Well, the heat wave is gone at last, and the forecast promises a chilly weak ahead. Today’s low down at the bottom of Ladera Frutal: 38. Up at the summit: 45. Today marks the first day of the 2005-06 winter season in which we will accumulate chill hours. Deciduous fruit trees–apricots, peaches, plums, etc.–require an accumulation of “chill” before they will bloom and set fruit the following spring. This is one of nature’s fascinating evolutionary adaptations. It serves to keep a tree from budding after a mid-winter warm-up only to have frost or freeze later kill off the blooms and thus the fruit (and, more importantly from an evolutionary perspective, the seed inside).

Over centuries of propagation and selection of naturally occuring variations, as well as selective breeding by horticulturists, we now have thousands of different varieties. Within any given species (plum, peach, etc.) the chilling requirements of varieties can vary dramatically. Ladera Frutal is located within what is normally considered a “low chill” region, because our winters are mild. However, because we are on a steep slope, the bottom of which is a canyon that drains from the interior foothills, we get to take advantage of micro-climates. Cold air sinks and follows canyons and notches in the hills as if it were water. Thus the lower slopes of the finca can receive anywhere from 500-800 chill units over the months from November through March, while the top may be more like 250-450.

If your location receives over 500, but under 1000, you are in a “moderate chill” environment, and you can grow a wider range of varieties and even whole species–like European pears–that are off limits for low-chill areas.

So, the finca has been planted with these microclimates in mind. This morning’s 7-degree spread in low temperature from top to bottom is not unusual, and the colder it is the greater the spread, because we get significantly cold weather (by local standards) only when the air is relatively dry and the winds calm. And dry, calm air is precisely the sort of condition that favors wide temperature variations over small distances–especially vertical distances. On a really clear night with northwest flow aloft, we could be in the upper 20s with a good frost down below (if it is not too dry), but remain well above freezing at the top.

So the planting strategy means pluots, apricots, cherries, and moderate-chill apples and peaches down below; persimmons, lychees, and low-chill varieties of apples and peaches in the middle; more-tender subtropicals (which can’t take frost) like avocados, bananas, mangoes, and really exotic stuff like sapotes up high. And citrus everywhere, but grapefruit (which can take and even benefit from moderate frost) down below, while most other citrus are in the middle belt to protect them from frost.

As mentioned before, I monitor temperatures and calculate chilling accumulations via a network of wireless electronic thermometers (with solar battery power), all hooked up the the computer.

Today marks the first day when a net-positive chill accumulation will occur. But how long till the next warm spell? When counting chill, one must subtract hours in the 60s and higher. The more hours in the 60s and up, the sooner dormancy breaks, chill or no chill. The more hours in the 30s and 40s, the longer dormancy holds and the better the next spring bloom and set will be. Watching it all develop is the spectator sport that keeps me going between the World Series and Spring Training.