On Jun 7, 2006, at 12:12 PM, Max Withers wrote: > Also, a new question: I have seen chill requirements expressed as hours > below 45 degrees (F), and hours below 40 degrees. Which is it? The > answer is crucial, at least in zone 17. > > There are many different theories on chilling hours even besides the two cutoff temperatures you mention (and some add negative chill for hours above 65 deg. or 70 deg., or a 50% deduction per hour above some upper cutoff). Regardless of whatever theory you ascribe to, you guys up there in (Sunset) Zone 17, and similar, have plenty. I believe that zone is the only place in the entire U.S. where you can successfully fruit both cherries and oranges every year--cherries requiring a lot of chill hours (at least 900 and more typically 1000 or more each winter) while simultaneously never dropping below about 25 deg. where you would lose the oranges. The Calif. Central Valley also gets a high number of chill hours, but only in very protected areas do they not risk losing their orange crop frequently. It's much more marginal for them for oranges and even more so for lemons than for you guys. Everywhere else in the U.S. that gets 1000 hours of chill per winter is in USDA Zone 7 or lower (except for some Zone 8 areas in the Pacific Northwest--but they can't grow or fruit oranges up there).