Don, I couldn't agree more, the vegetation does give them away as "false" filter jobs, but your cheap solution is as near correct as I've seen. We have to bear in mind that there are several "IR" filters out there that creep from the end of visible into IR, some which just allow IR, like the Wratten #87 and some which block out some or all IR.
Here is a Chart of IR filters courtesy of LINK
http://www.markerink.org/WJM/HTML/irfilter.htm
Wratten Schott B+W Hoya Tiffen 0% 50% Remarks
# 25 OG590 O90 25A 25 580 nm 600 nm Really a red filter
# 29 RG630 O91 - 29 600 nm 620 nm Dark Red
# 70 RG665 - - - 640 nm 680 nm Very Dark Red
# 89B RG695 O92 R72 - 680 nm 720 nm Almost Black, but not quite. (MOST COMMON REPLACEMENT FILTER FOR AN IR CONVERTED DIGITAL CAMERA)
# 88A RG715 - - - 720 nm 780 nm Anyone ever seen one?
# 87 RG780 - - 87 740 nm 795 nm Cuts off ALL visible light
# 87C RG830 O93 - - 790 nm 850 nm Usually called “Black”
# 87B RG850 - RM90 - 880 nm 930 nm Very expensive - $250 upwards
# 87A RG1000 O94 RM100 - 880 nm 1050 nm Blocks even some infrared.
nm = nanometers = wavelength.