A Christmas tree riddle is:
 
Q: Why are Christmas trees like bad knitters?
A: They both drop their needles.
 
The Christmas joke has been cited in print since at least 1994.
 
               
Google Books
The Father Christmas:
It’s a Blooming Terrible Joke Book
By Raymond Briggs
London, UK: Puffin Books
1994
Pg. 8:
Why are Christmas trees like bad knitters?
They both drop their needles.
 
8 December 1998, Northern Echo (Darlington, UK), “United Colours of Stockton” by Mike Amos, pg. 11:
Warming up now, the bairns wanted to know why a Christmas tree is like a clumsy tailor.
Because they both drop their needles.
     
14 December 2000, Houston (TX) Chronicle, “Just for Yuks / Please pass the puns” by Syd Kearney, pg. 1:
Why are Christmas trees like bad knitters?
They both drop their needles.
 
28 December 2001, Lincoln (NE) Journal Star, “The Nutcracker,” pg. 1:
Q: Why are Christmas trees like bad knitters?
A: They both drop their needles.
 
Kids Christmas Joe-ks (December 2003)
Q: Why are Christmas trees like people who can’t knit?
A: They both drop their needles
   
Google Groups: alt.humor.puns
Kids Puns of the Weak 12-14-04
Stan Kegel
12/14/04
(...)
Why are Christmas trees like bad knitters ?
They both drop their needles ! (Kids Jokes)
 
Google Books
The Whopping Great Big Bonkers Joke Book
By Puffin Books
London, UK: Puffin Books
2007
Pg. ?:
Why are Christmas trees so terrible at sewing?
They drop their needles everywhere.
   
Google Books
Holly’s Jolly Christmas
By Nancy Krulik
New York, NY: Grosset & Dunalp
2009
Pg. ?:
Why are Christmas trees so bad at sewing?
Because they always drop their needles.
 
Twitter
Bollywood Sargam‏
@bollywoodsargam
Christmas jokes - 156 Joke: Why are Christmas trees like bad knitters
?
They both drop their needles ! .... http://bit.ly/12F2pH
7:05 AM - 20 Jul 2009
 
Twitter
Ed Theriault‏
@editary
Why can’t Christmas trees ever learn to knit? They always drop their needles!
7:10 PM - 6 Dec 2009
 
20 December 2009, Los Angeles (CA) Times, “The Kids’ Reading Room: Jokes & Riddles,” pg. H4, col. 3:
Why are Christmas trees bad at knitting? They keep dropping their needles!
Chris, 8
Palos Verdes Elementary, Palos Verdes
         
Google Books
The Mammoth Book of Jokes 2
Edited by Geoff Tibballs
London, UK: Constable & Robinson Ltd
2012
Pg. ?:
Why are Christmas trees like people who can’t knit?
They both drop their needles.
 
22 December 2012, The Sun (London, UK), “Heard the best joke ever? It’s a cracker!,” pg. 8:
WHY are Christmas trees so bad at sewing? They always drop their needles!
   
Facebook
B+CO
December 20, 2013 ·
Why are Christmas trees so bad at sewing?
They always drop their needles! #crackerjokes #graphicdesign #lastdayofwork #christmas
     
Google Books
Gigglers: Christmas Jokes
By Toby Reynolds
London, UK: Scholastic Fiction
2015
Pg. 74:
Q. Why are Christmas trees bad knitters?
A. They drop their needles!
Q. If Christmas trees can’t knit, what can they do?
A. Needlepoint, of course!
 
Facebook
Knitting Daily
December 24, 2016 ·
Why are Christmas trees so bad at knitting? Because they always drop their needles! ~Andrea #knittingjokes #knittingpuns
 
Reddit—Clean Jokes
Why are Christmas trees so bad at sewing? (self.cleanjokes)
submitted December 21, 2017 by madazzahatterQuality “Ugh” Contributor
They always drop their needles.