Joe Asks: I am interested in planting four or five fruit bearing olive trees on my property. I would then cure the olives for home consumption. My problem is that I have a high boron content (12PPM) in my well water. Are olive trees sensitive to this level of boron in water? Do you recommend any varietals of olive trees that would be boron water resistant? Would dry farming be successful when the olive trees would receive no well water at all? By the way I live in the northeast side of Santa Rosa in Sonoma County California.

Paul Vossen, UC Davis Extension expert responds: Olives are classified as "somewhat tolerant" of boron in irrigation water, accepting water levels of boron of 1 to 2 mg/liter (roughly equivalent to 1-2 ppm). Water with 12 ppm will cause problems for olives and most other crops that are not tolerant to high levels of boron. Just because the well water is high in boron does not mean the soil is also high in boron. A soil analysis would be the only way to determine if there is a soil problem. I suspect that in Sonoma County there is a good chance that the soil will not have a high level of boron, so growing the trees without any irrigation may possible and really your only choice.

The amount of water required by a tree is dependent on climate, that is, how hot and dry and windy the growing season days are to cause the plants to transpire and keep themselves from burning up with drought stress. Olives are trees that are quite drought tolerant, in that they will not die if given little water, however if given insufficient water the trees will grow very slowly (taking 30+ years to reach full size instead of 10) and the fruit will be small (only a problem for table fruit). The rooting depth of your soil is the key factor for dry farmed fruit trees. Just because someplace can be dry farmed does not mean that other sites will also work. Success (growth and production of trees) is highly dependent on how much water is stored in the soil. A good rule of thumb is that most loam soils will hold about 2 inches of water per foot of rooting depth. So if you have a hillside location with 2 feet of rooting depth you will have 4 inches of water available for those trees for the whole season. If you have a deep valley soil with 5 + feet of rooting depth you will have 10 + inches of water for the season. If we get 40 inches of rain that means that all the rest ran off. An olive tree in the coastal climate of NE Santa Rosa, will require about 12 to 18 inches of water per season to be as productive as possible, that is growing well and producing large sized table fruit. For oil the trees might get by with only 10 to 16 inches. Little or no supplemental irrigation would be required to get adequate, but not maximum growth and production, if you have a deep soil. In a shallow soil, the trees would just grow very slowly, have severe alternate bearing, the fruit would be small, there might be fruit shrivel, and the fruit if used for oil, could be quite bitter.

By the way, the amount of water available for the trees is ONLY if you allow no cover crop or weeds to steal the water first. Excellent weed control is extremely important for dry farming any crop. For 4-5 trees put 6 inches wood chips under the trees out to the drip line (or at least a 6 ft. diameter circle) to smother all the weeds and hold as much moisture in as possible. Good luck