Regional Gardening Reports :: National Gardening Association

In the Garden:
Northern California Coastal & Inland Valleys
April, 2006
Regional Report

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Treat magnificent oak trees with the respect they deserve. This one is more than 150 years old.

Mighty Oaks

I love days when I learn something new. Ideally, every day should be like that, but perhaps some days I'm not paying attention, or maybe information just slips over my head. At least I was paying attention when Arborwell Tree Experts came to do a segment on oak trees for Henry's Garden, the show I produce for KRON TV.

The oaks at Henry's house are actually on county property that adjoins his lot. The lovely stand of California live oaks (Quercus agrifolia) are right next to the driveway on a very steep slope. It was not a problem for our agile cameraman, Art Takeshita. He was up and down that hill like a mountain goat!

The first thing that the Arborwell certified arborist pointed out was an amazingly long limb that seemed to be held in place by sky hooks. He said that oak is exceptionally strong, and that's the reason the British Royal Navy built their fleet entirely from this magnificent wood. The limb in question was at least 75 feet long, perhaps longer, and held fast to the huge trunk by a branch that had a circumference of about 6 feet. As it left the main trunk, it twisted and turned, following some divine plan, and it supported the foliage far out from the main body of the tree.

Interplay of Canopy and Roots
The arborist explained to Henry and me that oak trees do not have a main taproot but have a large, shallow area of roots that survive under the dense canopy of foliage and are protected by a thick layer of leaf litter. The roots only grow into the ground 10 to 18 inches deep! The larger the tree canopy, the larger the area of root growth. The canopy shades the soil that covers the roots, conserving water. In summer, after the winter rains have stopped, the trees stop growing to conserve both energy and water. I didn't know this, but you should never prune more than 15 percent of the canopy or you'll remove the shade that protects the roots from our dry summers.

One thing I did know was that you should never plant anything under an oak tree. Water during the summer drought season is deadly to these handsome natives and will result in various fungus and disease issues. If you have oak trees on your property, mulch under them to the outside edge of the canopy with river stone or some other decorative material that doesn't need irrigation.

Proper Pruning
Henry and I were instructed on the proper pruning technique, cutting only on the outside of the branch collar. The arborist illustrated a properly healed cut, and then pointed out a branch that had been cut back too far against the main trunk of the tree. The difference was astounding! The improperly pruned limb had rotted all the way into the heartwood.

It seems that oaks do not need intervention of mankind, thank you very much. Please give these trees the respect they deserve, and leave well enough alone!

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