The greenhouse is a good plan,you will need at least one vent as temps will be at least 40F warmer on a sunny day,and warmer than that in late winter and early spring.The thing that gets tricky is the temp flux on days where it is not totally sunny as temps will return to outside temp when sun is not shining and you can't heat at the same time as you could go over a hundred F,The trachy will be easier as it will handle high single digits and teens if kept dry,so anytime the temps is forecast to go over 35F during the day you don't need any heat and can open a flap before you go to work(if you leave home).The heating cables will not provide enough heat to warm your g/h,I've tried it 0.Iwould suggest a 125wt heat lamp,this will give you 20F and more if you use a tarp over the g/h.I pointed mine at a stone so plants were not getting heat blasted at them.I think your washy will provide the biggest challenge as it low range is quite a bit less than the trachy,which will be fine,the numbers I gave you are for one layer of plastic,when I put another layer on my g/h the temps warmed 70F over outside temp,you will need to watch carefully with a remote temp prob you can keep an eye on from the house.Good Luck

I think I'll go with the 125 watt heat lamps pointed at some large stone in the enclosure. When the temperature falls to 5 or below, I think I'll put in a 250 for those nights so I get more than 20F rise.

I would use a heat cable (as long as it can keep up). A lamp will give very uneven heat and will become too hot or not heat the entire area. You should also make sure there is some area to vent when using plastic. Using a black tarp over top like Jim mentioned is a good idea (keeps sunlight out - stops over heating from sun - both keep growth slow)

I'd get a small ceramic space heater with a built in thermostat. But it will be a daily struggle between keeping it from overheating on a sunny day and keeping it warm enough at night when the outside air temp will be the same as inside the unheated structure.

Just don't go on vacation :-)

The tarp I mentioned(a painting tarp), was only for nights when the heat lamp was not enough,the temp stayed even in there as the little g/h was close to the size of the plants so I was not heating any area I did not need to.But something that turtile said rings true, that I did not do,and that is this,on really cold days you could leave it covered completely shaded so you dont need to worry about the sun overheating it,the reason I did not do this is that I would get home in time to open the flaps,a little before noon,and I watched it like a hawk!Overheating is a bigger prob than it being to cold,I tried cables they only provide heat to the area they touch,I tried a 100wt lightbulb,3F-5F max,I tried a flood light,8F-10F the heatlamp gives you a 20F range,which will protect down to -10F at least in my case because I was only trying to keep them from going into single digits.My trachys saw 9F for a few hours more than once and where not damaged at all-Your washy will present much more of a challenge!B.T.W.a space heater will fry your plants if you have a small area.And yea,you will be staying home all winter!LOL ,all that info was for a 44w"x49h" umbrella g/h that was no mre than 18" taller and 6"-10"away on the sides(from the plants outer leaves

If you wish to protect your palms with little cost involved to heat the space they take up, you may wish to build some styrofoam boxes as shown below and heat them with two or three flourescent light bulbs. I have protected mine for over 12 years in zone 4 and they have survived -40 F. temperatures.

The cool thing about that design is it really takes whats going on outside and eliminates it,warm cold does not matter I would imagine with this design the temp swings are not to great on a day to day basis.Artictropical has really set the standard for his true zone 4 area,he has baically taken the idea of a leaf cage and improved on the only flaw(assuming your keeping the pile dry)in this method,no light.I don't think you will need more heat as far as using a higher watt heat lamp because adding more covering(insulation) blankets etc,will raise the temp as much as your going to need.One thing to keep in mind is that the numbers I'm giving you are for an area that is protected on the southside of my house from wind out of the north.What are your normal lows and what is expected in your area during a "normal winter? "What town do you live in?I would say again that your trachy will be fine with this kind of protection,the washy will be more of a challenge.I think anyone growing palms north of zone 7b should read palms don't grow here,and read through the website,growing palms in colorado springs usda zone 5b-The former discuses many kinds of protection techniques and the latter water and carbohydrate function in palms grown in zone 5b,or wherever,they use little if any protection in their dry mountain climate and detail why it is important that palms are kept dry through periods of extreme cold.

I'm in Saukville, WI. Uncovering them during the day to vent won't be a problem because my wife is a stay-at-home mom, so she's going to be my "palm monitor". I know it will be hard for the 2 washy's I have, and possibly the butia capitata, but I like challenge, and am determined to get them to survive. The one washy has put out 3 fans since I planted it in June, and the other has put out 2 since I planted it in July...so no transplant shock that I can see (which is good). The butia capitata is also growing....the trachy hasn't really shown any growth since I planted it in the end of July though....although no decline either. Coldest temperatures in last few years:

2008: -9F
2007: -14F
2006: -16F
2005: -4F
2004: -11F
2003: -6F
2002: -2F
2001: 1F
2000: -9F