I'm headed out this weekend for a 2 night hammock camping trip.  Temps are expected to drop to 23 degrees.  I have a Wiggy's 25 degree bag and a Warbonnet outdoors yeti under quilt also a 20 degree rating. I'm a warm sleeper so with the addition of some hand warmers thrown into the sleeping bag will my 20 degree bag be "enough" to stay comfortable?

Not even close, if I am sleeping in 20 degree weather I use a -20 sleeping bag or stuff one 20 degree bag inside another. You will be fine for a few hours but you will wake up with a chill in your bones that will take all day to get rid of.
But I am 6'5" tall so my feet commonly are cold anyway [poor circulation]. High Tech long jons will help but not the old cotton stuff, a warm cap will help as well.  I have a -40 bag that I use if it is going to be around zero degrees. Good luck.

I think you will be ok, but it depends on a lot of factors. I've not slept with a hammock setup, I have backpacked a fair amount with a tent and a 30 degree bag and found it to be adequate to outside temps dropping into the 20's.

But, there is no standard means of grading a bag as adequate for "X" degrees, and some manufacturers are better than others at rating their bags.  As well, each person sleeps differently.  And as a final caveat, in my case my tent always stays a few degrees warmer inside than the outside air.

All that means is I would be comfortable with taking my bag out in those temperatures, your bag isn't unreasonable but it doesn't hurt to take a backup blanket or something until you're more familiar with your setup.

Wear some base layer and a hat to bed, stay hydrated, and maybe snack on something like a snickers before turning in.

What's between the bag and the hammoc?

Your under-quilt is what is important.  You should be fine.  I would probably add a small section of CCF.  If it does get a little too uncomfortable, I've found the addition of that CCF covering my butt to my shoulders a nice addition.  Add your warmer sleep clothes (hat and socks) if you need to bump your rating up, but those that haven't used a quality under-quilt would be surprised at how effective they insulate the worst part of a hammock in cold weather.  

ROCK6

I've always found that if I don't have a bad that's rated at least 20 degrees colder than the temp at night...I'm cold.  Sure, a 20deg bag will keep you alive in 20deg weather.....but that doesn't mean your not going to be freezing you ass off while you survive

That's why you look for a bag that's EN-rated, most bags that aren't have ratings that don't bear much resemblance to reality.

so it turns out that my 25 degree bag and 20 degree under quilt were no where near warm enough.  The low Friday dipped to 22 with a 15 mph wind.  I was clearly under equipped for the first night of our trip and I paid a price for that.  On the positive side of things I found exactly where the limits of my bag are.  The second night of our trip the low was 31 with 5 mph wind.  I was quite comfortable and was more than warm enough.  Rough 1st night.  I asked my camping buddies if there were any extra layers that they wouldn't be using and nobody had any.  However the next morning the fattest stupidest member of the party remembered that he had an extra sweat shirt and light jacket that he didn't even need. I could have killed him right then and there in the forest and nobody would  have ever known.

Look for a silk liner, it adds several degrees of comfort to your bag and takes up very little space.

Comfort ratings vs survival ratings.
Learn them.
Also
Hammocks are colder . Under quilt it..or bag heavy.
Wind. Hang a poncho or second tarp under it as a wind break. It'll help.
I just use my ground sheet I use tarp camping.

I always use a military ground pad inside
my hammock, summer or winter. Last year,
I went on 2 backpacking trips with my son's
BSA troop. One in March, the other in may.
The low on the March trip was 45, the
May trip was colder, and we had rain &
wind.

On both of those trips my son & I used poncho
liners for under quilts & another as a top cover.
We don't have any high end/low temp bags, but
with the DIY under quilt, along with the sleeping
pad has made things very comfortable. I would
not like to sleep in a hammock or tent in the 20's.

John

No hammock...too much cold air underneath.

If roughing it, put some thick pad down on ground and run a GI sleeping bag cover to keep more heat in.

Yep, bag is too thin. Nothing worse than a crappy cold night.

I almost always backpack with hiking poles as they provide a good alternative to assist in various shelter setups.  Since hammock setups typically include a separate rainfly, you have multiple options.  I can either use trees (or any other vertical structure) or I can rig up my poles to use my hammock on the ground in a pseudo-bivy setup on the ground.  This is handy if above the tree line and cold/windy conditions.  





Now, part of my cold-weather hammock "system" includes both a short section CCF pad, my insulated inflatable pad, a 5oz bivy bag (Ti Goat Ptarmigan bivy) and a 15 ounce down under-quilt.  If going to ground, I have the ground insulation covered and the under-quilt adds additional top insulation to my sleeping bag.  The bivy cover is more for emergencies, but will add an additional 5-10 degrees of comfort rating.

ROCK6

Have you ever slept in sub freezing temps before?  Some lessons come hard

I wouldn't have a problem with that.  I would worry about hand warmers but I would have a good bennie cap and socks in addition to making sure I was totally dry when I got in the sack.  The thing with hammocks is you have air circulation all around your body so if there is any wind it will get really cold really quick (wind chill factor is real) if you are not in a shelter or sheltered area, some pointed out putting a puss pad in the hammock which also helps but it is still very compressed due to body weight so something to think about.  Usually in a hammock in the cold I throw all my clothing in the hammock before I get in to add some loft under my body and that seems to help, my personal experience in hammocks hasn't been below 30 so I'll be standing by for your AAR.  

I'm also assuming you will have some sort of overhead cover.

Rock,

what sticks are those and how do they hold up in the weight?  I don't think my Leki's would support my 225# fat butt in a hammock.

Those are Exped hiking poles.  They don't take your weight, they just stabilize the hammock and fly; your body is resting on the ground/ground pad.  It's no different than supporting the netting with some ground bivy bags; the tension is really just for the top of the hammock to keep the mosquito net off my face and to support the rainfly/tarp.  Hope that explains it...

ROCK6

Gotcha, that makes sense.