Place the plants in an office/house settings (that is, an area that looks like a typical office) near a big window that doesn't get direct sunlight - this will 1. let the visitor visualize the plant at the office and 2. give you beautiful soft light.

Clean and arrange the plant and background before shooting, turn the plant to see it's good side, put it in a nice looking flowerpot, clean it (yes, I said clean twice, it's important) and don't leave anything in the picture that doesn't belong.

For the plant "full-length" shot select an aperture that is small enough to make the entire plant sharp but wide enough to get the background just slightly out of focus (remember, you want it to be recognizable as an office).

For each full-length picture add some detail shots, a small image of a large plant will always look boring, add some flower pictures (or whatever is unique or good looking for that plant) - those should be typical flower shot, very small DOF, maybe macro shots.

Shoot in RAW, make sure your white balance is correct and edit every photo to make it look good (just a little levels, curves and color adjustment) - but don't over do it - the picture should still look exactly like the real plant, you don't want to deceive the customers).

But, before you start swapping photos (it's at the end of the answer because it's not photography related) make absolutely sure the visitors adwords is sending you are really potential customers (If you are a UK company make sure they aren't from India or something, check they didn't get from an unrelated keyword because od broad match, etc.) - I personally had a really bad experience with the traffic quality adwords used to send to my site.