Hi,
Your lab may have to be tied up, while you are gone, at least for awhile. If "dog scent" or repellant doesn't work, then consider using a long "wire" between two posts, and tying a "leader" to it, then to your dogs collar, allowing it to run between the posts; but, not near your new landscaping.
There are techniques you can use, to train it from not going to the new area.
I highly recommend a video by Ceaser Millan, a Professional Dog Behaviorist. He rehabilitates dogs, and trains people.
He has his own TV program, the Dog Whisperer, on the NGEO (National Geographic) channel, on Friday nights mainly, but also comes on some other days. This video is available from:
http://www.dogpsychologycenter.com/
I also ordered his new book, "Ceaser's Way", from Walmart online, around $15 US, and it's very good!
There are other good videos available on the web, but this one is really worth your attention.
It will be hard to "break it" from a habit like this, unless you are there all the time with it. Training has to be "immediate", and persistent. That's why I recommend the "running line" attached to two posts, since you are not home all the time. I do wish you the best, and good luck.