White paint is used in freezing climates to prevent the southern side of the tree trunk from being warmed in the sun to an extent where the sap will start running in the winter. The bark on the south side of a tree that has not been painted white will be severely scarred by the repeated freezing and thawing action. White paint also serves to reduce tissue damage on sunny days in hot climates. If the tree has enough canopy and branches to shade the trunk white paint isn't necessary.
I think Dubyadee's got the 'real answer,' Though I remember seeing this question before @ viewtopic.php?t=247
As I may have noted in my answer, around here (Willamette Valley) it simply 'looks nice'... I still use my fading white trunks to monitor ant activity, and as a base for Tanglefoot to stop them. There may be a legitimate use of 'whitewash' on Fuzzy kiwi fines; they're definitely susceptible to the early freeze damage described for trees ... though in 8 or so years, I've yet to do it...
Other than giving that Amish Orchard look, the white paint has probably kept of few of my younger trees from being backed into or run over; in fact - they're in need of a fresh coat!
Here in the hot inland valleys of Southern California I have to paint the trunks of young apple trees white to avoid sunburn. On young trees if left exposed, the sun will scorch the bark and kill it, and the pacific flathead borer hones in and lays an egg on the damaged bark. Since the tree can't "sap" the borer out there, it gets a foothold and quickly moves to the side of the damaged bark and starts feeding on new wood.
Besides protecting the tree from sunburn, the white paint makes it easier to see the "frass" from the borer so I can dig it out. We used to spray Lindane, but that's outlawed here now and had marginal results anyway. Bayer makes an insecticide that you mix with water and pour at the roots that's supposed to protect a tree for a year, but it hasn't worked to kill active infestations. So until then, it's white trunks all round the yard.
Once the tree is about 7 or 8 years old, the bark it pretty immune from sunburn. But up until then, borers are the leading killer of young apple trees here.
Kevin, excellent point on the Pacific Flathead Borer. I've lost several fruit trees to them over the years... The white exterior latex paint I use also allows me to spot their entrance holes and frass. With a single tree attack, if noticed soon enough, I'll poke an equal diameter stiff wire into the hole to crush the beetle, or, use the little red tube connected to some WD-40 and squirt that inside too! I figure, if it dosen't get the beetle, the tree's going to die anyway. And, it's worked!
I've had them attack apple, pear, European 'prune,' and Japanese 'plums.' And, as you described, only the young trees. The only sunburn damage I've had is after scraping off 2 inch thick layers of moss from the upper branches of some old apple trees. After pruning off their 'water suckers,' the limbs were exposed for the first time in decades -- guess I should have painted them white!
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