The only trapping device for triffids that I've heard about uses people and dogs for bait. While I might be tempted to use a few people I know for bait, I just have to draw the limit at dogs. One needs to have some limits, even for good causes. You might want to try putting up motion activated cameras with flash, but that is a bit expensive and relies on luck (without the bait) to get pictures. Since few good pictures exist, I suspect the d*&%$ things eat cameras.
Because of all the interest here, I did a little research and will present some info below. Note that triffids are not considered too much of a problem anymore, as there is much more concern related to a new, man-made pest called the RFID (pronounce rif-fid, more later). I think the Wikipedia quotes provide good information regarding why it is important to make sure that mature triffids do not get established in your garden.
Here are a few pics (from the Internet) and some more information from Wikipedia):
Mature Triffid; Triffic cultivation in Australia (deceased proprietor we think); Drawing of basic Triffid anatomy
Triffids hunting, Young Triffid, unknown species
"speculating that the triffids were the creation of the real-life Soviet biologist Trofim Lysenko. According to Masen's narration, the triffids first came to the attention of the Western world when a man named Umberto Christoforo Palanguez presented the Arctic & European Fish Oil Company with a mysterious vegetable oil originating from Russia. Once the scientists of Arctic & European realised how potent the oil was, Palanguez' offer to smuggle some seeds of the plant out of Russia was accepted. Palanguez disappeared, but Masen guesses that his plane carrying the triffid seeds was shot down by the Red Airforce, allowing the seeds to be carried all over the globe by wind".[Wikipedia]
"The first documented triffid outbreaks occur in Indochina, where they receive little press attention, until triffids begin appearing in Sumatra, Borneo, Belgian Congo, Colombia, Brazil and other equatorial regions. Although they develop faster in tropical zones, triffids soon begin establishing themselves in many regions outside the polar and desert regions. Once it is discovered that triffids are predatory creatures, they are culled in large numbers and almost exterminated until it is discovered that they are also the source of the valuable oil. Once it is established that docking their stingers renders them harmless, docked triffids soon become fashionable in public and private gardens. As it takes triffids two years to fully regrow their stings, captive triffids need to be pruned every year. Triffid farms are built in order to produce triffid oil, which is of greater quality when taken from undocked specimens" [Wikipedia]
"triffid can be divided into three components: base, trunk, and head (which contains a venomous sting)...adult triffids are described as measuring on average seven feet in height. European triffids never exceed eight feet, while those living in tropical areas can reach 10 feet...The base of a triffid is a large muscle-like root mass comprising three blunt appendages. When dormant/docile, these appendages are rooted into the ground and are used to draw nutrients, as with a normal plant. When active, triffids use these appendages to propel themselves along at a moderate walking pace."[Wikipedia]
"The upper part of a triffid consists of a stem ending in a funnel-like formation containing a sticky substance which traps insects, much like a pitcher plant. Also housed within the funnel is a stinger which, when fully extended, can measure 10 feet in length. When attacking, a triffid will lash out at its target using its sting, primarily aiming for its prey's face or head, and with considerable speed and force. Contact with bare skin can kill a person instantly. Once its prey has been stung and killed, a triffid will root itself beside the body and feed on it as it decomposes by tearing at its softened flesh with its stinger and pulling the rotting meat into its funnel."[Wikipedia]
"Aquatic triffids ... remain largely unseen, with the exceptions of their stingers which are described as being prehensile, unlike those of land based triffids" [Wikipedia]
Great advice Kate. How is your weather? Under snow yet?
I can;t imagine these old hands pulling anything without gloves...the very idea! 
