The lenses you own now are fine for the APS-C sensor body you now own. Shoot away and have fun. If you really want to move to the next level as a landscape photographer I'd recommend you save your money and get a full-frame 6D and then start moving toward these primes (listed in order of importance):

  1. 24mm f/2.8 - This is a simple lens that, for small aperture landscape photography, doesn't have to be an L-series. This is an economical (~$300 US) and reliable workhorse landscape lens that offers great sharpness and angle of view with minimal distortion. The wide-angle zooms can't make these claims.
  2. 135mm f/2.0L USM- Extraordinarily sharp lens at a great price (less than $1000 US).

I spent many years hiking around the Rocky Mountains with these two lenses (plus a "nifty fifty" that I rarely used), one body and a tripod. The versatile kit was light and portable. I found that I took the majority of my keepers, maybe 75% of them, with the 24mm and the rest with the 135mm. Use your tripod, cable release, high-quality polarizer, and lens hoods (increases contrast) and you will have all the equipment you need to make world class landscapes.

If you figure out how to make money from your landscapes, you might want to eventually add Canon's 35mm f/1.4. This fails the economy test, but worth it for long exposure night shots.