arborist, Paul Chaney, who I often check with, says this about oak pruning – “Even though the growth slows in the heat of the summer, I like to wait until later in the summer, or early fall, because foliage removal during extreme heat heaps one stress on top of another–and that can’t be good.” That’s here in So CA so it could be a little later north is okay too. (I did change the text to through fall)

  8. Pam Marino says:

    I recently bought a home in Pacific Grove with a lovely Coast Live Oak in the front yard. I noticed the company that prepped the house for sale covered the front yard with thick, black plastic, with tanbark on top. I was worried this would be bad for the tree, so a few weeks after I moved in I removed the plastic, leaving the tanbark. I believe the plastic may have been there for up to two months, but I’m not sure. Now some of the leaves are turning brown, and the tree does not look as healthy as surrounding trees in neighbors’ yards. Barring any other diseases or issues, does the tree have a good chance of recovering from having the roots covered by plastic?

    • Linda Richards says:

      Good move taking it off. This is a hard season for all trees (especially for us in SoCA with the high temps) so some shedding especially with the stress, is normal. A deep soak around the perimeter of the tree might help…. And let the company know that’s bad practice on native trees like that!

  9. Reggie Wilkins says:

    I have the same issue as Pam above but here in Altadena California. Maybe 100 to 200 year old Coast Oak was draped in artificial turf about two years ago. The tree is a border tree although the trunk is on their side. It defoliated about 80% of the tree. Turf is gone now but arborist is 50 50. Growing shoots from inside bottom like reverse gravity working to the bare ends. Big improvement but so many leaves died…

  10. Rose Riedel says:

    My 200 year old live oak leaves are turning yellow. No other tree around mine is.

    • Linda Richards says:

      I would contact an arborist… I have a msg into ours but haven’t heard back – but at 200 years old seems it needs its own consult…

  11. Evelyn Chiang says:

    I have a heritage valley oak tree in my back yard in San Jose area. It’s growing very close to my house, maybe less than 5 inches. Is there any danger? What do I need to do? Will the unusual wet winter weather cause more threat?
    please help

    • Linda Richards says:

      seems if it’s a heritage tree that it’s protected by the city and so you could have a city arborist come look at it? Or if not under city jurisdiction, I would still get an arborist to look at it. Good luck….

  12. victoria v says:

    Hi, Can you tell me how often a mature oak tree needs to be trimmed/ pruned ? annually or every other year?

    • Linda Richards says:

      really, as little as possible. Think about them in the wild – they don’t get any pruning. Mainly for safety reasons and maybe dead branches removed. A mature tree should only have 5% foliage removed, and not often at all. I’ve heard several well respected arborists say this. Mature trees expend a lot of their valuable energy just trying to survive and any pruning is stressful om them. Thanks for the great q!

  13. Honey Hill says:

    I live in Folsom, Ca. (Sacramento area) and there is a large old oak to the side of my house. It has many dead branches hanging down, and has lots.of weeds (fetch) around under it, and many small rocks also. Should the weeds be cut, and raked up , and the rocks removed?, leaving soil and leaves, at the base, and the dead branches and limbs trimmed in the Fall?? It only gets water from nature(although it is in a small ravine) please advise asap

    • Linda Richards says:

      I am not an expert. My understanding is the weeds are good to remove but small rocks are ok as long as they are not right at the base (think of what you see out in nature; also, it sounds like the tree has done well with the rocks for awhile.) And yes, leaving the tree’s leaves on the ground, and disturb the soil as little as possible. We only use ISA certified arborists for our trees – you can find links to them at http://www.isa-arbor.com/findanarborist/arboristsearch.aspx — Safety, such as removing dead branches, is a number one issue. In general, except for safety issues, mature oak trees need little pruning – they need as much foliage as possible for their big bulk.

  14. Do you think a coast live oak could grow in a rocky area where bedrock is about 10″ below the surface. It’s a flat area in San Francisco that would get at least 6-8 hours sun. I have a beautiful little 3′ tall oak and need to plant it somewhere. I could continue to water it till it would get established if it could establish itself.

    • Linda Richards says:

      Hi Lori, I’m not an expert but I do see on the question/answer on this website – https://www.wildflower.org/expert/show.php?id=8708 – that a study showed 3 live oaks growing well down into bedrock. They also have important lateral roots, especially as the tree ages, so that is important too. And yes, deep infrequent watering to help it get established would be ideal. Thanks for writing…

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