Given that you're working with a 1.6x crop factor, my recommendation would be an ultra-wide angle lens with some measure of zoom. The focal length of any lens you use is going to be longer in practice because of the crop factor. So, for instance, a 17-40mm lens on your camera body will effectively function as a 27-64mm lens. Something like a 10-22mm will end up effectively working as if it were a 16-35mm. So, when you take the crop factor into consideration, a 10-22mm will give you roughly the same effective focal lengths as a 17-40mm would for somebody using a full-frame camera without crop factor.
Indeed, not all landscapes call for an especially wide angle of view. And, as pgriz mentioned, ultra-wide-angle lenses will exhibit some measure of perspective distortion and they'll be unsuitable in some circumstances where more distant details of a landscape shouldn't be reduced to specs in your final shot.
However, when you do have a landscape that really benefits from a wide angle of view (as is fairly common in landscape photography), there's really no good substitute for an ultra-wide-angle lens. You're left trying to take two, three or four exposures panoramically with longer focal length and then stitch them in PS. That method works, and it can produce great results, but it also tends to complicate things unnecessarily in many cases and adds much more post-processing time. Instead, it would've been great to simply be able to swallow up a sweeping view of the landscape with a single shot using a nice, ultra-wide lens. Also, the perspective distortion of an ultra-wide angle lens is not all bad by any means. In fact, it can be used skillfully to make some shots much much more dramatic.
So, although an ultra-wide angle lens is not always the best choice for a given shooting scenario, I think it would be pretty tough to try and make a case that a landscape photographer doesn't need one or should put off getting one in favor of some other lens. Ultra-wide angle lenses have been part of the landscape photographers go-to toolkit for years, and with good reason. And, as I've shown in the beginning of this post, an ultra-wide angle on your camera isn't as crazily ultra-wide as it would be when you consider that you've got a heavy crop factor to deal with. It ends up being mostly equivalent to a 17-40mm focal length range in practice, so it's not quite as insanely "ultra" as it seems.
Assuming that you're currently happy with the 18-55 (at least for the time being), something like a 10-22mm will also give you a bit of overlap in your range of lengths. This ensures that you don't end up with an annoying gap in your available focal lengths (at least in the wider end of the spectrum; you'll still have the gap between 55mm and 75mm).
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