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In this zone I will add palms that in a Mediterranean climate will survive, fairly reliably, temps as low as 20F. There are a few that can survive temps much lower and those will be listed first:

Group A (the super cold hardy palms).

Rhapidophyllum hystrix: this is the most cold hardy of all palms surviving temps as low as 0F, and probably 5 degrees lower. Even lower temps may cut this palm to the ground, but as it is a suckering palm, it may come back from freezes as low as -10F. The length of the freeze is important, though, and any prolonged freeze that low will freeze the soil may kill this palm. But leaf damage happens at or below 0F. This palm is unique in that it also tolerates a tropical climate as well, making it the most adaptable palm species in existence. Some argue Nanorrhops, because of its tremendous drought tolerance, is equally adaptable, but that's like comparing apples to oranges.

Trachycarpus fortunei: this palm cannot tolerate 0F, but has been known to survive 5F without significant leaf damage, as long as the low was only brief. Seedlings of this species are not nearly as hardy, however, and can be damaged at 10F. This palm is grown all along the western coast of the US up into Canada, and seeing snow on it without leaf damage is not unusual. This is not the most ideal palm for tropical climates, so its hardiness may be viewed differently in a cold, humid climate.

Chamaerops humilis: maybe just a bit less cold hardy than Trachycarpus, and certainly more likely to grow back if frozen to the ground. Probably can tolerate 5-7F. Not sure if C cerifera form is any more or less hardy, but grows a lot slower. Tolerates high humidity, but sometimes doesn't look all that great.

Serenoa repens: 10F is probably the lowest this palm can tolerate without leaf damage. Some say the bluer/silver leaf forms are a bit more cold hardy. Florida results for this palm may be different than the lower humidity environments in California or the Mediterranean. It may be a better choice for cold 'humid' climates than cold dry climates, as the latter do not promote fast growth. In low humidity climates where there is no alternative hot days, this palm will often sulk and eventually die. So its cold hardiness along the east coast may be more impressive than along the west coast of the US.

Trachycarpus wagerianus: though listed as damaged at 20F, many growers in the northwest US report this palm can tolerate temps into the low teens without damage. This palm seems to be pretty cold tolerant even as a seedling.

Group B: the hardy palms

Nannorrhops ritchiana: though not as cold tolerant as the super cold hardy palms, this palm can survive an amazing variety of climates, from dry desert, to tropical humid, to cold Mediterranean. However, it is not as happy in that last climate and extended periods of cold may do this palm in. It does seem to survive brief periods of cold even a bit below 15F however. Some argue this is the most adaptable palm in existence because of its wide climate tolerance- arguably wider than any other palm's.

Brahea armata: Though suffers from extended freezing temps, this palm can tolerate temps as low as 15F without significant or any leaf damage as long as they are brief cold snaps. However, seedlings are NOT nearly so sturdy. Like Serenoa, this is not a good palm for growing in climates that never get hot, but unlike Serenoa,it also resents humidity... so it's cold tolerance is rarely tested except in cold desert areas.

Sabal minor: this palm, or some forms of this palm, seem to be able to withstand temps in the teens with little damage. Palms burned badly often come back, even with near total defoliation, so it has to be listed as one of the most cold hardy palms. This palm is highly tolerant of humidity as well, so it is one of the more adaptable palm species.

Jubaea chilensis: cold tolerant to about 15F, and seems nearly equally tolerant as a seedling. Though it tolerates cold periods, it does not like to stay at thoses temps, so it can be a difficult palm to grow in areas that do not get some warmth. And it is nearly impossible to grow in humid climates.

Butia capitata: cold tolerant also to 15F, and seems to tolerate continued cold a tad better than Jubaea. Also tolerates a good deal of humidity, making this palm one the most overall hardy, useful palms and easily the most adaptable pinnate-leaf palm.

Butia eriospathe: not enough information on this palm to know if it is really any less or more cold hardy than B capitata- probably about the same. Certainly as tolerant of high humidity, though, and happily grows in Hawaii.

Butiax Jubaea: probably about as hardy as either parent species, but certainly more tolerant of humidity than Jubaeas. THis is a pretty slow palm, though.

Butia paraguayensis: ditto for what was said about B eriospathe

Butia yatay: probably just a bit less cold hardy than the other Butias, but not really known for sure.

Butia archeri may be equallly as cold tolerant, but no experience yet.

Trithrinax brasiliensis: could be considered about the same as the last 6-7 palms in cold tolerance but seems to tolerate more humidity than some of these.

Washintonia filifera: seems to be more cold tolerant than its Mexican cousin, and about the same as most of these other palms (15F). Not a good humid-climate palm, though. Doesnt' even like living along the coast of California. This is a true desert-lover.

Phoenix dactylifera: can survive the teens, but usually with some leaf damage. However, will often grow back if cut to the ground. Very drought tolerant, but also tolerant of high humidity.

Phoenix theophrasti: seems similarly cold tolerant- some consider it the same species as P dactylifera anyway.

Phoenix canariensis: least cold tolerant of the hardy palms, this palm can barely survive temps below 20F for any length of time. It is also not all that happy in tropical climates, that P dactylifera seems to thrive in. Palmbob 15:15, 20 July 2007 (PDT)