Is there less infrared light in winter or is it just that there is less vegitation to reflect IR light?
The IR you capture is near infrared (NIR). It has nothing to do with thermal infrared, which begins at five to seven times longer wavelength only. Source of outdor NIR is the sun, just as visible light. So, any source brightness variations are the same as in the visible.
You'll definitely get the IR effect, but there won't be any vegitation to turn that shimmering white. However, I'm sure you'll still get some unique effects in winter, especially if there is a snowfall.
IR in winter is beautiful. There's little to no IR in "skylight", that bluish light that tends to "fill in" all your shadows in shots of snowy scenes. This was a winter IR shot... http://www.pbase.com/the_wiz/image/14490400
Curtis - are you in the northern hemisphere or the southern hemisphere? If you're on a favourable planetary tilt, you may have more infrared light The tones from infrared film are really beautiful, particularly for winter sky detail.