On December 22, I noted how one can take advantage of the lower position of the sun in mid-winter and plant deciduous fruit varieties near tall evergreens in a way as to maximize the accumulation of chilling hours. That post contained a photo of the hedgerow located within the corralito, which is itself located within the grapefruit grove at the lowest part of the finca.

The above photo is a close-up, taken today, of the middle portion of the hedgerow after I pruned the trees. Notice that the lower and back-side branches, on which I was careful not to prune off too many flower buds, remain in shade, even though the upper parts of the trees are in sun.

The tree in the foreground is a Moorpark apricot (the best, in my assessment). The two beyond it are Earli Autumn and Autumn Glo apricots. The latter two are new varieties with an unknown chilling requirement, but probably over 500 hours (but certainly not the 800 shown in the Bay Laurel Nursery catalog). Moorpark probably has a chilling requirement of around 600 hours, which is more than we can count on accumulating over our short and mild winters. However, by maximizing winter shade, one can obtain more chill than would be the case in a location that remained sunnier in the winter months, and thus can greatly expand the range of varieties that can be grown.

All three trees fruited last year, and all the fruit was on lower or back-side branches. Chilling accumulation works through the individual buds, not the tree as a whole or the roots.*

Today around 11:15 a.m. when the photo was taken, the temperature differential between sun and shade was enormous: 79 in the sun, but only 71 in the shade. The termperatures will converge somewhat later in the day, but the difference in late morning indicates how much more slowly the shaded area warms up. [UPDATE: The temperatues did not converge. The high in the shade was 82, while out in the open it was 89. I should have known: shade/sun high temperatures converge less the lower the humidity.]

In December, 2004, the average lows and highs were 43 and 67 in the sun, but 41 and 66 in the shade. For January, 2005, the figures were 44/67 versus 43/65. (The lows are affected as well as the highs partly due to the large trees trapping cold air that keeps on flowing downhill out in the open, but also because with less daytime heating, the shaded region also does not stay as warm overnight.)

This difference of a degree here and there may not sound like much, but can accumulate to 100 or more estimated chill hours for the season compared to what the electronic half-hourly count indicates for the thermometer in sun. The greater chill is at least as much a product of the slower warm-up in the shaded area as it is of differences in the ultimate high (or low) temperature over any 24-hour period.

Below is the manual thermometer that I keep in the shade of some grapefruit trees just outside the corralito.

The photo below shows the electronic thermometer that relays to the Davis Instruments Weather Pro console (and ultimately to the Ladera Frutal computer station) and proviudes half-hourly termperature readings used to compute chilling accumulation.

Note that while this thermometer, visible on the pole to the right of the photo, is in the sun, it has a plastic shield around it. This prevents sunlight from shining directly on the sensor and giving a false reading. In other words, it ensures that what is being measured is ambient air temperature and not the intensity of reflected solar radiation.**

The above photo (more easily viewed in a larger version) also shows the networking system that transmits the data up to the office. Off to the far left of the photo is a second temperature station (as a backup) on a pole that is slightly askew (and almost obscured by the white trunks of grapefruit trees that have been cut down for re-grafting). On the lower part of that pole is a wireless data repeater. Also visible through or between native shrubbery up the slope are three other posts (some of white PVC) on which other repeaters–five in all–are arranged to relay the data.***

This is shaping up to be a much lower-chill winter than last, when we reached around 550 hours according to the half-hourly data transmitted through the network (and thus perhaps 650 in the shaded area). As of the morning of January 1, 2006, we were sitting at around 174 hours at the lower part of the finca (and 68, with a prior peak before the warm spell of around 95, up here in the more subtropical climes of F&V HQ). A year ago by January 1 we were already closing in on 300 hours below (and 190 at the office). Unless a good cold snap comes soon, the higher-chill varieties are unlikely to fruit next summer. Thus it is a good thing I have planted lower-chill varieties, too, to the extent possible.

One last photo below shows a Hunza apricot, which I planted last January. The Hunza comes from the high moutain valley of the same name in Pakistan. This tree is outside the corralito, but nearby. Notice how it is planted at the edge of a shady area. As it grows, much of the tree will be shaded. Hunza probably has a very high chilling requirement, but given how fantastically sweet and toffee-like the dried fruit of this apricot is (and with an edible kernel, too, with reputedly cancer-fighting properties), it is worth experimenting with.

*I mention roots because I have heard of people packing ice around deciduous plants to maximize bloom, but freezing the soil is hardly useful to buds located some distance above.