As everyone who lives in California knows, this has been a fairly unusual year weather-wise. In the fall there was abundant rainfall, but this was followed by a January when the tap shut off almost completely. Then in March it started raining and snowing almost continuously and has only recently stopped. There was a week or two of warm weather, but now in mid-April, it has turned cold again. Here at UC’s Sierra Foothill Research and Extension Center in Yuba County, almost all of the oaks have now leafed out. The trees are especially beautiful at this time of year since it seems that each of the different oak species has a distinctive color of green. However this new foliage is quite tender and vulnerable to frost damage. Damage to new foliage occurred in Northern California three years ago when there was a late frost and many trees had their new foliage killed. Interestingly it seemed to only occur at mid elevations between 1000 and 2000 feet. At lower elevations, temperatures were not cold enough to affect the foliage and at higher elevations, the trees were phenologically further behind and hadn’t yet leafed out. Fortunately oaks have evolved under conditions where leaf damage regularly occurs (also as a result of low intensity fires or insect defoliation) and normally suffer little long-term damage from late freezes.
Another risk at this time of year is cattle poisoning. Oak trees contain toxic chemicals in newly emerging foliage, including tannins and phenols, that can be lethal to cows if the foliage is a very high percentage of their diet. A couple of decades ago there was a late spring snowstorm that happened after the oaks had leafed out. In addition to knocking many limbs and branches to the ground, the snow covered up available grass so there was little for livestock to eat besides the young buds and foliage. As a result, about 2,700 cattle died due to oak toxicity. In the unlikely event such conditions occur again, livestock operators can prevent lethal damage by feeding their animals hay or other supplemental feed when the natural grass is covered up. But it is important to start hay supplementation immediately and not to wait until cattle get sick or die. A delay of only a day or two can easily result in many more deaths and ill cattle. If cattle are in conditions where toxicity is a longer-term possibility, the use of calcium hydroxide in a supplement can prevent sickness. The addition of 10% calcium hydroxide (hydrated lime) to a supplement will still be palatable to cattle. Then if the cattle will consume about two (2) pounds of this supplement per day it will prevent many cases of oak toxicity. This supplemental calcium hydroxide has to be consumed before exposure to be effective.*
* Information about oak toxicity was gleaned from an article by UC Extension Veterinarian, John Maas in the January 2008 issue of California Cattlemen’s Magazine.